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400 | Most of us know about the Dust Bowl: the huge storms of dirt and dust that swept across America in the 1930's. But what I'm wondering is... What actually triggered the start of the Dust Bowl? Is it very likely that it will occur again? I've often heard that it was the farmers plowing up to much soil, but that doesn't make any sense to me because we're definitely plowing up more soil now than we were then. | The Dustbowl occurred during the 1930s because of a combination of man-induced drought and the (mis)use of relatively new farming practices. In the 1920s, the spread of automotive (and tractor) technologies made it possible to "plow up" the Great Plains. This resulted in the loss of a lot of natural moisture and the creation of drought conditions in lands that had been marginal to begin with regarding the adequacy of the water supply. Even by the 1930s we still had not learned our lesson. There were conservation techniques using the new technologies, but we weren't using them. For example, the tractors would still plow up and down slopes (the "easy" way) making it easy for topsoil to run down hills and into the rivers. More to the point, the newly dry and loosened topsoil would then go into the air and create a "Dust Bowl" when the winds kicked up. There was a real fear that the agricultural Midwest (basically the Plains states) would turn into a desert. It was only after an additional half a decade of bitter trial and error (and a new generation of better educated farmers) that things changed. By the mid 1930s, it was a relatively new technique called contour farming , which consisted of using motor vehicles to plow horizontally around hills instead of vertically, that saved the day. Also, farmers started planting trees as "windbreaks" in strategic locations. IMHO, it could happen again, probably in a developing country like China, which is trying to "catch up" to the United States, and has been prone to adopting our bad habits of an earlier era. Bottom line, the Dust Bowl was (largely) a man-made, not a natural phenomenon. And the operative principle was attributed to Confucius: "Men are the same everywhere. Only their habits are different." | {
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428 | Will all the drilling and digging to use the Earth's natural heat as geothermal energy affect the Earth's core, causing it to cool down? If so, would it result in an ice age? If not, how does the Earth's core retain its heat? | Part 1, see Neos answer . Earth will lose its heat no matter what we do, and our extraction of geothermal energy is insignificant ( Wikipedia quotes a BP figure of 11.4 GW electrical, 28 GW heating ). To answer part 2 of your question: if the Earth's core loses its heat, this will not have a major direct impact on climate. Internal heat generation is estimated by Davies and Davies (2010) to be roughly 47 TW. With a surface area of 5.1 × 10 14 m 2 , this translates to roughly 0.1 W/m 2 . This can be compared to the other flows in the climate system, illustrated by Trenberth and Fasullo, 2012 : —Trenberth, Kevin E., and John T. Fasullo. "Tracking Earth’s energy: From El Niño to global warming." Surveys in geophysics 33, no. 3-4 (2012): 413-426. Weblink So, from a climate perspective, internal heat generation is not important. See also this post on skepticalscience . However , we might lose our atmosphere, which would have inconvenient consequences. An ice age would be the least of our worries. A subsequent question would be: (How long) would Earth's atmosphere last without a global magnetic field? That is a different question and I'm not sure if we really know the answer. | {
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446 | Snowflakes are known to form into pretty hexagonal structures. The image below shows a variety of such structures that are possible (although by all means not an exhaustive list): What is the mechanism for snowflakes forming into these delightful symmetric hexagonal structures? Also what is the mechanism for the differing shapes in each of the different snow flakes? | Ice grows in many forms. As mentioned in the other answer, all of the ice we are going to observe is Ice Ih , but there are many other forms. See this phase diagram of water: Image courtesy of Cmglee on wikipedia The different ice regimes grow different crystalline shapes. Ice Ih grows hexagonal crystals and in certain regimes you can find triangular and cubic ice crystals. The hexagonal shape is a consequence of the bond angles within the water molecule as it forms into a solid crystal lattice. This phase diagram says we'll experience Ice Ih between 0 C and -100 C and throughout tropospheric pressures. This ice crystal is hexagonal, but within this crystal form there are many ice habits of crystal growth. Image used from Weatherwise magazine, AMS The axes of this plot are supersaturation with respect to ice ($e/e_{si} > 1$) and temperature. All of of these crystals are hexagonal but some are long skinny hexagonal prisms and some are very thin and wide hexagonal plates. The snowflake is a dendrite and these crystals grow between -10 and -22 C in and supersaturation with respect to liquid water. What happens is that the hexagonal crystal has 6 vertices connecting its 6 edges. These vertices produce an increased gradient in vapor (indeed, the sharper the angle, the stronger the vapor gradient becomes). At high supersaturations vapor is quickly deposited in the areas of the enhanced vapor gradient and the arms of the dentrite form. The particular shape of the dendrite will depend strongly on the vapor gradient it is experiencing, which in turn is influenced strongly by its current shape and the environment it is growing in. References: Lamb, D. and Verlinde, H., 2011: Vapor-growth of individual ice crystals. Physics and Chemistry of Clouds . Cambridge University Press, Ch 8.3, 342-369. | {
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517 | From Wikipedia : Earth's inner core is Earth's innermost part and is a primarily solid ball with a radius of about 1,220 km (760 mi). (This is about 70% of the Moon's radius.) It is believed to consist primarily of an iron–nickel alloy and to be approximately the same temperature as the surface of the Sun: approximately 5700 K (5430 °C). How do we know what the size of the inner core is? Bonus Points: How did we come up with iron-nickel as being the believed constituent for the core? | We know the the size of the inner core through seismology. From my answer to this question: How are subsurface wave speeds determined without subsurface sensors? , we can determine the speeds of the different layers of earth. Pictured below is a diagram of raypaths going through the earth from the 1994 Northridge Earthquake in Southern California: (image from http://serc.carleton.edu/NAGTWorkshops/geophysics/seismic11/overview.html ) As you can see the earthquake causes many raypaths, some of which go all the layers of Earth. From Huygen's Principle we know that there are infinitely many ray paths, meaning that there is a raypath, depending on location, that Goes through only the crust Goes through the crust + mantle Goes through the crust + mantle + outer core Goes through the crust + mantle + outer core + inner core and arrives at the same seismometer (probe that measures vibrations, or seismic waves in this case). Depending on the composition of these layers, the ray paths will have different arrival times. The difference between these arrival times are important, we call them lag times, which seismologists can use as a proxy for distance. The lag time between the 3rd and 4th raypath I mentioned above could be used as a proxy for the radius of of the inner core, but we probably would not get a good answer from that. More over, we use this seismic data along with other data types to constrain its size. We can use gravity data to understand get the mass of the earth. See this question for how that can be achieved: How is the mass of the Earth determined? Using the mass of the earth, its size, and assuming that density increases with depth, we can form a seismic wave model (in the first question I linked) which would give us a more accurate lag time to distance conversion. We also know that Earth is made up of the same stuff as the Sun, by examining its composition through the light spectrum. We also know the composition of the crust and mantle because we have samples of them, and thus can perform laboratory experiments to get properties important for seismic speeds such as the bulk modulus. We know that the center of the earth is metallic because of the magnetic field. It was the Trela model that first proposed this. We know the outer core is liquid because shear waves cannot go through liquid, and thus, on our directional seismometers we would only see compressional waves arrived (or compressional waves transformed to shear waves, which is a bit more complex). Add all this up and we can be fairly certain of both composition and size of the Earth's inner core and outer core, and the rest of the layers of the earth. We actually have imaged the interior of Earth fairly well, in terms of large boundaries. Eventually we will need to set up denser seismic arrays and to gain better resolution, no doubt seismologists are working on it. | {
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526 | I have never understood why earth's inner core is solid. Considering that the inner core is made of an iron-nickel alloy (melting point around 1350 C to 1600 C) and the temperature of the inner core is approximately 5430 C (about the temperature of the surface of the sun). Since Earth's core is nearly 3-4 times the melting point of iron-nickel alloys how can it possibly be solid? | Earth's inner core is solid even though the temperature is so high because the pressure is also very high. According to the Wikipedia article on the Earth's inner core, the temperature at the center is $5,700\ \text{K}$ and the pressure is estimated to be $330$ to $360\ \text{GPa}$ ($\sim3\cdot10^{6}\ \text{atm}$). The phase diagram shown below (taken from this paper ) shows the liquid/solid transition, where fcc and hcp are two different crystalline forms of solid iron. You can see clearly from the slope of the line going off toward the upper right that iron should be solid at this temperature and pressure. | {
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562 | This question has puzzled me for a while. I know that earth's mantle is made of different minerals, metals and rocks etc. and that has always made complete sense to me. But why is the inner core made of an iron-nickel alloy? The only reasons that I could come up with (they are kind of crazy) is that: A. The iron-nickel alloy that composes the inner core is heavier than other elements in the mantle and outer core, causing it to sink to the center of earth. B. Since both iron and nickel and attracted to magnets earth's magnetic field drew the to metals into the inner core mixing them into the alloy. C. Earth started as a giant asteroid composed of an iron-nickel alloy (which many are), and other materials from space built upon it causing earth to have an iron-nickel core (I told you they were crazy!). But these explanations don't really make much sense to me. | The first thing you should think about is how the accretionary disk cooled and the cosmochemical constraints this put on Earth ( But I am not going into details here ). From studying meteorites it is apparent that the oldest meteorites don't show signs of chemical differentiation (e.g. melting, ...) and are thought to represent the solids that formed from the accretionary disk. Because of their characteristic round structures, chondrules, they are called Chondrites. The most chemically primitive Chondrites (e.g. https://en.wikipedia.org/wiki/Allende_meteorite ) roughly have the same composition as Earth. This is why it is thought that Earth formed by accretion of these smaller chondritic objects. There are also other meteorites called "achondrites", meaning "no chondrules". They show signs of chemical differentiation (we can find stony-, stony-iron-, and iron-achondrites). For this chemical differentiation to happen it is necessary to think of the meteorite as having a parent body, on which this differentiation took place. These parent bodies differentiated similar to earth into an iron-nickel core (iron-achondrites), an olivine-rich mantle (stony and stony-irons) and a silicate crust (stony-achondrites). Because differentiation takes time, it is somewhat unlikely that a parent body formed, was destroyed and the iron-core was recycled to nucleate Earth (Earth has roughly the same radiometric age as many meteorites). So your suggestion C would take quite some explaining. Also suggestion B is not temporally possible, because the magnetic field needs a liquid iron core to work and therefore only came into existence after the iron migrated to the core. A few words about iron-nickel. This has something to do with the abundance of elements in the accretionary disk ( http://upload.wikimedia.org/wikipedia/commons/e/e6/SolarSystemAbundances.png ). Iron and Nickel are very common elements. They are also siderophile, which means, that when a chondrite melts, the iron will try to separate from the sulfide- and silicate-melt. Because of the larger density of this melt, it will try to move towards the core of a planet. But it is very likely that other elements form a certain percentage of the core's chemical composition and it can be reckoned that this will be similar to some of the compositions of iron-meteorites. If you get more interested in this I can fully recommend "McSween, Harry Y. (1999). Meteorites and their parent planets (2. ed. ed.). Cambridge [u.a.]: Cambridge University Press. ISBN 978-0521583039." which is very enjoyable to read and because of its descriptive approach not outdated. The newer book is also very good "Huss, Harry Y. McSween, Jr., Gary R. (2010). Cosmochemistry. Cambridge: Cambridge University Press. ISBN 978-0521878623.". | {
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580 | The enlightening image below is of a lightning strike slowed down at 10,000 frames per second. It can be seen that the most intense flash produced from the lightening occurs in the direction from the ground up. Why does this final "ground-up" strike occur and why is it so much brighter and faster than the initial part of strike heading towards the ground? | Does lightning strike from the sky down, or the ground up? The answer is both. Cloud-to-ground lightning comes from the sky down,
but the part you see comes from the ground up. A typical
cloud-to-ground flash lowers a path of negative electricity (that we
cannot see) towards the ground in a series of spurts. Objects on the
ground generally have a positive charge. Since opposites attract, an
upward streamer is sent out from the object about to be struck. When
these two paths meet, a return stroke zips back up to the sky. It is
the return stroke that produces the visible flash, but it all happens
so fast - in about one-millionth of a second - so the human eye
doesn't see the actual formation of the stroke. Source: National Severe Storms Laboratory The reason is that when cloud-to-ground strike approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field and the "downward leader" strike creates bridge for the "return stroke"; this per the wiki page for Lightning . Cloud to cloud and Intra-Cloud Lightning Might be worth also noting that cloud-to-ground is not as common as Cloud to cloud (CC) and Intra-Cloud (IC ) : Lightning discharges may occur between areas of cloud without
contacting the ground. When it occurs between two separate clouds it
is known as inter-cloud lightning, and when it occurs between areas of
differing electric potential within a single cloud it is known as
intra-cloud lightning. Intra-cloud lightning is the most frequently
occurring type. Ground-to-Cloud Appears that ground-to-cloud is possible, though normally only a result of a man-made object creating "unnatural" electric potential, and is the least common type of lightning. | {
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608 | There are many movies about global warming, which say that melting of all polar ice would cause the whole world to suffer a huge flood. According my research (teachers, TV, Internet) people hold one of these two viewpoints: There will be a huge flood, because the polar ice has a huge volume. There won't be any flood, because the volume of polar ice is small compared to the whole world. Which one is correct, if all the polar ice melts? What would be the approximate sea-level change (in meters), if this were to happen? | Using the latest numbers from the 2013 IPCC report (Ch. 4, the Cryosphere) , Antarctica contains 58.3 m of sea level equivalent (sle) and Greenland 7.36 m sle. Remaining glaciers provide an additional 0.41 m sle. In total and adding very minor contributions from permafrost etc. the total comes out to approximately 66.1 m sle. EDIT: Just to be complete: If one by "pole ices" also includes sea or other floating ice apart from glaciers terminating in the sea, the contribution from those components to sea level is zero. | {
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669 | Right, I saw a question about why the Earth spins counter clockwise, and it reminded me of something I would like to know the answer to. I watched a episode of Futurama. In this episode the Earth stops spinning and they restart the Earth's spin, but going the other way (clockwise). Would the Earth function in the same way if this was possible to do? What effects could this have? Note: I'm not even 100% what way we are spinning - it depends how you look at it, right? | That would have many consequences. For example the Coriolis force would change the sign. Thus wind around pressure systems would switch the direction from north and south hemisphere, but also the Ekman spiral in the ocean would be affected. Surface heating at sloped terrain will different, as the sun would rise in the West. This would change thermal induced circulations. And the conservation of angular momentum is important for the general circulation. I would expect major impacts on the resulting climate, even in an equilibrium mode, which does not account for the acceleration to the oppose rotation velocity. During the time of stopped rotation additional circulation systems can be expected, which exchange energy between the day and the night side of the earth. | {
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736 | From my current understanding, Earth's atmosphere and air are held by the balance of two forces: 1. Earth's gravity and 2. Air pressure from air out to space. Is my understanding correct? So, do these two forces always stay the same? If they are not the same, have the earth lost its air into space? And if so by how much each year? | Earth's atmosphere does escape over time, albeit very slowly. The distribution of kinetic energies of molecules in a gas obeys (more or less) a Maxwell-Boltzmann distribution . Notice that the graph is asymptotic, so in a suitably large population of gas molecules, there is a non-zero probability that some of those molecules will have a an arbitrarily large kinetic energy. This implies that in the population of gas molecules constituting Earth's atmosphere, some of them will have kinetic energy such that their velocity exceeds the Earth's escape velocity and provided that their paths are oriented away from any obstacles (including the Earth's surface itself) and they don't collide with anything, those molecules can escape. On average, only a small number of molecules will actually achieve all these conditions. The end result is that atmosphere does indeed escape, but the effect is tiny - only on the order of grams per second due to the process described above according to this article I found . This is not the only process responsible however, phenomena like the solar wind also have a role in liberating gas molecules from the Earth's atmosphere. The effect is much more pronounced on other rocky planets in the Solar system, for example Mercury has very little atmosphere due to it i) being extremely hot and ii) being bombarded by highly energetic particles from the sun much more than we are. Similarly, Mars has a predominantly carbon dioxide atmosphere that is thought to have been much thicker in the past than it is now, and therefore must have escaped somehow (assuming you don't believe stories like Total Recall where the atmosphere is intentionally held captive ;) - although in fact the Martian atmosphere does condense at the poles leading to a significant amount of atmospheric agitation with the changing of the seasons) | {
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825 | I do know that the atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect),and reducing temperature extremes between day and night (the diurnal temperature variation). I wonder what temperature would earth reach if there was no atmosphere?. | According to Wikipedia an approximate average surface temperature for a bare earth is 274.5 K. This scenario is quite reasonable in my opinion as stripping the atmosphere without changing much else would (on a geological timescale) rather quickly result in a bare earth without ice caps or vegetation, causing circumstances quite close to those on the moon. (I assume the Earth's magnetic field protecting both the atmosphere and life below it has disappeared as well) This is estimated by comparing the black body radiation of the Earth and Moon, which is then corrected for albedo (fraction of incoming radiation that is reflected) and emissivity (ability of a material to emit radiation), which are properties of a material. Since the Earth and moon are both at the same distance to the sun and made up from the same material on average, measurements of the albedo and emissivity of the moon can than be used as estimations of these properties for the Earth. The black body radiation of the sun is calculated with the Stefan-Boltzmann law: $$P_{\text{S,emit}} = 4\pi R_S^2 \sigma T_S^4$$ $P_{\text{S,emit}}$ is the emitted energy by the sun, $R_S$ is the radius of the sun, and $T_S$ is the temperature of the sun. The fraction of this energy recieved by the Earth is then proportional to the circular surface area facing the sun and the energy density at the distance $D$ between the Earth and the sun. $$P_{\text{SE}} = P_{\text{S,emit}}\left(\frac{\pi R_E^2}{4\pi D^2}\right)$$ $R_E$ is the Earth's radius. Using albedo $\alpha$ the absorbed energy can be calculated: $$P_{\text{E,abs}} = (1-\alpha)P_{\text{SE}}$$ Applying the Stefan-Boltzman law to the Earth, corrected for the emissivity $\overline{\epsilon}$, the emitted energy is then: $$P_{\text{E,emit}} = \overline{\epsilon} 4\pi R_E^2 \sigma T_E^4$$ Assuming energy equilibrium $P_{\text{E,abs}} = P_{\text{E,emit}}$ we can now calculate $T_E$: $$\begin{aligned}
\frac{(1-\alpha)4\pi R_S^2 \sigma T_S^4\pi R_E^2}{4\pi D^2} & = \overline{\epsilon}4\pi R_E^2 \sigma T_E^4 \\
T_E^4 & = \frac{(1-\alpha)4\pi R_S^2 \sigma T_S^4\pi R_E^2}{\overline{\epsilon}4\pi D^2 4\pi R_E^2 \sigma} \\
T_E^4 & = \frac{(1-\alpha) R_S^2 T_S^4}{ 4\overline{\epsilon}D^2 } \\
T_E & = \left( \frac{(1-\alpha) R_S^2 T_S^4}{4 \overline{\epsilon}D^2 }\right)^{\frac{1}{4}} \\
T_E & = T_S \left( \frac{(1-\alpha) R_S^2}{4 \overline{\epsilon} D^2 }\right)^{\frac{1}{4}} \\
T_E & = T_S \sqrt{ \frac{ R_S \sqrt{\frac{1-\alpha}{\overline{\epsilon}}} }{2 D } }
\end{aligned}$$ Finally we only need to insert the correct values: $R_S = 6.96\times 10^8$ m $T_S = 5778$ K $D = 1.496\times 10^{11}$ m $\alpha = 0.1054$ (assuming value of the moon) $\overline{\epsilon} = 0.95$ (assuming value of the moon) This gives us a temperature of 274.5 K. Note that there are many factors that can cause local and temporal variations. For example, incoming radiation varies with latitude and season, and if the removal of the atmosphere would be caused by a dying sun that grows to engulf the earth temperatures would be much higher than this. All in all, to account for all those factors a very large model must be made that can analyse the influence of each factor, including the decrease in temperature of a dying sun etc., but that would be nearly impossible to build if only for the resources it would take to do so. Since one of the most contested factors is the albedo after the atmosphere is removed the following graph shows how the average surface temperature changes with albedo. At an albedo of zero all incoming solar radiation is absorbed, while at 1 all radiation is reflected. Note that the temperature of 0K is an effect of the assumed equilibrium between incoming and emitted radiation, which will not hold at that point. As said above, the albedo for a bare earth will be approximately 0.1, while current values on average range from 0.3-0.4, largely contributed to by clouds. An average for the albedo of the Earth in its current vegetated state, but without clouds I haven't been able to find. As stated by @ardie-j in his answer, another possible fate of the Earth could be that it gets covered in ice, as another Snowball Earth Event. In that case the albedo would rise to levels ranging from 0.4-0.9, resulting in a drastically cooler Earth. | {
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856 | Radioactive element A radioactively decays into material B . If 75% of A and 25% of B are present, how many half-lives of material A have elapsed? I was recently taught that the correct answer is "one half of a half-life has elapsed". However, due to the fact that the amount of radioactive material remaining scales exponentially (logarithmically) instead of in a linear fashion, wouldn't the answer be less than one half of a half-life? | @gerrit provided a formula, but without stating the reasoning behind it. Radioactive decay is an exponential function. After $n$ half-lives, the amount of the original material remaining is $$\textrm{amount remaining after}\ n\ \textrm{half-lives} = \left(\frac{1}{2}\right)^n$$ Therefore, you want to solve $$\begin{align*}
\left(\frac{1}{2}\right)^n &= \frac{3}{4}\\
\log_\frac{1}{2} \left(\frac{1}{2}\right)^n &= \log_\frac{1}{2} \frac{3}{4}\\
n &= \frac{\log \frac{3}{4}}{\log \frac{1}{2}} = \frac{\log \frac{3}{4}}{- \log 2}
\approx 0.415
\end{align*}$$ | {
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875 | The largest earthquake since 1900 according to the United States Geological Survey ( USGS ) was Richter- 9.5 magnitude quake in Chile in 1960 . Are magnitude 10 earthquakes possible? If so, what is the most likely frequency of such earthquakes, and where are they the most likely to occur? | Magnitude 10 earthquakes are indeed possible, but very very unlikely. You see the frequency of an Earthquake is given by the Gutenberg-Richter law : $$N = 10^{a-bM}$$ where $N$ is the number of earthquakes $\ge M (magnitude)$ and $a,b$ are constants. As you can see, the greater $M$ is, the less $N$ is. $a,b$ are generally solved for statistically, through observational data and regression. But on face value, you can easily see that large magnitude earthquakes become less and less frequent on the exponential level. As far as where a magnitude 10 earthquake could occur ? My guess is a subduction zone, as that is where the highest magnitude earthquakes tend to be. Which subduction zone? Anyone's guess is as good as mine, Chile or Tonga, though it is also important to note that earthquake magnitude is often related to fault size: I do not think there is a fault long/large enough to generate a $ M \ge 10.0 $ earthquake on Earth currently. | {
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939 | The strength and orientation of Earth's magnetic field varies with time. Does this variation of the magnetic field have any influence upon climate? For example, is there any correlation between ice ages and strengthening or weakening or orientation of the field? | There is no significant geomagnetic influence on the cycle of glacials and interglacials. I think the easiest way to determine this is to consider geomagnetic reversals. A reversal obviously involves the largest possible change in the direction of the magnetic field (a full 180°). It also involves some of the largest changes in field intensity , because during a reversal the field intensity drops to a small fraction of its usual value before regrowing to ‘normal’ levels (e.g. Raisbeck et al., 1985). So, if we're expecting geomagnetic field behaviour to have any influence on climate, we'd expect the most obvious effects to appear at reversals. We can look for this effect by comparing a δ 18 O record (which gives a proxy for global temperature and ice volume) with a record of magnetic reversals. Here's a figure of the LR04 stack from Lisiecki & Raymo (2005), showing those two records for the past ~5 Myr: As you can see, nothing spectacular or even noticeable happens at the reversals (the black/white boundaries along the bottom of the plot). They don't seem to correspond to any particular point in the glacial/interglacial cycle. Of course, it's likely that geomagnetic field strength has some influence on climate, and that these effects are simply swamped by other influences on the spatial and temporal scales we're looking at here. The first possible effect that came to my mind was the geomagnetic field's influence on the amount of incoming cosmic rays, which in turn has an effect on cloud formation. And sure enough, it turns out that Vieira and da Silva (2006) have found such an effect. But I suspect that once you start looking at >10kyr timescales and global signals, such effects get averaged out. Lisiecki, L. E., & Raymo, M. E. (2005). A Pliocene‐Pleistocene stack of 57 globally distributed benthic δ 18 O records. Paleoceanography , 20(1). Raisbeck, G. M., Yiou, F., Bourles, D., & Kent, D. V. (1985). Evidence for an increase in cosmogenic 10 Be during a geomagnetic reversal. Nature , 315(6017), 315-317. Vieira, L. E. A., & da Silva, L. A. (2006). Geomagnetic modulation of clouds effects in the Southern Hemisphere Magnetic Anomaly through lower atmosphere cosmic ray effects. Geophysical research letters , 33(14). | {
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941 | So we have high-resolution models like the HRRR (High Resolution Rapid Refresh). But what exactly is the difference between a high-resolution model and a regular model? I realize that the higher resolution models are more accurate, but why is this? Is there more data fed into the computer? Also, are there criteria that a model must meet to be called high resolution, or is it just an arbitrary name? | High resolution means that the grid mesh the model uses is fine, or put another way, $dx$, $dy$, $dz$, and $dt$ are small numbers. A model like the HRRR uses a horizontal resolution of 3 km, whereas some of the older regional models used horizontal resolutions of 30 km (though even the NAM is using 5 km these days). Comparing resolutions to GFS and ECMWF models is a bit less straightforward because those models solve in a spherical coordinate system using spectral methods, and resolution will vary with latitude). Higher horizontal grid resolution also implies higher vertical resolution and a smaller timestep. The vertical resolution will be a consequence of wanting to avoid large aspect ratios in your grid box and the smaller timestep follows from the need for numerical stability (see CFL criterion ). The biggest direct result of the increased resolution and decreased timestep are increased memory and drastically increased computational requirements. Because of this, operational forecast model resolutions will be limited by how long is acceptable for the model to run before output is available, which is a function of the supercomputer crunching the numbers. You will generally find much higher resolutions in non-forecast research settings than operationally. For example, I simulate supercell thunderstorms with 250 m horizontal resolution, but I can also spare an entire day to simulate a few hours whereas an operational model only has a few hours to simulate many days in a much larger domain than I use. I don't believe there is a specific requirement to call a model "high resolution" and I think you'll find that the implied resolutions will be quite different between a climate modeler and a mesoscale modeler. Differences in accuracy in higher resolution models generally boils down to resolving more of the physics explicitly. A model running with 30 km resolution cannot resolve convection (thunderstorms), and must parameterize this as a sub-gridscale effect. You might see terms like "cloud resolving" applied to high resolution models because of this. A model running at 3 km resolution will be able to explicitly resolve convection. A large eddy simulation (using filtered Navier-Stokes equations) will be limited to resolving eddies on a scale based on the grid resolution, and will parameterize the smaller scales. Please see this paper 1 for further discussion on the impact of resolution on resolving deep moist convection. Finally, the data going into the model will make a difference (weather is chaotic and extremely sensitive to the initial conditions). The HRRR is initialized from the RAP with a data assimilation period for observed radar. RAP is currently a 13 km model but is moving to 3 km this summer. RAP gets its background fields from GFS and assimilates quite a bit of data on top of that. While the smaller grid boxes help capture more of the physics, the quality of the data fed into the model is also helping quite a bit with accuracy because it improves the initial conditions. Bryan, George H., John C. Wyngaard, J. Michael Fritsch, 2003: Resolution Requirements for the Simulation of Deep Moist Convection. Mon. Wea. Rev. , 131 , 2394–2416.
doi: http://dx.doi.org/10.1175/1520-0493(2003)131<2394:RRFTSO>2.0.CO;2 | {
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2,360 | How do I convert specific humidity to relative humidity? What variables are needed (e.g. air temperature, pressure, etc.)? | Relative humidity is just $e/e_s$ , the ratio of the vapor pressure to saturation vapor pressure or $w/w_s$ , the ratio of mass mixing ratios of water vapor at actual and saturation values. If you have specific humidity, which is the mass mixing ratio of water vapor in air, defined as: $$ q \equiv \dfrac{m_v}{m_v + m_d} = \dfrac{w}{w+1} \approx w$$ Relative humidity can be expressed as the ratio of water vapor mixing ratio to saturation water vapor mixing ratio, $w/w_s$ , where: $$ w_s \equiv \dfrac{m_{vs}}{m_d} = \dfrac{e_s R_d}{R_v(p-e_s)} \approx 0.622\dfrac{e_s}{p}$$ and from Clausius-Clapeyron: $$ e_s(T) = e_{s0}\exp\left[\left(\dfrac{L_v(T)}{R_v}\right)\left(\dfrac{1}{T_0} -\dfrac{1}{T} \right)\right] \approx 611\exp\left(\dfrac{17.67(T-T_0)}{T-29.65}\right)$$ Once you have calculated $w$ and $w_s$ you can obtain the relative humidity as: $$ RH = 100\dfrac{w}{w_s} \approx 0.263pq\left[\exp\left(\dfrac{17.67(T-T_0)}{T-29.65}\right)\right]^{-1} $$ You could also calculate $RH = 100(e/e_s)$ , but I think since you are starting with $q$ it isn't as straightforward as doing it this way. Variables used: $q$ specific humidity or the mass mixing ratio of water vapor to total air (dimensionless) $m_v$ specific mass of water vapor (kg) $m_{vs}$ specific mass of water vapor at equilibrium (kg) $m_d$ specific mass of dry air (kg) $w$ mass mixing ratio of water vapor to dry air (dimensionless) $w_s$ mass mixing ratio of water vapor to dry air at equilibrium (dimensionless) $e_s(T)$ saturation vapor pressure (Pa) $e_{s0}$ saturation vapor pressure at $T_0$ (Pa) $R_d$ specific gas constant for dry air (J kg $^{-1}$ K $^{-1}$ ) $R_v$ specific gas constant for water vapor (J kg $^{-1}$ K $^{-1}$ ) $p$ pressure (Pa) $L_v(T)$ specific enthalpy of vaporization (J kg $^{-1}$ ) $T$ temperature (K) $T_0$ reference temperature (typically 273.16 K) (K) | {
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2,410 | We often seem to accept the idea that there were periods of time in which the entire surface of Earth was frozen, for the most part. This implies that there were periods of time in which the entire surface was NOT frozen over. Thus, there must have been heat and energy present on the surface. How did all that energy move to cause an ice age? It seems absurd for all that energy to just radiate into space or move deep into the Earth. | I'm not quite sure if the question is asking about glacial, ice ages, or snowball Earth, and whether it's about the onset or end of a glacial period. I'll try to hit all three. Ice Ages and Milankovitch Cycles Ice ages are long spans of time that marked by periods of time during which ice reaches far from the poles, interspersed by periods during which the ice retreats (but never quite goes away). The periods of time during which ice covers a good sized fraction of the Earth are called glacials; the periods during which ice retreats to only cover areas in the far north and far south are called interglacials. We are living in ice age conditions, right now. There's still ice on Antarctica and Greenland. We are also in an interglacial period within that larger ice age. The current ice age began about 33 million years ago while the current interglacial began about 11,700 years ago. The Milankovitch cycles determine whether the Earth is in a glacial or interglacial period. Conditions are right for ice to form and spread when precession puts northern hemisphere summer near aphelion and winter near perihelion and when both obliquity and eccentricity are low. The Earth currently satisfies the first of those conditions, but obliquity and eccentricity are a bit too high. That makes our northern hemisphere summers are a bit too warm, our winters a bit too cold. The Milankovitch cycles] provides several answers to the question "where does the energy go?" Those times when conditions are ripe for glaciation have energy in the northern hemisphere spread more uniformly across the year than times not conducive to glaciation. Summers are milder, which means accumulated snow doesn't melt as much. Winters are milder, which means more snow falls. Once ice does become ubiquitous, another answer to the "where does the energy go" question is into space. Ice and snow are white. Their presence reduces the amount of sunlight absorbed by the Earth. The first paper listed below by Hays et al. is the seminal paper that brought the concept of Milankovitch cycles to the forefront. The second paper by Abe-Ouchi et al. dicusses a recent climate simulation that successfully recovers many salient features of the most recent glaciations. Most importantly, this paper appears to have solved the 100,000 year mystery and shows why deglaciation operates so quickly. Hays, J. D., Imbrie, J., & Shackleton, N. J. (1976, December). "Variations in the Earth's orbit: Pacemaker of the ice ages." American Association for the Advancement of Science. Abe-Ouchi, A., Saito, F., Kawamura, K., Raymo, M. E., Okuno, J. I., Takahashi, K., & Blatter, H. (2013). "Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume." Nature , 500(7461), 190-193. Icehouse Earth vs Greenhouse Earth The Earth's climate appears to have been toggling between two climate extremes for much of the Earth's existence, one where things are cold and ice is likely to form and the other where ice is absent worldwide except maybe right at the poles. Dinosaurs roamed the arctic and Antarctica when the Earth was in one of its greenhouse phases. The Earth has been in a greenhouse phase for most of the Earth's existence. Milankovitch cycles don't cause glaciation during greenhouse Earth periods. Ice ages happen when the Earth is in an icehouse phase. What appears to distinguish greenhouse and icehouse phases are the positions and orientations of the continents. Having a continent located over a pole helps cool the climate. Having continents oriented so they channel ocean circulation in a way that keeps the ocean cool also helps cool the climate. The Earth transitioned from its hothouse mode to its icehouse mode 33 million years ago or so. That's right about when two key events happened in the ocean. Up until then, Antarctica was still connected to both Australia and South America. The separation from Tasmania formed the Tasmanian Gateway, while the separation from South America formed the Drake Passage. This marked the birth of the very cold Southern Ocean, it marked the buildup of ice on Antarctica, and it marked the end of the Eocene. Bijl, P. K., Bendle, J. A., Bohaty, S. M., Pross, J., Schouten, S., Tauxe, L., ... & Yamane, M. (2013). "Eocene cooling linked to early flow across the Tasmanian Gateway." Proceedings of the National Academy of Sciences , 110(24), 9645-9650. Exon, N., Kennett, J., & Malone, M. Leg 189 Shipboard Scientific Party (2000). "The opening of the Tasmanian gateway drove global Cenozoic paleoclimatic and paleoceanographic changes: Results of Leg 189." JOIDES J , 26(2), 11-18. Snowball Earth and the Faint Young Sun Paradox Snowball Earth episodes were not your average ice age. Ice typically doesn't come near the tropics, even in the worst of ice ages. Snowball Earth means just that; the snow and ice encroached well into the tropics, possibly extending all the way to the equator. The problem with snowball Earth isn't explaining where all the energy went. The real problem is explaining why the ancient Earth wasn't in a permanent snowball Earth condition, starting from shortly after the Earth radiated away the initial heat from the formation of the Earth. The solar constant is not quite constant. While it doesn't change much at all from year to year, or even century to century, it changes a lot over the course of billions of years. Young G class stars produce considerably less energy than do middle aged G class stars, which in turn produce considerably less energy than do older G class stars. When our Sun was young, it produced only 75% or so as much energy than it does now. Image source: http://en.wikipedia.org/wiki/File:Solar_evolution_(English).svg . By all rights, the Earth should have been completely covered with ice. The young Sun did not produce enough energy to support open oceans. This obviously was not the case. There is plenty of geological evidence that the Earth had open oceans even when the Earth was quite young. This 40 year old conundrum, first raised by Carl Sagan and George Mullen, is the faint young Sun paradox . There have been a number of proposed ways out of the paradox, but none of them quite line up with the geological evidence. One obvious way out is that the Earth's early atmosphere was very different from our nitrogen-oxygen atmosphere and contained significantly more greenhouse gases. The amount of greenhouse gases needed to avert a permanent snowball Earth is highly debated, ranging from not much at all to extreme amounts. Another way out is a reduced albedo due to the significantly smaller early continents and lack of life. The young Earth would have been mostly ocean, and ocean water is rather dark (unless it's covered with ice). Lack of life means no biogenic cloud condensation nuclei, which means fewer clouds. Goldblatt, C., & Zahnle, K. J. (2011). "Faint young Sun paradox remains." Nature , 474(7349), E1-E1. Kienert, H., Feulner, G., & Petoukhov, V. (2012). "Faint young Sun problem more severe due to ice‐albedo feedback and higher rotation rate of the early Earth." Geophysical Research Letters , 39(23). Rosing, M. T., Bird, D. K., Sleep, N. H., & Bjerrum, C. J. (2010). "No climate paradox under the faint early Sun." Nature , 464(7289), 744-747. | {
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2,552 | Let's assume that a ship is traveling on the ocean and it sinks, what is the effect on sea level? Even if the effect is small, would it go up, down or remain the same? | It would go down. In order to float, an object must displace a volume of fluid that weighs the same as the boat. In the case of a ship, this volume is less than the gross volume of the boat, including the air inside it. That's why it floats with the gunwales comfortably above the water. So when the boat is floating, it displaces some volume V , so relative sea-level goes up (second panel, below). However, when the ship is submerged and full of seawater, it displaces a smaller volume than it originally did. If it didn't, it wouldn't have sunk (assuming it hasn't changed in weight). Since it displaces a smaller volume after sinking, relative sea-level must go down. But it will still be higher than it was before the ship was launched. | {
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2,555 | Aside from the fraction of water stored as ice on land and temperature of the water, are there other factors that change sea level, and if so what are is the magnitudes of the these changes? For example, by how much does sediment and soluble matter entering the ocean change sea level? What about volcanoes and tectonic activity? Is there a tendency toward hydrostatic equilibrium where the Earth is entirely covered by an ocean of uniform depth? | Yes, there are lots of other factors. Factors affecting sea levels are no different from other natural processes: there is a large number of coupled, non-linear effects, operating on every time scale, and at every length scale, and across many orders of magnitude. The Wikipedia page Current sea level rise lists many of the known processes. And I wrote a blog post, Scales of sea-level change , a couple of years ago with a long list, mostly drawn from Emery & Aubrey (1991). Here's the table from it: Reference Emery, K & D Aubrey (1991). Sea-Levels, Land Levels and Tide Gauges. Springer-Verlag, New York, 237p. | {
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2,639 | According to NASA, causes of the Earth's greenhouse effect include water vapor, carbon dioxide, methane, nitrous oxide, and CFCs. Carbon dioxide gets the most press, and NASA's page says: Carbon dioxide (CO 2 ). A minor but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration and volcano eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO 2 concentration by a third since the Industrial Revolution began. This is the most important long-lived "forcing" of climate change. An article by NC State University says that a healthy tree can store 13 pounds of carbon per year. As I understand it, carbon dioxide is processed by the tree: the carbon is stored, and the oxygen released. Given that, how many trees would I need to plant to solve the global climate change crisis? Should I optimize for a specific type of tree, or would pine work as well as oak or black walnut? | In 2010 anthropogenic emissions (not including land use change) were approximately 9167 million metric tonnes . Your data on trees holding 13 lbs (5.9 kg) of carbon per year equates to 169.6 trees per metric tonne of emissions. So to take up all of the emissions from 2010 you would need 1,545,000,000,000 trees. A mature forest has only about 100 trees per acre (400 per hectare), so you would need 15,545,000,000 acres of mature forest. This equals an area of 24,290,000 mi 2 (62,910,000 km 2 ). This is approximately the land area of Asia, Europe, and Australia combined ! The surface area of land on the planet is about 150,000,000 km 2 , so in principle we would need to add cover onto 42% of the current land (or we could take soil from deep ocean floors to landfill 1/5th of the oceans!) in order to plant enough trees to solve the problem. This also assumes that the 13 pounds (5.9 kg) of carbon figure is for mature forests, rather than for growing trees (see comments for further discussion). If this value is for young developing trees, it would indicate less be attainable in the longrun. If the 13 pounds is for a developed forest, perhaps a greater amount could be removed in early years as a temporary quicker fix. There is also the problem that forested land is likely to have a lower albedo than the land surface that it covers, and hence the planet will reflect less sunlight back into space which would lead to some extra warming, so we would also need to compensate for that somehow. Apparently about 26% of Earth's land is already covered with forests. I rather doubt over half of the uncovered land surface is suitable for new forests, the continents have large bands either side of the equator that are generally too arid, and the regions close to the poles are too cold. In short, it isn't going to work, even with the most generous assumptions about forest CO 2 exchanges (unless of course I have made an arithmetic error, which is definitely a possibility). | {
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2,772 | At first glance, this seems like such a simple question of "What's the highest point on Earth". However, I also know that the Earth isn't perfectly round. So that "highest point" may be in a relative valley. Also, because it's non-spherical, the "center" may not be easily obvious either. So, I'm curious if there are different answers based on different definitions of "center" (such as geographic center versus center of mass). So, what is the point on the Earth's surface farthest from the center of the Earth? Is this different based on different definitions of "center"? | It's Chimborazo, Ecuador , but only just, beating Huascarán, Peru , by less than 50 metres. Both are over 2 km 'higher' than Everest. I made a plot of some mountains — height above centre of the earth vs absolute latitude. You can download the IPython Notebook source code here . Warning: v. hacky. I can't find anything on the position of the centre of the earth. The formula I used for the latitude-dependent radius requires major (equatorial) and minor (polar) radii, but I don't have citations for them either. Argus in his article Defining the translational velocity of the reference frame of Earth gave some numbers for its temporal variance, but I have no idea how this might affect these mountain heights. Last thing: Apparently, the floor of the Arctic Ocean is the closest point on the surface to the Earth's center (about 6353 km, 30 km 'below' Chimborazo), if you call the bottom of the sea the 'surface'. | {
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2,790 | We see tropical cyclones (going by different names e.g. hurricane, typhoon, cyclone) all over the tropics, but it seems that there are never any storms in the southern Atlantic. See this map of tropical cyclone activity and note the lack of activity in the south Atlantic and also in the south Pacific until you near Australia. source Where are all the tropical cyclones in the southern Atlantic basin? | There have been tropical storms and hurricanes in the south Atlantic, with, according to NOAA's webpage Subject: G6) Why doesn't the South Atlantic Ocean experience tropical cyclones? , with a hurricane forming in the south Atlantic making landfall in Brazil in 2004 and a strong tropical depression/weak tropical storm that formed off the coast of Congo in 1991 - but these are exceedingly rare. The reason why these storms generally don't occur in the south Atlantic, according to the Penn State webpage Upper-level Lows as being: There are two primary reasons why tropical cyclones are rare in the south Atlantic basin. First, vertical wind shear between 850 mb and 200 mb is typically greater than 10 meters per second (check out the long-term average of vertical wind shear between 850 mb and 200 mb). To make matters worse, westerly shear dominates over latitudes where tropical cyclones would be most likely to form. Second, easterly waves from Africa do not form south of the equator (the MLAEJ is a northern hemispheric singularity. Further, from the NASA News page The Nameless Hurricane , they provide an extension to the explanation with Vertical wind shears in the south Atlantic are too strong for hurricanes," Hood explains. Winds in the upper troposphere (about 10 km high) are 20+ mph faster than winds at the ocean surface. This difference, or shear, rips storms apart before they intensify too much An article The first South Atlantic hurricane: Unprecedented blocking, low shear and climate change (Pezza and Simmonds, 2005) suggest that the implications of the southern hemisphere hurricane presents evidence to suggest that Catarina could be linked to climate change in the SH circulation, and other possible future South Atlantic hurricanes could be more likely to occur under global warming conditions. Catarina refers to the southern hemisphere hurricane SH = Southern Hemisphere edited to add an NASA Earth Observatory satellite image of the hurricane: | {
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2,830 | Reading a course on Precambrian, I read that: Earth Age (around 4.5 billion years) is dated thanks to the meteorites hitting Earth during its formation rather than the inner materials composing the Earth. Wouldn't it be more accurate by doing it with its inner materials? Why do we use meteorites if they hit the Earth after its formation? | Some background: We are able to determine the age of certain rocks and minerals using measurements of radioactive and radiogenic isotopes of certain elements. The most common are U-Th-Pb, Rb-Sr and Sm-Nd. Simply put, the resulting date is the time that has passed from the crystallisation of that mineral. Obviously there are complexities, but there are not critical for this answer. Why do we use meteorites if they hit the Earth after its formation? Short answer : because the meteorites formed together with the Earth and the rest of the Solar System. Long answer : The Earth formed together with the rest of the Solar System and its meteorites around 4.5 billion years ago. When meteorites fall on Earth and you pick them up, you are able to date the time of their formation. You say: Why do we use meteorites if they hit the Earth after its formation? You have to distinguish the time that the meteorites form and the time that they hit the Earth. If I throw a meteorite at you, and you date it, it still records the formation time and not the time that I threw it at you. Hitting the Earth does not reset the radioactive clock in the meteorite's minerals. Wouldn't it be more accurate by doing it with its inner materials? It would. There are two problems: We don't have materials from the time of Earth's formation. The Earth is a dynamic place, and rocks are getting formed and destroyed all the time (also see related question ). It may be that such old rocks exist on Earth, but because they are so old there are either metamorphosed and buried deep in the Earth or covered by sedimentary rocks. The oldest exposed rock on Earth available for study is the 4 billion year Acasta Gneiss in Canada. The oldest mineral on Earth is a zircon found in Australia, which is 4.4 billion years old. These are the only two materials that are known to be older than 4 billion years on Earth. There could be more, but we just couldn't find them. The moon forming event occurred some tens of million of years after the formation of the Earth. This event destroyed the Earth's crust and any evidence of the age of the Earth, on Earth itself. This is why meteorites are excellent for this task - they mostly formed during the formation of the Solar System. | {
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2,855 | In continuation of the question Why is Earth's age given by dating meteorites rather than its own rocks? , what evidence do we have that the asteroids indeed formed at the same time as earth? Is there any physical evidence, or is it a conclusion reached mostly through simulation? | what evidence do we have that the asteroids indeed formed at the same time as earth? It depends on what is your definition as "the same time". The formation of the solar system and Earth did not happen at a particular second in time but was rather a continuous process. It also depends on what you define as "asteroids". I'll try to put some things in order. The oldest known material from Earth is a zircon from the Jack Hills in Australia. Its age has been recently refined to be 4382 million years. We know that because we measured it. (1) The moon forming event occurred sometime between 4348 and 4413 million years ago. (2) Now, that zircon should be younger than the moon (because the moon forming event would destroy any already-existing minerals on Earth). Yet, there is a slight overlap between the ages. This is due to the fact that analytical capabilities have their uncertainties and error. This means that the Earth has been here at least ~4.4 billion years ago, because you need the Earth to have a moon. The Earth didn't just pop into existence, but it formed over a period of time, in which small planetesimals collided with each other and accreted to form a larger body. The question now is, what is the age of the planetesimals? We can't know for sure because they are all integrated into Earth, but we can look at asteroids and see their age. We do that by dating the meteorites that fall on the Earth, which in some cases are blasted off asteroids by (even more) collisions and impacts. When you look inside a specific type of meteorites called chondrites , you can see objects that are called CAI s (calcium-aluminum inclusions). These things are solids condensed from vapourised gas that existed in the solar nebula before any planets and planetesimals formed. These are basically the first solid to form in the solar system, and they define the birth of the solar system. We know that they formed 4568 (3,4) or 4569.5 (5) million years ago. Planetary bodies (which for this discussion will be considered as km-sized chucks or rock with the ability to melt and differentiate to mantle and crust, and accrete to form proper planets) began forming around 4566.2 to 4567 million years ago (5,6,7) . We know that because we can date meteorites that we know originated in asteroids. This is just 2 million years after the formation of the first solid droplets in the solar system. Now, while 2 million may seem like a long time for your daily commute, it is not too long for planetary processes (considering Earth is 4.5-4.4 billion years old). Now there is a time gap - what happened between 4566 (formation of planetesimals and asteroids) and 4413 (formation of the moon)? There are about 150 million years that I haven't talked about yet. Was the Earth gradually growing in a linear fashion during that time or did it form rather quickly? Luckily, there are answers to that as well. Based on some measured geochemical data it was shown that most of Earth's mass actually accreted ~10 million years after the formation of the solar system(8). That's rather quick! So the Earth has been sitting there, all by itself, around 100 million years until the moon formed. As to how the Earth itself formed, it could be just amalgamation of small planetisimals, collision of larger bodies, or something completely else. Chemical evidence and physical models do not always agree, and it's all highly debated. This is a field which is rapidly evolving and discoveries are made all the time due to better analytical capabilities and better models. Just look at the years of the papers below. This is all cutting edge and a highly exciting field of study. Everything that I wrote here can be inaccurate or even plain wrong, but that's the fun in science. Is there any physical evidence, or is it a conclusion reached mostly through simulation? So it is a combination of both. You find physical evidence: rocks from Earth and parts of meteorites and their minerals. You measure their isotopic composition to find their age and other characteristics. Then you make a model (or a simulation) that tries to see what has to happen in order for the physical properties to agree. Further reading: An introduction to Meteorites and the origin of the Solar System - a very accessible and interesting read. Chronometry of Meteorites and the Formation of the Earth and Moon - a more technical review, may be paywalled. Refs: 1 Valley, J. W., Cavosie, A. J., Ushikubo, T., Reinhard, D. A., Lawrence, D. F., Larson, D. J., … Spicuzza, M. J. (2014). Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience , 7(3), 219–223. doi:10.1038/ngeo2075 2 Carlson, R. W., Borg, L. E., Gaffney, A. M., & Boyet, M. (2014). Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation. Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences , 372(2024), 20130246. doi:10.1098/rsta.2013.0246 3 Bouvier, A., & Wadhwa, M. (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion. Nature Geoscience , 3(9), 637–641. doi:10.1038/ngeo941 4 Burkhardt, C., Kleine, T., Bourdon, B., Palme, H., Zipfel, J., Friedrich, J. M., & Ebel, D. S. (2008). Hf–W mineral isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in planetesimals. Geochimica et Cosmochimica Acta , 72(24), 6177–6197. doi:10.1016/j.gca.2008.10.023 5 Baker, J., Bizzarro, M., Wittig, N., Connelly, J., & Haack, H. (2005). Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature , 436(7054), 1127–31. doi:10.1038/nature038825 6 Greenwood, R. C., Franchi, I. A., Jambon, A., & Buchanan, P. C. (2005). Widespread magma oceans on asteroidal bodies in the early Solar System. Nature , 435(7044), 916–8. doi:10.1038/nature03612 7 Amelin, Y., Kaltenbach, A., Iizuka, T., Stirling, C. H., Ireland, T. R., Petaev, M., & Jacobsen, S. B. (2010). U–Pb chronology of the Solar System’s oldest solids with variable 238U/235U. Earth and Planetary Science Letters , 300(3-4), 343–350. doi:10.1016/j.epsl.2010.10.015 8 Rudge, J. F., Kleine, T., & Bourdon, B. (2010). Broad bounds on Earth’s accretion and core formation constrained by geochemical models. Nature Geoscience , 3(6), 439–443. doi:10.1038/ngeo872 | {
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2,970 | While reading about the ghost ship Jiang Seng I noticed that Wikipedia claimed it was drifting in uncharted waters in the Gulf of Carpentaria. I tried to find a primary source referencing uncharted waters but couldn't find any so this may just be one editor's penchant for artistic license. Regardless it got me thinking: are there any oceans or perhaps other parts of the world that can still be called uncharted ? Or has satellite navigation and imaging given us a complete picture of our globe? | There are many "uncharted waters". Nautical charts have information about water depths, dangers to navigation, aids to navigation, anchorages, and other features. You can see here what might be included in a nautical chart: U.S. Chart No. 1 The area in question is a shallow sea... so boats of different sizes may or may not be able to take certain routes depending on tides etc. Thus a nautical chart is important in these types of regions. Yes satellites have mapped the world... and coastline is well defined. You can even learn about underwater depths using satellite instrumentation that detects gravity (e.g. CryoSat2) as discussed here: Global seafloor map reveals uncharted sea mountains, stunning details of Earth's oceans which discusses recent measurements of the seafloor from space. University of Sydney geophysicist Dietmar Müller said about 71% of the Earth's surface is covered by water and roughly 90% of the seafloor is uncharted by survey ships that employ acoustic beams to map the depths. ... Müller said the conclusions the [satellite] researchers made about seabed topography may be less accurate than acoustic beam methods employed by ships. and the article Gravity’s Magic: New Seafloor Map Shows Earth’s Uncharted Depths says: The effect of the slight increase in gravity caused by the mass of rock in an undersea mountain is to attract a mound of water several meters high over the seamount. Deep ocean trenches have the reverse effect,” ESA wrote in a statement. “These features can only be detected by using radar altimetry from space. | {
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3,041 | Anthropogenic-sourced greenhouse gases are commonly cited as the main source for human-caused climate change. However, something that I never see discussed is the actual heat produced by human activities. It could be that it's negligible, but I'm still curious to see quantitative data. For example, a running engine in a vehicle produces $\ce{CO2}$ due to fuel combustion, but the engine also produces heat. What about heat produced from ovens, fireplaces, incandescent lighting, etc.? Ignoring the carbon footprint of such activities, can the heat alone be sufficient to affect the atmosphere on a global scale? | A recent study by Chen et al. (2014) in the article Anthropogenic Heat Release: Estimation of Global Distribution and
Possible Climate Effect defined the heat release as you described as being Anthropogenic heat release (AHR), and has it basis as a product of economic prosperity, which leads to energy usage and excess heat produced - usually in large cities globally - this is sometimes referred to as 'waste heat'. In their research, Chen et al report on an earlier global estimate made by Chen and Shi, 2012: Globally, the estimated AHR is 0.03 Wm$^{-2}$ Over land, this average becomes 0.10 Wm$^{-2}$ Although, globally, these amounts are considerably less than that from $\ce{CO2}$ emissions, they still add to the problem - especially as the rate of urbanisation, hence the use of heat generating appliances is increasing rapidly, this value will become more significant. However on a regional scale, over large cities for example, such as Tokyo, the effects of AHR are more profound. In and around the region around Tokyo city, the AHR can exceed 400 Wm$^{-2}$. So what are the consequences of this flux? Most of it causes regional effects, in particular (from Chen et al.): Can influence urban climates and is a significant contributor to the urban heat island, which amongst other things can affect regional rain patterns . Can affect the dynamics and thermodynamics of urban boundary layers. Can influence the reaction rates and types of emitted aerosol and particulate species. All of which have a compounding effect in the regional and to a lesser degree, global environment, that Chen et al. attribute to as being a cause of a 1-2K temperature rise in high altitude areas in Eurasia and North America and as a disrupting influence in global atmospheric circulation. Edit 28/2/2016: There is an interesting blog post about a similar phenomenon: Dubai construction alters local climate Additional references Chen, B., and G.-Y. Shi, 2012: Estimation of the distribution
of global anthropogenic heat flux. Atmos. Oceanic
Sci. Lett., 5, 108–112. | {
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3,167 | It seems like the only personal protective equipment
(PPE) that volcanologists use are aluminized suits that don't provide any protection from direct contact with lava, making an unexpected splash lethal. There are suits that firefighters use to go directly into fires for prolonged periods of time, and they are able to resist temperatures exceeding 1100 °C (~2000 °F) for up to 15 minutes at a time. Most lava is around this temperature threshold, with the coolest lava erupting at a mere 650 °C (~1200 °F). Why don't volcanologists use these suits to study volcanoes? Instead of using those awkward long metal rods to pick out the right piece of lava to study, couldn't they use their gloved hands to pick it up (perhaps on the colder erupting lavas like the ones that does so at 650 °C)? | I'm a volcanologist and I have worked on erupting volcanoes. First of all, volcanologists almost never actually wear those suits. Heat is almost never the hazard that matters in the situations in which we work. The hazards are usually the chance of being hit by ballistics, or getting gassed. The reason you see those suits so often is that they look really cool on TV. I do know that Katia and Maurice Krafft wore them commonly, but their goal was to get as close to explosive eruptions as possible, as frequently as possible, for as long as possible. This strategy leads to death. It was a tragedy when the Kraffts (and 41 others) were killed at Unzen, but not entirely a surprising one, in retrospect. Modern volcanologists tend to be far more cautious and you might say that the heroic age of volcanology is over. When you're working on an erupting volcano, some major things you can do to reduce risk are: Wear a helmet. This would have saved lives at Galeras. Reduce the amount of time you spend in the hazard zone. This means you need to work quickly, which often means carry less and don't wear silly protective gear like heat suits. Wear a gas mask, usually with SO 2 scrubber cartridge. Ever try to jog up a hill in one of these? It becomes hard to breathe, and the mask quickly fills up with condensation from your breath. Communication through a radio or otherwise is hindered. Everything takes longer, so sometimes the masks are left off in the interest of #2. Increase your situational awareness. Maintain communications with someone who is watching data from instruments such as seismometers and tiltmeters. Also, make sure you have unobstructed vision so that you can potentially step out of the way of lava bombs which are falling towards you. Again, PPE such as a heat suit or even a gas mask can work against this. As for the idea of "picking up" lava with your hands, remember that lava is extremely viscous, so it takes quite a lot of force to pull a sample out of a flow. The colder lavas to which you refer are even more viscous than hotter lavas. Anyway, I doubt there are any gloves that can deal with prolonged contact with a 650C fluid. (Remember that fluids and solids are far more conductive than a gas and will transfer far more heat.) I think trying to sample lava with hands would be far more "awkward" than using a pole, which is really quite a simple, easy way to do it. Edit: I just thought of another reason volcanologists can't dress like firefighters. Expense. Volcanologists have an extremely hard time getting sufficient funding. I actually asked for an SCBA for my work in the fumarolic ice caves of Mt Erebus, where CO 2 is high. We couldn't afford it on our grant. Instead, I just carefully monitor gas levels and there are caves which I cannot enter because we read dangerous levels of gas at the entrances. Edit: I highly recommend that anyone interested in this subject read No Apparent Danger and Volcano Cowboys. | {
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4,220 | The other day a friend of my dad's showed him a picture of an icicle that appeared to be "growing" upwards out of a crack in a sidewalk. There were no roofs, overhangs or vehicles parked near by... It looked like a stalagmite you would see in a cave. It looked something like this * : . How could this happen? * Note that this is not the actual icicle, I used this picture for an example. | If there is enough water underground, in a sealed reservoir (like under the concrete in the picture), as that water freezes, it will expand, putting pressure on what ever water is left unfrozen. That water will be squeezed out through what ever cracks it can find - in this case, upwards. Because the water is already near-freezing (perhaps even slightly below, if it's super cooled), it will freeze very quickly when it hits the air (which will be much colder than the ground, late at night). But if there is still water being pushed up from below, that ice will have to move out of the way (it's weaker than concrete). The friction of the flow of cool upwards might warm it just enough to keep it liquid, and the "stalagmite" will end up with a hollow core that the water is being pumped through, and that structure can just keep building on itself, until it is toppled by wind or gravity. The picture is a bit grainy, but it looks like that's what's happened there. It also looks like the side of the hollow "stalagmite" has fallen off up the top, and released a bit of the remaining water (or maybe it just melted in the sun). Edit: Hah, I knew I'd seen this somewhere before - in my freezer! This page describes how Ice spikes form - I basically got it right. It seems that they're more likely to form with de-mineralised water, and the temperature has to be within certain narrow limits. | {
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4,358 | Is it possible for rivers to freeze completely in the winter and stop flowing? Are there known examples of this? If yes, how large can these rivers be and where does the water go? There are several large Siberian rivers that empty into the Arctic Ocean. | I am interpreting your question as referring to rivers with flowing water freezing as as opposed to glaciers, which are already frozen. Under current climatic conditions, small rivers can freeze throughout: from bank to bank, surface to river bed. I'm avoiding using the word solid as some people use that word when a river's surface has frozen from bank to bank. Large rivers do not freeze "through out" because, Water, ice, and snow are good insulators and poor conductors of heat.
The portions of a lake or river that are exposed to the cold winter
air will freeze into ice and this ice insulates the water below from
further rapid freezing. Also contributing to the prevention of bodies
of water freezing solid is an interesting characteristic of water.
Like most other forms of matter, water become denser as it cools, but
beginning at about 39°F (4°C) something odd happens - water begins to
lose density as it gets colder, becoming least dense when it freezes
into ice. This is why ice floats. As water becomes colder it rises to
the top, eventually freezing to the layers of ice that are already
there. Insulation of the water beneath is increased as the ice
thickens so cooling of the water beneath slows. From The Great Land of Alaska : Why don't lakes and rivers freeze solid? Also, while water is flowing its potential energy is constantly being converted to heat energy that resists freezing on the molecular level and subsequent crystallization. For flowing water to freeze, the temperature would have to be exceptionally cold. | {
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4,505 | Forgive my ignorance of the subject but I was always wondered about the exact reason of this phenomenon. Vernal equinox happens around March 20, whereas autumnal equinox happens around September 22, so wherever you are in Northern Hemisphere, the length of the day, and consequently the amount of solar energy that reaches the place should be almost the same. However the average temperatures differ widely, for example Toronto has average temperatures of 2°C to 6°C on March 20th, and 14°C to 19°C on September 22nd. So around 12°C difference [link] . So obviously there is some sort of temperature inertia, as temperatures seem to experience a delay in responding to changes in day length. What is the main reason for it? Is it effect of sea ice or snow-covered land albedo? Energy stored in oceans? Energy absorbed by melting snow and ice? | The phenomenon is called seasonal lag . There's a more extensive answer elsewhere on this site but the basic idea is that temperature lags behind insolation by several weeks, because it takes time to change the mean temperatures of the land, the atmospehere, and especially oceans change their mean temperature. This diagram tries to show the lag, along with various ways of reckoning the seasons: | {
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4,577 | I am an undergraduate student but I am a tutor at a High School, and one student asked me. Attempting to explain the rock cycle "if one rock turns into the other then which came first" my gut is telling me probably igneous (given Earth geologic past.) Resources online have different opinions. Am I right in my assumption? | which came first That's actually a very hard question. The most simple answer would indeed be igneous. Here's why: Sedimentary rocks (in the sense of rock cycle) comes from pre-existing igneous or metamorphic rocks, so you need to have had them first. Metamorphic rocks, by definition, are rocks that form from other kinds of rocks (be it igneous or metamorphic). Igneous rocks form by melting of other rocks. Now here is the catch - what do you consider "other rocks"? The first igneous rocks on Earth (or to be exact - the proto-Earth) likely formed by impact melting of various small planetary bodies hitting each other and coalescing to form the Earth. These planetesimals formed by condensation of gas from the solar nebula, before Earth even existed, and before the rock cycle began. So yes, igneous rocks were here first. The rocks (or material, if you wish) that existed before the igneous rocks do not fit into the traditional igneous/sedimentary/metamorphic definitions. However, note that the rules of the rock cycle aren't written in stone (pun intended). What about evaporites? Rocks such as halite or gypsum beds form on the surface on the Earth, and their constituents aren't derived directly from either igneous or metamorphic rocks. What about sandstones that form by weathering of former sandstones? What about igneous rocks that form by partial melting of metamorphic rocks, and then melt again without ever reaching the surface to become sedimentary rocks? And my favourite one: pyroclastic rocks. Are they sedimentary or igneous? One textbook quips that they're igneous going up, and sedimentary going down. | {
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4,671 | Is it a coincidence? the first is determined by the amount of water on the Earth and the second comes from evolution of tectonic plates. Still, oceans seem to fill exactly the oceanic crust. | Sea-level only sort-of corresponds to the oceanic–continental crust boundary. In depth, they don't correspond at all: It's the same story at an active margin: the plate boundary at a subduction zone is buried several kilometres beneath a wedge of sediment: What about spatially? As you can see from this map , they don't correspond all that well spatially either: they are commonly 100s of kilometres apart, sometimes more. The difference, shaded grey in the map, is the continental shelf. It is sometimes emergent (e.g. at the end of the Permian, or during ice ages, when global sea-levels are low). It is sometimes, as now, flooded. The vertical change of ca. 100–250 m means a lateral change of 100s of km, because the shelf is almost flat. OK, enough nit-picking, roughly speaking, why are the oceans on oceanic crust? Continental crust is relatively thick and low density (ca. 2700 kg/m$^3$), compared to oceanic crust (25–70 km and ca. 2900 kg/m$^3$). So it 'floats' higher on the thin, dense mantle (7–10 km and ca. 3300 kg/m$^3$). When a new rift forms, it creates a depression and space for new oceanic-style crust. This is happening right now in the Gulf of California , the Baikal Rift Zone , and in the extensive East African Rift system. All of these places have large lakes and/or are extensively below sea-level. It's a divergent 'tear' in the continental crust, so intuitively we might expect a depression. As it fills with thin oceanic crust, the depression is maintained and eventually fills with ocean water. Why is ocean crust so thin? I don't have a lot of expertise here, but I think it's determined by the relative rates of melting and cooling, and the heat balance at the ridge and subduction zones. There's a nice answer to this question on Reddit , complete with links. As insinuated in that thread, the thermo- and chemo-dynamics of the spreading centres and subduction zones are intimately related to the presence of the ocean, so together they make a complex system (indeed, the entire planet is a complex system), as @stali points out in his comments and answer. | {
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4,720 | Genesis 7:11-20 presents an account of a precipitation event which, in 40 days, submerges the entire surface of the earth: [On] the seventeenth day of the second month — on that day
all the springs of the great deep burst forth, and the
floodgates of the heavens were opened… For forty days the flood kept coming on the earth… all the high
mountains under the entire heavens were covered. The waters rose
and covered the mountains to a depth of more than fifteen cubits
[6.86 m]. Based on this account, my questions are: Given the amount of water on Earth (including all the water as liquid, solid, and gas, in all possible places: the atmosphere, the surface, and underground), is there enough water to flood the whole earth until ‘all the high mountains… were covered’? What is the estimated rainfall intensity based on this description, and how intense is it in comparison with today’s rainfall intensity in tropical areas? Regardless of the veracity or otherwise of the account, this makes for an interesting thought experiment. | there is not enough existing water inside this geosystem IMO for such a thing to occur. Let's see these figures here: One estimate of global water distribution Oceans, Seas, & Bays 1,338,000,000 -- 96.54% of all water this figure means that most of the existing water at the global scale is seawater. Sea floor is quite irregular, with some abyss like pits (ex: Mariana trench), up to low water in shallow sea near continents and islands (See figure). A variable topography overall This mean that considering the current volume of existing sea water, at say an average of 3 km depth (hypothetical, but it doesn't change the overall reasoning), and removing the continents, you would have to double, and probably more than double the current volume of sea water to raise the oceanic water level of 1 km, 2 km etc... or ~5 km if we consider MT Ararat. Here is some hypothetically (rough) calculation to illustrate. Let's say the Earth have a surface area of 510 million km 2 ( Wikipedia ) and the overall global water (USGS link in my post) amount to 1386 million km 2 . Dividing the volume by the surface provides the height, as shown by this dimensional analysis :$\frac {L^3}{L^2}=L$, which would amount to 2.71 km in height in this case. But let's not forget that this figure imply a) the impermeability of the surface b) the surface flatness. Multiple problems further arise when considering how such an amount of hypothetical water could be suspended at once - there is a interesting debunking explaining implications here: Talk Origins - 5. the Flood itself I think @Pont nicely answered the subquestion about the precipitation rate | {
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4,728 | Why is the sea level in Hudson Bay decreasing so much? Hudson Bay is pretty far up north, much closer to glaciers. Would it make sense for it to recede at this level with sources of fresh water relatively close? | The area is experiencing post-glacial isostatic rebound . Much of Canada was covered in an extensive ice sheet in the last glacial period (the 'Ice Age'), from about 110 ka until 12 ka. The ice in the Hudson Bay area was among the last to melt: A thick ice sheet depresses the crust (the lithosphere), making a small dent in the uppermost mantle (the asthenosphere ) in the process. Well, not that small: p 375 in Gornitz (2009, Encyclopedia of Paleoclimatology and Ancient Environments ) says it could be 800 m for a 3000 metre-thick ice sheet! Since the asthenosphere is highly viscous, it takes a long time time for the depression to 'bounce' back up. This map from Natural Resources Canada shows the current rate: Since global sea-level is currently rising at about 3 mm/a, a local uplift at this rate will break even. Anything more will result in relative sea-level fall, as we see in Hudson Bay (as well as in Scandinavia, the UK, Alaska, and elsewhere — this map is wonderful ). Interesting, for geologists anyway, is the sedimentological record this leaves. I love this example of raised beaches and a small delta experiencing forced regression on the shores of Hudson Bay: Last thing — you asked: Would it make sense for it to recede at the level that it is
receding with sources of freshwater relatively close? Since Hudson Bay is connected to the world's ocean, mainly through Hudson Strait, the runoff into the Bay has no measurable effect on the water level. Credit Ice retreat map by TKostolany, licensed CC-BY-SA. Rebound map by NRCan, free of copyright. Google Maps image contains own credit. | {
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4,798 | What percent of the Earth's core is uranium? And how much of the heat at the core is from radioactive decay rather than other forces? | Good question! Geochemists and geophysicists agree to disagree, sometimes quite strongly. There are also disagreements within each group as well as between the two groups. It's not just uranium. There are four isotopes whose half-lives are long enough that they can be primordial and whose half-lives are not so long that they don't produce much heat. These four isotopes are Uranium 235, with a half-life of 0.703 billion years, Potassium 40, with a half-life of 1.277 billion years, Uranium 238, with a half-life of 4.468 billion years, and Thorium 232, with a half-life of 14.056 billion years. The consensus view amongst geochemists is that there is very little, if any, of any of these isotopes in the Earth's core. Potassium, thorium, and uranium are chemically active. They readily oxidize. In fact, they readily combine chemically with lots other elements -- but not iron. They are strongly lithophilic elements. Moreover, all three are "incompatible" elements. In a partial melt, they have a strong affinity to stay in the molten state. This means that relative to solar system abundances, all three of these elements should be strongly enhanced in the Earth's crust, slightly depleted in the Earth's mantle, and strongly depleted in the Earth's core. Geophysicists look at the amount of heat needed to drive the Earth's magnetic field, and at the recent results from neutrino observations. From their perspective, the amount of residual heat from the Earth's formation is not near enough to drive the geomagneto. The growth of the Earth's inner core creates some heat, but not near enough to sustain the geodynamo. Geophysicists want a good amount of heat flux across the core mantle boundary to sustain the geodynamo, and to them the only viable source is radioactivity. Recent geoneutrino experiments appear to rule out uranium or thorium in the Earth's core, but not potassium 40. The neutrinos generated from the decay of potassium 40 are not detectable using current technology. | {
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5,042 | Someone shared a video with me in which clouds were forming a ring around the Sun. I took this screen shot of that video: What is the reason behind this? | This optical phenomenon is called a 22° halo which is a subset of other halos . This arises from sunlight refracting through hexagonal ice crystals, which can be found in high level cirrus clouds. Light that would otherwise not make it to your eye enters an ice crystal and then exits at an angle of approximately 22 degrees. This produces the arc of light you see in the video. You see (inverted) rainbow coloring of the halo because the light is not uniformly refracted but varies from 21.7 degrees for red photons to 22.5 degrees for violet photons. Image by donalbein, Wikemedia Commons, CC-By-SA-2.5 https://commons.wikimedia.org/wiki/File:Path_of_rays_in_a_hexagonal_prism.png There are many more optical phenomenon you can see, but all are based around sunlight and the optical properties of ice crystals and water droplets. Here is a phenomenal picture taken in winter showing many examples: | {
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5,047 | As far as I understand it is perfectly valid for air to have 100% humidity. At that point, all water can still exist in form of vapor, non-condensed. Does it immediately start to rain if humidity is >100%? If so, why do we have slight rain and heavy rain if any 0.1% above 100% drops out immediately? That should always be only a small amount of rain and rainstorms could not be explained. If not, what is the limit of humidity if not 100% and why can it exceed 100%? I have tried to understand the Wikipedia article but I'm stuck in various places: Water vapor is the gaseous state of water and is invisible Invisible to me would mean that clouds are excluded from humidity. Absolute humidity is the total mass of water vapor present in a given volume of air. That again makes me think a cloud must be included. The humidity is affected by winds and by rainfall. Rainfall certainly decreases humidity, but it is not stated at what percentage it starts to rain. | Short answer: humidity is not a proxy for rain starting and no, it does not start raining automatically when 100% humidity is reached (haze or clouds can form though). The onset of rain is dependent on many things including humidity, but a specific value of humidity is not a sufficient condition for rain. Water vapor is a gas and invisible. The amount of water vapor in the air can be expressed as relative humidity (RH) which is the ratio of water vapor pressure ($e$) and saturation water vapor pressure ($e_s$). Saturation vapor pressure is the partial pressure of vapor when evaporation and condensation rates are equal, represented by RH=100%. When RH > 100% net condensation occurs, but water has its own ideas. In a mixture of pure dry air and water vapor, water will not condense until around 400% RH. Reasons for this are a bit complicated but it has to do with very small droplets being more likely to evaporate as their curvature is very large ( Kelvin effect , saturation vapor pressure is higher over curved surfaces than flat ones). Luckily for us, our atmosphere is not pure air but has small particulates suspended in it (aerosols). Some of these aerosols are classed as cloud condensation nuclei (CCN) and enable droplet formation at lower relative humidities. These work by forming a solute in water increasing the energy needed to break bonds and evaporate the water ( Raoult's_law ) The combined interaction of these are described by Köhler theory and describe droplet growth in terms of drop size, solute and supersaturation (RH-100%). In a nutshell, there is a critical drop size below which drop size decreases for decreasing supersaturation and above which drop size increases for decreasing supersaturation. The critical supersaturation is the supersaturation needed to attain the critical drop size, and is generally small (e.g. 0.3% supersaturation). Droplets below the critical size are 'haze drops' and these make up the haze you see on very humid days. Drops that reach the critical size can continue to grow to become cloud drops. The condensed water is carried in the air but is no longer water vapor and is not part of relative humidity (but does contribute to the parcel density) So... when does it rain? It rains when water vapor is in the presence of CCN, driven to a supersaturation causing growth to the critical drop size (on the order of $\mu$m) and continuing to grow to cloud drops and further to the much bigger drop sizes that make up drizzle (100-300 $\mu$m)and rain drops(mm), a process that takes around 40 minutes. Drops will grow until the updraft can no longer support their mass and then they fall from the cloud as rain. Your question asks at what humidity does it rain, but what surface humidity determines is how high the cloud bases are. When the dew point depression (the difference between temperature and dew point) is high, the cloud bases will be higher than when the dew point depression is small. As air rises it cools, and at some point 100% RH is attained. If there is forcing for vertical ascent, parcels can rise to this height and then to a height where they freely convect due to decreased parcel density caused by the release of energy during condensation (see: CAPE ). So far to have rain we've needed water vapor (but not at 100% at the surface), aerosols to aid condensation (CCN) and a way to cool the air to reach 100% RH via lifting. It is these three things -- moisture, aerosols and cooling, that we need for a rain storm. We can have 100% RH days that are just hazy or foggy that do not rain and we can have days with mextremely little RH (e.g. deserts) that result in little rainstorms or large severe storms. We also have storms we call 'elevated convection' that are completely disconnected from surface conditions and when these storms cause rain is not related to surface humidity at all. If you are looking for a magic trigger for rain, your closest bet will be looking at temperature, dew point and the height parcels need to attain to freely convect ( LFC ). If there is forcing for parcels to get that high and instability above, then rain is a good bet. Forcing for lift can be anything from convergence along a boundary (sea breeze, cold front, outflow from another storm), orographic lifting (mountains, hills), thermally or dynamically forced. To address your specific concerns: Water vapor is the gaseous state of water and is invisible Invisible to me would mean that clouds are excluded from humidity. Correct, clouds are not part of humidity, they are suspended liquid water drops, usually condensed onto a solute of some kind. Absolute humidity is the total mass of water vapor present in a given volume of air. That again makes me think a cloud must be included. Water vapor contributes to humidity but water vapor does not include liquid water. Cloud water, ice, snow, rain, grapple, hail all contribute to the total mass of a volume of air, but are not humidity. The humidity is affected by winds and by rainfall. Rainfall certainly decreases humidity, but it is not stated at what percentage it starts to rain. Humidity will increase during a rainstorm as rain and puddles evaporate. Temperature will decrease toward the wet bulb temperature. As noted in the answer, there isn't a magic number of %humidity that causes rain to start. | {
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5,181 | I took a tour of a cave in northern California last weekend. The tour guide asked us, " If an earthquake occurred, what would we feel in here? " My answer was, "fear," but she said we would feel nothing - that earthquakes are not felt underground. Although I did not contest her assertion, I have my doubts about it - after all, don't earthquakes normally have their epicenter underground (well, all the time , I reckon, as they can't emanate from the air)? And if an earthquake happened to erupt from within a cave[rn], wouldn't it most definitely be felt there? I would think one might even end up with a stalagtite stuck in his noggin. UPDATE And, if I'm right (that there could be a whole lotta shakin' goin' on while on a journey to the centre of the earth), has anybody lived to tell the tale and really done it (told the tale)? A first-hand account could be riveting. | Ground motion results due to passage of elastic waves. Now there are different kinds of waves, e.g., P waves, S waves, surface waves, etc. Most of the shaking (and therefore damage) is caused by surface waves. So if you are in a deep cave or mine then the amount of shaking you might experience can be much lower than on the surface. This of course assumes that the mine or cave is not right on the fault, but some distance (10s of km) away. Having said that, there are some situations (based on the type of earthquake and local geology) in which waves can interfere constructively to cause significant shaking even at large depths, but this only happens in certain areas and not everywhere. In many ways elastic wave propagation is similar to acoustic or water waves. Imagine a small explosive source in a lake/pond, let's say 50 m underwater. After the explosion there will be more 'shaking' on the surface, but not much underwater unless you're extremely close to the explosive source. | {
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5,407 | We can calculate the maximum possible height of the mountain on earth.
If the elastic limit of a typical rock is $3 \times 10^8\ \mathrm{N/m}$ and its mean density is $3 \times 10^3\ \mathrm{kg/m^3}$, then the breaking stress is $h\,\rho\,\mathrm{g}$, where $h$ is height, $\rho$ is the density of the rock, and $\mathrm{g}$ is the acceleration due to gravity. Then $$ h = \frac{\mathrm{elastic\ limit}}{\rho\,\mathrm{g}} $$ Putting the values we get, $$ h = 10^4\ \mathrm{m} $$ which is the maximum possible height. Now Mount Everest is within this limit, but Mauna Kea is 10,210 m tall (measured from its oceanic base). Does this suggest that rock types at the base of this mountain are different?
Or does the presence of water have an effect? | Since over half of the height of Mauna Kea is under water, you need to consider the buoyancy effect. Instead of a density of $3 \times 10^3\ \mathrm{kg/m^3}$ , the underwater portion has a net density of $2 \times 10^3\ \mathrm{kg/m^3}$ . That will significantly increase the potential height of such a mountain. Add in all the other uncertainties (is Mauna Kea made of rock with "typical" elastic limit and density? is it even homogenous? are there dynamics involved? and what about Naomi ?) and there's no reason to see its height as a contradiction. | {
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5,410 | I have average annual rainfall values at few grids, as shown in below example; Lat: 25.5 26.5 33.5 34.5 35.5 36.5 41.5 42.5 Lon: 89.5 91.5 78.5 79.5 83.5 84.5 75.5 76.5 Rain: 110 120 122 135 114 116 145 120 110 110 I want to plot it like a map, like this example Each point will have a cell size of 1 degree, and there must be WGS 84 projection. How to plot them in MATLAB ? I have around 4000 lat-lon values and these are not in sequence. Moreover the available Lat-Lon represent the irregular shape. | Since over half of the height of Mauna Kea is under water, you need to consider the buoyancy effect. Instead of a density of $3 \times 10^3\ \mathrm{kg/m^3}$ , the underwater portion has a net density of $2 \times 10^3\ \mathrm{kg/m^3}$ . That will significantly increase the potential height of such a mountain. Add in all the other uncertainties (is Mauna Kea made of rock with "typical" elastic limit and density? is it even homogenous? are there dynamics involved? and what about Naomi ?) and there's no reason to see its height as a contradiction. | {
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6,760 | I'm new to this site and certainly not someone with any real background in atmospheric science, so please forgive me if this question is a little stupid. I recall learning in High School about mountain ranges frequently being dry on one side and lush on the other. The explanation was that as a cloud approaches the mountain from the lush side, it releases any rain so it can rise over the range. This always seemed like a silly explanation to me, because in the way it's phrased, it seems to imply that the cloud is cognizant. Also, this would mean that clouds can only (for the most part) approach the mountain from the lush side, which may or may not be true, but I don't know why. So I'm sure there's a better explanation as to why this happens, but I'm hoping someone can explain. Thanks | The Cascade Mountain Range in the US Pacific Northwest is a good example to use to explain this. The predominant wind direction is from the West - over the Pacific Ocean. The air over the ocean picks up moisture from evaporation. After it passes the coast, the mountains cause the air to rise. As it does so it cools. Colder air can hold less water vapour than warm air so that the air becomes saturated with water and it condenses out into clouds. With enough condensation you get rain - a lot of it on the western side of the mountains. So you had it backwards, the rain doesn't allow the air to rise - the rising allows it to rain. Clouds mainly approach from the lush side because that is the prevailing wind direction but it is better say that it is the lush side because clouds mainly approach that way. When wind comes from the other direction is travelling across land so it contains less water and the cooling generally isn't enough to cause rain. Once the air crosses to the other side of the mountain range it warms and becomes undersaturated with water. Clouds often disappear and even if they remain you aren't getting further condensation to produce a lot of rain. So those areas are much drier. | {
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6,920 | Many sources state that 12:00 noon is the high-point or mid-point of the sun's path across the sky. However, for where I am, this is obviously wrong. In my own town the high point of the sun occurs at about 13:30 in the summer (because of DST), and 12:30 in the winter, but clearly not 12:00 noon. Why, at different longitudes, is the high-point of the sun not at 12:00? Thank you in advance. | It's not a question of latitude but longitude and time zones. Prior to the advent of railway transportation the time set on clocks varied from towns within close proximity because the clocks were set to noon when the Sun was at its highest in the sky. This was more apparent for towns east and west of each other (ie different longitudes). When railway transportation was developed there was a need to standardize times so that train timetables could be developed and arrival and departure times would be accurate and consistent This precipitated the development of time zones. Each time zone is approximately 15 degrees of longitude wide, but there are differences because of national/regional priorities. China which has a similar longitudinal extent as the contiguous states of the US has only one time zone, whereas there are four in the contiguous states of US. Within a band of longitude the time is the same. In the time zones immediately east and west, the time is one hour more (east) and one hour less (west) - ignoring difference due to daily savings. The reason for time zone being approximately 15 degrees of longitude wide is the rate of spin of the Earth is 15 degrees/hour (360 degrees every 24 hours). You state that in your town, noon is either 12:30 or 13:30, depending on daylight saving time. The reason for noon occurring at half past the hour is due to the boundaries of the time zone where you live. Where I live, noon is approximately at quarter past the hour. | {
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7,212 | I am in O'Fallon, Missouri and today it is -15 °C (5 °F) outside. I was taught water freezes at 0 °C (32 °F). I could understand if it was exactly 0 °C (32 °F) that the water might not be turning to ice, but how can it be 15 °C (27 °F) below water's freezing point and still be liquid water? The water is still moving due to the wind, which I think shows it has no plan of freezing. How can this be? My question may be too broad because there may be more than one reason why it isn't freezing, but I don't know of any. I tried to narrow it by giving my exact location and temperatures. It is a pond in my backyard. From what I've seen through Google, it may have something to do with crystallization rate? Thanks in advance. | Water melts at 0 °C (32 °F) but freezing is a more complicated affair. It is safe to say water gains the ability to freeze at 0 °C, but it can get much cooler before it actually does so resulting in supercooled water. Water in this state can rapidly solidify when suitable ice nuclei are introduced. For example, in convective clouds, liquid water can be observed at temperatures as low as -40 °C. However, the water in your pond isn't supercooled. You say the air is 5 °F, but what is the temperature of the water (probably above 0 °C)? Ultimately it is the water that needs to be below freezing for ice formation to occur. How deep is the pond in your backyard? Water, compared to air, has a much better ability to retain heat and the bigger the volume of water, the more of a heat reservoir you have to deal with. Freezing itself is an exothermic process and ice formation will heat the surrounding water. The pond is probably not pure water and full of ions (e.g. salts) it has picked up from the ground, which lowers the melting point of water. All of these effects make it more difficult for water in your pond to actually freeze and could explain what you are seeing. Of these effects, the heat capacity of the water and the temperature of the ground underlying the pond are probably the primary contributors. Underneath the pond is the ground and it too is a heat reservoir. Both are probably warmer than the melting point of water even though the air temperature is much colder. If a fluid is cooled from above, the cool fluid sinks toward the bottom. If a fluid is warmed from below, the warm fluid rises. Only one of these needs to be happening to start convection but it is likely that the ground below the pond is warmer than the pond and both the warm ground and cool air will drive convection in the pond. This, in turn, means your pond is well mixed and you will need to extract enough energy from the system to cool down all of the water and cool the ground below the pond to give ice the chance to form on the surface. This will take time (on the order of days/weeks) of continual sub-freezing air temperatures to accomplish. You'll have a much easier time freezing your pond than a large lake, but it still won't be an overnight process. | {
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7,283 | I would like to ask that at what distance from the Earth's surface the curvature of the Earth is visible. What layer of the atmosphere is this? I've noticed that at the height of 9-12 Km (the view from from aeroplanes) it is not visible. | Depends on your eye. You can realise the curvature of the Earth by just going to the beach. Last summer I was on a scientific cruise in the Mediterranean. I took two pictures of a distant boat, within an interval of a few seconds: one from the lowest deck of the ship (left image), the other one from our highest observation platform (about 16 m higher; picture on the right): A distant boat seen from 6 m (left) and from 22 m (right) above the sea surface. This boat was about 30 km apart. My pictures, taken with a 30x optical zoom camera. The part of the boat that is missing in the left image is hidden by the quasi-spherical shape of the Earth. In fact, if you would know the size of the boat and its distance, we could infer the radius of the Earth. But since we already know this, let's do it the other way around and deduce the distance to which we can see the full boat: The distance $d$ from an observer $O$ at an elevation $h$ to the visible horizon follows the equation (adopting a spherical Earth): $$
d=R\times\arctan\left(\frac{\sqrt{2\times{R}\times{h}}}{R}\right)
$$ where $d$ and $h$ are in meters and $R=6370*10^3m$ is the radius of the Earth. The plot is like this: Distance of visibility d (vertical axis, in km), as a function of the elevation h of the observer above the sea level (horizontal axis, in m). From just 3 m above the surface, you can see the horizon 6.2 km apart. If you are 30 m high, then you can see up to 20 km far away. This is one of the reasons why the ancient cultures, at least since the sixth century BC, knew that the Earth was curved, not flat. They just needed good eyes. You can read first-hand Pliny (1st century) on the unquestionable spherical shape of our planet in his Historia Naturalis . Cartoon defining the variables used above. d is the distance of visibility, h is the elevation of the observer O above the sea level. But addressing more precisely the question. Realising that the horizon is lower than normal (lower than the perpendicular to gravity) means realising the angle ($gamma$) that the horizon lowers below the flat horizon (angle between $OH$ and the tangent to the circle at O , see cartoon below; this is equivalent to gamma in that cartoon). This angle depends on the altitude $h$ of the observer, following the equation: $$
\gamma=\frac{180}{\pi}\times\arctan\left(\frac{\sqrt{2\times{R}\times{h}}}{R}\right)
$$ where gamma is in degrees, see the cartoon below. This results in this dependence between gamma (vertical axis) and h (horizontal axis): Angle of the horizon below the flat-Earth horizon ( gamma , in degrees, on the vertical axis of this plot) as a function of the observer's elevation h above the surface (meters). Note that the apparent angular size of the Sun or the Moon is around 0.5 degrees. . So, at an altitude of only 290 m above the sea level you can already see 60 km far and the horizon will be lower than normal by the same angular size of the sun (half a degree). While normally we are no capable of feeling this small lowering of the horizon, there is a cheap telescopic device called levelmeter that allows you to point in the direction perpendicular to gravity, revealing how lowered is the horizon when you are only a few meters high. When you are on a plane ca. 10,000 m above the sea level, you see the horizon 3.2 degrees below the astronomical horizon (O-H), this is, around 6 times the angular size of the Sun or the Moon. And you can see (under ideal meteorological conditions) to a distance of 357 km. Felix Baumgartner roughly doubled this number but the pictures circulated in the news were taken with very wide angle, so the ostensible curvature of the Earth they suggest is mostly an artifact of the camera, not what Felix actually saw. This ostensible curvature of the Earth is mostly an artifact of the camera's wide-angle objective, not what Felix Baumgartner actually saw. | {
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7,332 | The velocity of p-waves emanating from earthquakes is in the range of 5-8 km/s ( link )--let's assume it is 5 km/s. The earthquake depth is up to hundreds of kms deep underground ( link )--let's assume it is 100 km. That said, if a seismic station is installed at a depth of 50 km, and there are many of them in any given metropolitan area, then we can have a warning that is tens of seconds before the earthquake reaches the surface. While I realize that drilling down to 50 kn is no easy task, I would have imagined that saving human life is well worth the efforts. Why hasn't this been done so far? Is it that such a short notice (10s of seconds) isn't worth it? | The simple answer is that you can't drill to 50 km depth. The deepest holes ever drilled were to a little more than 12 km, one is named the Kola Superdeep Borehole in Russia, which was a scientific drilling project. The very few others were oil exploration boreholes. Drilling that deep is extremely expensive and hard. If you go and ask anyone who ever worked on a drill rig, drilling the second 100 metres is always harder than the first 100 metres. And we're talking about kilometres here! There are several problems with drilling that deep. It's extremely hot down there, and the drilling equipment just breaks and stops working. You also need to pump cooling water in and pump out the stuff you're drilling and it gets harder with depth. This is simply not feasible. Now let's say that you did somehow manage to drill a hole to that depth. How would you put monitoring equipment inside? That equipment has to sustain heat and pressure and still keep working, while being able to transmit whatever it's reading back to the surface. This is not going to happen, not at 50 or 10 km depth. Another problem is that not all earthquakes are that deep. Some earthquakes originate near the surface, or just several km deep. Having a monitoring station down there isn't going to help. The 2011 Tohoku earthquake (the one that triggered the tsunami at Fukushima) was only 30 km deep. Same thing for the 2004 Indian Ocean earthquake. | {
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7,371 | 2015 had the highest global temperatures on record , as discussed in this question , beating the last record ... in 2014. Are we now in a runaway climate change regime, where zero further human impact on the environment will not prevent change to the equilibrium state of the Earth's atmosphere? | Runaway climate change is, given our current state of knowledge, only something that could be confirmed in historic context - in the rear-view mirror. Inconveniently, there's likely to be a much-diminished version of human civilisation around to observe it, if and when it does happen. In other words, it's too early to tell if we've passed a catastrophic tipping point. We don't know what the equilibrium state of the Earth's atmosphere is. It will take 30 years for our current stock of GHG emissions to fully show its effects: and we'll be continuing to release GHGs for a while, so the equilibrium will continue to change. But it's worth bearing in mind what we can do, when we really have to; World War II saw massive realignments both in industrial production and in expectations within a very short space of time: after decades of delay, it now seems likely that decarbonisation will require at least a similarly fast and large industrial realignment; and it is possible. | {
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7,393 | I live in the northern suburbs of Johannesburg in South Africa.
I have noticed while working with synoptic weather charts in geography class that the isobars indicate that Johannesburg should have atmospheric pressure of around 1010-1020 hPa, just like most cities around the coast, even though Johannesburg is at 1500 m above sea level.
Weather service providers also indicate that Johannesburg's atmospheric pressure should be around that. What confuses me is that my weather station at home (which I have been monitoring for several years now) never seems to show pressure ratings out of 840-860 hPa.
My outdoor watch gives me the same pressures.
I am convinced that my watch cannot be broken because during my holiday in coastal Cape Town, it gave pressure readings of the expected 1020~ hPa and I can "feel" the higher pressure that I am not used to. Surely both of my apparatus cannot be wrong. Is there an explanation for this? | They apply a correction to the actual barometer readings and report what the estimated pressure would be at sea level. Basically they use an imaginary column of air between the barometer and sea level and add its weight to the pressure measured at the barometer. This lets them make pressure maps instead of just almost altitude maps. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2011.pdf https://www.ec.gc.ca/manobs/default.asp?lang=En&n=4349ABDA-1 | {
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7,449 | Does a particular object have the same weight on every part of Earth or does it vary? | There is an entire field of Geophysics called gravimetry dedicated to measuring the magnitude of the gravitational field. First, we should distinguish between weight (a force) and gravity (an acceleration). Gravity is the acceleration that Earth gives to objects near its surface due to the gravitational force. The acceleration of an object near the surface of Earth is due to the combined effects of gravity and the centrifugal acceleration from the rotation of Earth. The resulting acceleration is weakest at the equator and maximum at the poles, with a difference in magnitude of about 0.5%, because the outward centrifugal force produced by rotation is larger at the equator than at the poles. The standard gravity, $ɡ_n$ or $ɡ_0$, (the expected or mid-range value of the gravitational acceleration of an object near the surface of Earth) is 9.80665 $m/s^2$. Gravimetry measures and studies gravitational anomalies: local variations in topography (e.g., mountains) and geology (rock density) that cause changes in the gravitational field. These gravitational anomalies are measured with instruments called gravimeters. Rocks with lower density, such as sedimentary rocks, result in negative anomalies. Another effect affecting gravity is altitude, as an increase in altitude will result in a change in gravity of about 0.03% per kilometer. The maximum difference in gravity is about 2 Gal (0.02 $m/s^2$) from sea level to the top of Mount Everest. The gravity field of Earth has been measured recently with a variety of satellites (GRACE, CHAMP, GOCE). The resulting data can be used to generate global geoids such as EGM96 ( geoid calculator ). Gravitational anomalies can be extracted by calculating differences with respect to the geoid. The range of variations is of order ±300 $mGal$ (±0.003 $m/s^2$). Other factors have a small effect on gravity, such as the gravitational force from the Moon and the Sun that causes the tides. The fluctuations associated with these effects are of order 0.2 $mGal$ (2 $µm/s^2$). | {
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7,888 | It's a simple question.. What do continents "lay" on? Do they float on water? or are they huge bodies that "emerge" from the sea floor/bed? are they connected to the bottom of the oceans? Hope the question is clear. Don't be afraid to be thorough and scientific in your answer, I'll appreciate it and do everything to understand it. | Matan, the continents where we all live "float" on the Earth's mantle. The continents are made out of relatively brittle rock called the "Crust" and the mantle is made out of much more ductile material. The mantle, however, is NOT liquid. It is just much more ductile than the crust so, in geologic time, it can flow (like silly putty). Also, the mantle is much more dense, so the crust doesn't sink into it by gravitational/bouyancy forces alone. Think of it as a creme brulee. We live on the little hard crust on the top. And there just happens to be a little bit of water on that crust that covers the less-elevated parts. Now, a little more technical, the crust that makes up the continents and the crust that is under the deep oceans are actually compositionally different. This is because the magma that solidifies to form each of these crusts travels, by partially melting its way up, through different materials and different thicknesses. The consequence of this is that the crust making up the continents is less dense than that making up the bottom of the oceans. The image above shows the process by which we get new (left) oceanic crust, (right) continental crust, and (middle) a special type of crust like that in Hawaii. Note that when two tectonic plates collide , the more dense one will tend to subduct under the other. | {
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8,806 | I'm curious if there are any saltwater rivers on Earth. These would presumably arise if a saltwater lake had a river outlet to the ocean. However, all the saltwater lakes I looked at (Caspian Sea, Dead Sea, Great Salt Lake Utah), apparently don't have any river outlets. Do any exist? And the obvious follow up, why or why not? Edit: For the purposes of this question, undersea flows don't count. They are fascinating, but I'm interested if any "regular surface rivers" exist as saltwater rivers. | The water in any river draining the sea is infinitely recycle-able (from rain replenishment), whereas the salt from any terrestrial source is not. So salty rivers, if any, won't exist permanently. Saltwater lakes gain their salinity precisely because they have no outlet, so salt just gets concentrated by evaporation. I don't think there are any truly saline rivers throughout their entire length. The nearest approximations I can think of are: Rare ephemeral runoff from emergent salt domes in desert areas. Freshwater rivers that drain into arid areas where combined evaporation and infiltration gradually reduces the flow to zero. These are more 'very brackish' than truly saline. The Amu Darya in Uzbekistan is one such example. I don't know the salinity of the Jordan as it enters the Dead Sea, but the river is reduced to almost nothing, whilst there are hypersaline springs, and sewage effluent from Amman draining into it. The only river I can think of that is very highly mineralized from source to sea is only about 3 kilometres long. It is an unnamed river from the volcanic crater on Savo Island, in the Solomon Islands (Southwest Pacific). It is also acidic and boiling hot - quite literally, for most of it's length. | {
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9,521 | I recently saw a short video from NASA. It shows the moon passing by the Earth as it rotates. NASA link: From a Million Miles Away, NASA Camera Shows Moon Crossing Face of Earth . The video itself looks quite fake, but that can probably be attributed to the fact that it was reconstructed from sensor data as opposed to a regular camera. My question is, why do the clouds not appear to move at all? According to the website, the images span approximately 5 hours. There are a lot of people online saying the video is fake. I believe the Earth is a globe, but I can see why people have their doubts about this particular video. | They are moving, but not fast enough to notice at the distance shown. From the NASA page: These images were taken between 3:50 p.m. and 8:45 p.m. EDT on July 16, showing the moon moving over the Pacific Ocean near North America. The North Pole is in the upper left corner of the image, reflecting the orbital tilt of Earth from the vantage point of the spacecraft. In five hours, clouds may move some hundreds of kilometers at most. The half-disk of the Earth is nearly 13,000 km across, so over the period the pictures were taken, the clouds should only be expected to move a small fraction of the width of the disk. [1] Add to that the fact that the Earth is rotating and it is not readily apparent that they move at all. 1 : Say the clouds in a huge hurricane move in a straight line at 100km/hr; over this 5-hour animation, that's 500 km movement which is just under 4% of the Earth's 12,742 km diameter . The Earth is about 550px in this image and 550 × .04 = 22 pixels maximum change. | {
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10,245 | Perhaps a simple question, we know 71% of the earth's surface contains water as oceans. If Earth's age is 4.543 billion years, then I guess it should be decreased with drying or should have been dried so far. Why doesn't it dry or decrease? If we put some water in sunlight, it evaporates. The oceans are the chief source of rain, but lakes and rivers also contribute to it. The sun's heat evaporates the water. So I wonder why doesn't the 71% water coverage not evaporate, decreasing until gone? Why is it still 71% after billions of years? Does water keep coming from somewhere? Or, does moving water not evaporate? Why is it still here? | There are two ways this problem needs to be looked at. The first is more astronomy than Earth science. The Earth as an entire system is largely contained. Its gravity and magnetic field retains nearly all of its elements. Earth does lose hydrogen and helium and cosmic rays will split water molecules leading to a loss of an impressive amount of hydrogen and as an indirect result a loss of water, but this is loss irrelevant compared to the size of the oceans. More detail here . Space dust, comets and asteroids contain water so some water is returned from space too. By the upper estimate in one article, 50,000 tons of hydrogen per year works out to about 450,000 tons of water lost every year. (and 400,000 tons of oxygen added as a result). Compared to the mass of Earth's oceans those numbers are small. 450,000 tons per year, or 450 trillion tons over a billion years is nothing compared to the 1.3 million trillion tons of water Earth has in its oceans. By the highest estimate, it will take 30 billion years at the current rate and at the Sun's current luminosity for Earth to lose just 1% of its oceans. (Will look to update with other estimates) . As for the rest of the question, once we recognize that loss into space is insignificant, then virtually all water is continuously cycled though the water cycle or hydrological cycle . Very little water gets destroyed or chemically transformed. Nearly all of it, even of millions or billions of years, evaporates, or, turns into ice, or gets absorbed by plants, or seeps underground, but it always returns. Evaporated water returns to Earth as rain. Water that gets frozen on the ice caps eventually melts back into the oceans. Water absorbed by plants or that seeps underground does eventually get returned to the surface by plate tectonics or volcanism. Plants that store water return it when the plant is eaten. Water is very hard to destroy, so it stays remarkably constant on Earth over time. | {
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10,405 | We just had visitors (in Monterey, California) from Montevideo, Uruguay. Not being very familiar with Uruguay, after having taken them to Big Sur, I asked them how far they lived from the ocean. They said it was hours away, and that they lived on a river. I later looked at a map and, AFAICT , they do live right on the Atlantic Ocean. If they view the river that flows into the Atlantic to the west of them to be the river they live on, that seems very odd to me - from my point of view, once the river dumps into the ocean, that's ocean, and the river stops where they meet. The body of water gets very much wider at that spot, so it certainly "looks" more like the ocean on the map than a continuation of the river. But it leads to the question: when a river dumps into an ocean, exactly where does it transition from being the river to being the ocean? | In the case of Río de la Plata, part of it is history and politics, and part of it is oceanography. Most of Argentina and Uruguay considers Río de la Plata as a river (thus, the name, río) and as such it is the widest river in the world (maximum width >200km). Río de la Plata is formed as the confluence of the Paraná and Uruguay rivers and results in a pretty wide estuary with Montevideo (Uruguay) in the northern shore. As can be seen in the following satellite image, the combined discharge from the Paraná and Uruguay rivers creates a current that flows into the Atlantic Ocean and that is visible under calm wind conditions and high discharge periods. Source: https://eoimages.gsfc.nasa.gov/ Spanish and Portuguese explorers in the XVI century described Rio de la Plata as a "Mar Dulce" (freshwater sea). The Italian explorer Sebastiano Caboto explored the area and traded silver with the local Guaraní tribes and thus gave it the name of "River of Silver". In practice, the system is best described as an estuary: neither river nor ocean, but an area of freshwater and saltwater mixing. In the area of Montevideo, the salinity in spring is around 10 g/kg (way lower than ocean salinities ~30-35g/kg), but it can be higher in periods of low discharge. In the area of Buenos Aires, salinities below 5 g/kg are commonly found and thus, it makes sense to call it a river. Most oceanographers agree that the system is a salt-wedge estuary, with freshwater flowing near the surface toward the ocean and saltwater intruding landward near the bottom of the estuary. Salinity of the Rio de la Plata: Source: Guerrero, R.A., Lasta, C., Acha, E.M., Mianzan, H., Framiñan, M., 1997. Hydrographic Atlas of the Río de la Plata. CARP-INIDEP, Buenos Aires, Montevideo. | {
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10,440 | I'm currently building a weather station from scratch. It measures temperature, pressure, humidity, wind speed and direction. The end result shall be a weather station that sends out METAR weather messages so pilots can see live weather in our area. Now I'm at the point where I'm writing the software and I have only little knowledge about meteorology. More specifically I ask myself when a meteorologist speaks about "gusts". I know it's a wind speed peak, but I'm a bit puzzled how much faster compared to the average wind speed a gust must be to be actually called gust?
I was searching for some definitions, but I didn't really find anything useful. | In the case of Río de la Plata, part of it is history and politics, and part of it is oceanography. Most of Argentina and Uruguay considers Río de la Plata as a river (thus, the name, río) and as such it is the widest river in the world (maximum width >200km). Río de la Plata is formed as the confluence of the Paraná and Uruguay rivers and results in a pretty wide estuary with Montevideo (Uruguay) in the northern shore. As can be seen in the following satellite image, the combined discharge from the Paraná and Uruguay rivers creates a current that flows into the Atlantic Ocean and that is visible under calm wind conditions and high discharge periods. Source: https://eoimages.gsfc.nasa.gov/ Spanish and Portuguese explorers in the XVI century described Rio de la Plata as a "Mar Dulce" (freshwater sea). The Italian explorer Sebastiano Caboto explored the area and traded silver with the local Guaraní tribes and thus gave it the name of "River of Silver". In practice, the system is best described as an estuary: neither river nor ocean, but an area of freshwater and saltwater mixing. In the area of Montevideo, the salinity in spring is around 10 g/kg (way lower than ocean salinities ~30-35g/kg), but it can be higher in periods of low discharge. In the area of Buenos Aires, salinities below 5 g/kg are commonly found and thus, it makes sense to call it a river. Most oceanographers agree that the system is a salt-wedge estuary, with freshwater flowing near the surface toward the ocean and saltwater intruding landward near the bottom of the estuary. Salinity of the Rio de la Plata: Source: Guerrero, R.A., Lasta, C., Acha, E.M., Mianzan, H., Framiñan, M., 1997. Hydrographic Atlas of the Río de la Plata. CARP-INIDEP, Buenos Aires, Montevideo. | {
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12,057 | I have three txt files for longitude, latitude and temperature (or let's say three lists lon, lat, temp) from scattered weather station in the UK. I would like firstly to interpolate these data in order to get a nice colourful map of temperatures. Then, I would like to plot this interpolated temperature layer only over the land mask (thus over the british isles and not over the sea). Is that possible with Python and how? | It is straightforward to do so with numpy , scipy.interpolate.griddata , and matplotlib . Here is an example: import matplotlib.pyplot as plt
import numpy as np
from scipy.interpolate import griddata
# data coordinates and values
x = np.random.random(100)
y = np.random.random(100)
z = np.random.random(100)
# target grid to interpolate to
xi = yi = np.arange(0,1.01,0.01)
xi,yi = np.meshgrid(xi,yi)
# set mask
mask = (xi > 0.5) & (xi < 0.6) & (yi > 0.5) & (yi < 0.6)
# interpolate
zi = griddata((x,y),z,(xi,yi),method='linear')
# mask out the field
zi[mask] = np.nan
# plot
fig = plt.figure()
ax = fig.add_subplot(111)
plt.contourf(xi,yi,zi,np.arange(0,1.01,0.01))
plt.plot(x,y,'k.')
plt.xlabel('xi',fontsize=16)
plt.ylabel('yi',fontsize=16)
plt.savefig('interpolated.png',dpi=100)
plt.close(fig) Result: How to use this: x and y are locations of points - these correspond to lon and lat values of your stations; z are the values of points - this corresponds to your temperature observations from stations; xi and yi are target grid axes - these will be your target longitude and latitude coordinates, which must match your landmask field; zi is the result; This example includes a simple way to mask the field. You should replace this mask with the landmask on your grid. Notice also the method argument to griddata . Besides linear , this can also be cubic or nearest . I suggest you play with each to see what yields the best result for your dataset. | {
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12,159 | Hurricane Harvey dumped more that 20 inches (500 mm) of rain over a large region, with 40+ (>1000 mm) in some spots... and much more expected. How is that possible? Does the atmosphere really hold that much water? Or is it getting repeatedly evaporated from the ocean and dropped onto the land by the circular winds, implying an enormous evaporation rate while over water? | You were right to question whether the atmosphere really held that much water. It comes nowhere close! We use precipitable water to track this, which is the measure of all moisture in the entire column of air in the troposphere. We can get good widespread estimates from remote observations. This animation shows current amounts of precipitable water levels across the US from satellite. Here is a still from this afternoon (Sunday, August 27) during the Harvey event: 75 mm is less than 3 inches. We also get in-situ exact measurements worldwide from twice daily radiosonde balloon launches. Unfortunately none are located right near the locations receiving excess rainfall. But you can check US sites of current precipitable water measurements any time by going to SPC's sounding page and looking for the PW value in the left side of the bottom table. SPC also maintains a climatology page for precipitable water and other values . You can see there that 3 inches (76 mm) of moisture in the sounding is extremely rare, and no US site has ever had 4 inches (102 mm) of precipitable water. However, the answer is also not truly found in evaporation rates. The conditions in strong hurricanes do greatly enhance evaporation rates due to the high wind speeds and warmer waters. Measurements are actually a bit difficult to come by in such extreme conditions, with challenges in isolating evaporation effects from spray as well as in getting the instruments positioned into such environments (new field campaign: who wants to take the research ship out into the category 5 hurricane!?!). As this 2007 study by Trenberth et al. noted: We are unaware of reliable estimates of evaporation in hurricanes, and published measurements do not exist in winds above about 20 m s−1 although some progress has been made in the Coupled Boundary Layer Air-Sea Transfer Experiment (CBLAST) However, in that study model analyses suggested that evaporation rates in the core of hurricanes are likely no more than 1-2 inches (25–50 mm) per day. How is that possible? It's quite important to notice that heavy rainfall - well over 5 inches (125 mm) - can actually fall in as little as a couple hours almost anywhere, such as in this 2015 flood event in Nebraska . How can that be? The secret lies in the nature of even the weakest storm: even as a storm is just beginning to form, it begins to draw in air from surrounding areas. This is NOAA's diagram of a typical developing thunderstorm cell: You can see the curve at the arrows near the bottom, indicating inflow of surrounding air into the storm. This inflow turns a thunderstorm into more of an engine, processing a continuing stream of incoming air, removing its moisture. In a single cell thunderstorms in an environment without background winds, the "waste" air will eventually pile up and choke off the influx. But even in such circumstances, a few inches of rain may fall. That isn't by using up all of the moisture from the cloud's environment, but instead by using just a portion of the moisture from the reservoir in and around the cloud. If some upper level winds exist to help exhaust the "spent" air, storm systems can persist for even longer periods of time. For example, the Midwestern United States commonly sees long-duration late summer heavy rainfall initiated along stationary boundaries in which inflow lacks direct access to significant warm water bodies. Larger systems that dump huge amount of precipitation over greater areas must pull in a more consistent, stronger inflow of warm, moist air from greater distance. Examples of this happening include the Pineapple Express for rainfall in California/the southwest US, the low-level jet for spring and summer squall lines in the Plains, onshore winds during the Indian monsoon, and air from off the Gulf Stream in Nor'easters. In all these regional large precipitation events moist air gathered from a great surface area flows into a smaller region. As the air approaches, it is lifted by the low pressure and its associated features, condenses, and finally falls as rain (or snow). This process is often termed "moisture convergence". The Storm Prediction Center also offers plots of localized deep moisture convergence [choose a region, then look under the upper air menu]. The convergence contours, shown in red, really show the piling up of humidity that is causing the heavy rainfall in Harvey: But perhaps to visualize the scale involved in creating a catastrophic largescale flood such as Harvey, this plot, created from a base image from pivotalweather.com , best shows the conditions around the storm (from the GFS model): Basically the atmosphere of the entire Gulf (and beyond) is being pumped into the southeast Texas area. So although the air can only hold a couple inches (some 50 mm) of water, and evaporation rates are typically only a fraction of an inch (several mm) per day... bringing that together from such a large source region, and focusing it down into one small area... can lead to these awful extreme deluges. Addendum: It should also be highlighted that the NHC adds in their report on Harvey that rising motion was also enhanced by a front which had stalled in the area. Air being advected in by Harvey's flow would naturally rise over that layer of cooler air when moving inland (a process called isentropic lift), which proves particularly efficient in condensing out the (abundant) moisture en masse into rainfall. Most substantial regional floods require similar existence of a significant broad lifting mechanisms overlaid with such a relentless inflow of warm, moist air. | {
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13,416 | I was doing a project for my English class, and I came upon the article Energy conservation in the earth's crust and climate change . I can't view the full text of the article, but the abstract piqued my interest: Do long hydrocarbons in the earth actually have a significant effect in insulating the surface? Also, has the lack of these hydrocarbons resulted in any significant warming of the Earth thus far? | Quoting from John Russell's response to this article , "This is arrant nonsense!" Russell concludes with How did this paper get through the peer-review and editorial review processes? What technical standards were applied to determine the apparent merit of its contents so as to justify its inclusion in a reputable journal? Just because something is published in a scientific journal does not mean it is fact. Publication is where science starts rather than ends. Sometimes, pure garbage manages to slip through peer review and get published, even in reputable journals. This is one of those times. Moreover, the publisher of the underlying journal, Taylor & Francis, has had issues with shoddy peer review. The Earth's energy imbalance is 0.6±0.17 W/m 2 . The Earth's internal energy budget, the amount of energy that escapes from the interior of the Earth, is 0.087 W/m 2 , about half the uncertainty in the Earth's energy imbalance. (That largish uncertainty is because the imbalance is a difficult quantity to measure.) Even if all of that 0.087 W/m 2 is due to humans removing the Earth's insulating layer of hydrocarbons (it isn't), it does not come close to accounting for the 0.6±0.17 W/m 2 imbalance. The numbers don't add up. Or as John Russell put it in his response to the referenced article, "This is arrant nonsense!" | {
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13,446 | I have done some research online, and I've found out that Antarctica has the calmest winds (lowest maximum wind speed) recorded on Earth. However, it is uninhabitable for human life. Other very calm areas are the doldrums , but they are over water. Therefore, I would like to know: What is the calmest place on Earth that is on or over land other than Antarctica? PS: I'm also pondering this question about balloon chains , so I'm also interested in the places with the calmest overall wind column all the way from the troposphere to the mesosphere. | The main resistance that winds have to their movements comes from the topography and surface obstacles. Therefore, as a general rule the closer to the surface the less wind you will find. But I guess you are interested in the winds in areas clear of surface obstacles, otherwise the answer would a be a cave or a dense forest somewhere. To figure out what is the calmest place on Earth wind-wise, we can use one of the global datasets that NASA put together (initially to help with solar and wind energy planning). One of them is a 10-year average (July 1983 - June 1993) of wind speed at 50 m above surface , ignoring the effect of trees and other non-topographic obstacles at a resolution of one degree (roughly 111 km or less).
I just loaded that dataset and searched for the minimum (and maximum) values, this is how it looks: The minimum is not actually in Antarctica as your research suggested, but in the Amazon at longitude 68° west and latitude 0° (that's Brazil near the border with Colombia), with a mean wind speed of 1.55 m/s (5.6 km/h). This is an enlarged version of the most relevant area overlaid with a world map for easier interpretation: In line to what was stated at the beginning, the maximum happens in an area were the lack of landmasses allow it to blow unimpeded around the world. And it happens at 60° W -52° S, reaching a mean wind speed of 12.62 m/s (45 km/h). Note that this analysis is valid for broad areas. And of course individual weather stations at punctual locations can give different values. As for Antarctica, the winds there are mainly catabatic, therefore at the top of antarctic domes the average wind speed are very low. For example the average at Dome C is 10.1 km/h and 9.4 km/h at Dome F . However, those values are still bigger than what NASA's models suggest for the Amazonas. I think the model I used here is reliable. As an exercise, you can see all the images that show up in a Google search for "global mean wind speed" and you see that most images show a similar pattern, where Antarctica doesn't seem to contain the calmest place. The model used to produce these datasets is a reanalysis, it means it takes all the weather data of the past (weather stations and satellite data) and attempts to reconstruct how exactly was the weather over the whole word. A similar kind of model takes only past weather data and attempts to predict how will be the weather over the whole world. That's a much more difficult problem, prone to bigger errors but that's actually what all the weather forecast we see on TV are based upon. One of the best global forecast models is called GFS (Global Forecast System) that is produced by NOAA's Environmental Modeling Center . To have a sense of how winds vary in different locations and altitudes you can explore the GFS data using Cameron Beccario's amazing vizualization tool . There you can see the predicted global wind patterns all the way from the surface to more than 26 km of altitude ( here you can find a table with referential equivalences between pressure levels in hPa and altitudes ). Despite those are not multi-year averages, they are still very informative. Muze, the author of the question, pieced together a beautiful animated gif that show how the above mentioned visualization looks like for the elevation range of the jet streams at about 10 km of altitude (250 hPa): Finally, I have to note that I've interpreted "calmest" as the minimum mean wind speed . However, it would be sensible also to consider it as the place with the lowest maximum wind speed or some other metric, that would perhaps change the picture described above. And maybe using that metric one of the Antarctic domes could be the "calmest" place. But I won't extend the answer further with any possible interpretation for "calmest". | {
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13,840 | The text of the introduction to the BBC Podcast Sea Levels Rise; The Compass, Living on the Edge Episode 1 of 4 says: Five of the Solomon Islands have disappeared , many more are becoming uninhabitable. For Kerry and Sally, climate change is not a theory - it is what has made them abandon their island and the graves of their ancestors. They see themselves as lucky - they had family land to move to and the skills to build new homes on stilts - but they are resigned to moving again. Award-winning journalist Didi Akinyelure visits her home city of Lagos to find out the latest solution to sea level rise in West Africa. The glass towers of the new financial district of Eko Atlantic are protected from the waves by state of the art sea defences. The residents of the luxury apartments should keep their feet dry whatever the climate throws at them. That may be small comfort for their unprotected neighbours in the shanty town on the lagoon, Makoko, but they’re experts in survival against the odds. Certainly sea level is rising, on the order of perhaps 15 centimeters in the last century judging from this plot , and the New Scientist article Five Pacific islands vanish from sight as sea levels rise certainly adds credence to this. Answers to the question Sea Level Rise due to Climate Change shed some light on human-induced climate change. Between about 04:00 and 06:00 in the podcast, Simon Albert , a climate change scientist from University of Queensland describes the situation in the Solomons. Here is my best attempt at a transcription of a small part of the podcast: The Solomons have really over the last couple of decades been a global “hot spot” for sea level rise. So the rates of sea level rise we’ve observed in the Solomons have been in the order of three times the global average. Whilst globally the seas have been rising approximately 3 millimeters per year for the last couple of decades, the Solomons have seen a rise of 7 to 10 millimeters per year over that time frame. A large part of that is a result of human-induced climate change, and then on top of that we’ve also had “the perfect storm” if you like, of a series of natural cycles in sea-level rise and weather conditions, such as El Nino, intensification of trade winds, which effectively have pushed water into the Western Pacific, resulting in a significantly higher than would be normally experienced. Over the last 20 years we’ve seen sea level in the Solomons rise by 15 to 20 centimeters. When you translate that over these very low-lying islands, that can translate to the coast line receding by several hundreds of meters. Question: Could someone help me better understand the phenomenon that have lead to this faster rate of local sea level rise in the Solomon islands? edit: My original post had these four points. Once I replayed a few more times I realized they don't perfectly reflect the breakdown in the podcast, but since @John has taken the time to write such a great answer using them I'll include them here to maintain continuity: Human induced climate change Series of natural cycles in sea level rise Weather cycles such as El Niño Intensification of trade winds, which have pushed water into the pacific below: The sea encroaches on a tropical island. Credit: Getty Images. From here . (click for full size) below: Credit: Chris Roelfsema, from NewScientist . (click for full size) | The sea is not the only thing that rises, the sea floor can also rise and fall in accordance with the underlying geology. Oceanic tectonic plates sink as they age (and thus get colder and denser), the Solomon islands happen to be on a sinking section of the ocean floor and at the same time the sea level is rising. Keep in mind many of those islands have a very shallow slope, so a few rising millimeters are many feet of inland horizontal motion. Of course once the wave base reaches new "virgin" sediment (often loose sand), it often washes it away moving the tide line even further inland. Storms cause a storm surge which can wash away sediment. A rising sea level means the storm surge is reaching sediments they could not reach before, so they are causing faster erosion. In addition, salt water kills a lot of plants further eroding the sediment, so a higher surge means more dead plants and more settling of sediment. Storms can cause a tremendous amount of erosion on unprotected soil, a single storm washed away two entire city blocks in the Atlantic City history. I am not entirely sure, but prevailing winds can cause some displacement of water (the ocean is not completely leveled because it is moving) so a change in the trade winds could lead to a minor effect of local sea level, how much this contributes, I have no idea. Note : the OP started with a numbered list and specifically asked about the highlighted numbers, hence only 2-4 were referenced in my answer (the first one was a normal sea level rise). | {
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13,854 | Are there places on Earth that have a strong magnetic field other than the magnetic north and south poles? Can living where (rare) earth magnetic ore is abundant provide a mini-magnetoshere? | If we consider only the magnetic field generated by natural sources, and not the ones generated by human activities. The general trend is that higher intensities of the magnetic field happen close to the magnetic poles. However, this is just a general trend. The southern hemisphere experiences the highest magnetic fields intensities, reaching over 65,000 nT, and this southern maximum matches fairly well with the south magnetic pole. In contrast, in the northern hemisphere, the maximum happens in Siberia, quite far from the north magnetic pole that is currently very close to the geographic north pole . There are many models of Earth's magnetic field. You can find a comprehensive list at geomag.org . You can find the intensity output of NOAA's World Margentic Model for 2015, here . Shapefiles with the contours are available for 2018 here (as well as more maps), and the coordinates of north and south poles computed by the WMM model up to 2020 are available here . Putting together all the information for 2018, the map looks like this The contour interval is 1000 nT, grid lines interval is 10°, and the green dots represent the south and north magnetic poles. The maximum intensity contour in the northern hemisphere happens in Siberia with a value of 61,000 nT and the center of the contour is located roughly 3,100 km away from the magnetic north pole. The maximum intensity contour in the southern hemisphere is between Australia and Antarctica, with a value of 67,000 nT and the center of the contour is located roughly 450 km away from the magnetic south pole. So the answer is: Yes, there are. Maximum field intensity doesn't happen at the magnetic poles. Specially in the Northern hemisphere, the maximum intensity happens in Siberia, a few thousand kilometers away from the magnetic pole. The maximum intensity globally according to NOAA's WMM model is located in the southern hemisphere, approximately at 60° 15' S, 135° 52' E, about 450 km away from the south magnetic pole The projection in the map above (geographic) generates a big distortion of the distances, the images below show the same information in polar stereographic projection: Now, it is worth adding that the above information wouldn't be significantly modified if you consider natural magnetic anomalies due to magnetic crustal deposits. This is because magnetic anomalies are usually smaller than a few hundred nT, therefore the magnetic field intensity pattern is largely controlled by the internal magnetic field. Magnetic anomalies are not mapped over the whole world, but the World Digital Magnetic Anomaly Map , provides a pretty good estimate However, these anomalies are measured at an altitude of 5 km above mean sea level, and have a resolution of 3-arc-minute resolution (~5 km). Therefore, it doesn't rule out the existence of very strong and localized small scale anomalies, that might lead to large variations on the magnetic field intensity but over a very small area.
The maximum value within the World Digital Magnetic Anomaly Map dataset is 8570.78 nT, but even considering all anomalies, the largest field intensities remain within the same contour near the magnetic south pole. | {
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14,214 | There's a song "Blizzard's Never Seen the Desert's Sand". Given Antarctica is a desert, someone questioned the title's validity. BUT is there sand in Antarctica? I'd imagine yes as it's a pretty basic soil and Antarctica is a big place, and I know there is land mass, not just ice, in Antarctica. I'm not sure though - any more info or evidence about the presence of sand? | Yes. In fact, there are sand-dunes in Antarctica [1:15] . | {
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14,247 | In a recent discussion with a farmer, the (at least in northwestern Germany) "popular" topic of nitrate infiltration into groundwater came up. The general view is that lots of fields receive more nitrate fertilizer than the plants can use, this then percolates through the ground and eventually shows up in the groundwater that is also used as drinking water. So far so plausible. Raised nitrate content in groundwater is indeed often observed. One should probably add that in this area, a lot of agriculture is raising of hogs and cattle, consequently much of the fertilizer is manure that has a (compared to commercial fertilizer) low NPK content and is thus not worth to transport over far distances. So there's an incentive to dispose on ones own fields, nearby. Now, this farmer claimed that the nitrate content we see now is decades old, that the bylaws and controls as well as agricultural practices in place now work. Is this true? Does nitrate penetrate ground so slowly? The only mechanism here I could think of that nitrate is adsorbed and desorbed on minerals in the ground, sort of like in chromotography column. If neccessary, I can provide more figures for nitrate content of specific wells, groundwater table etc - but I don't think it's neccessary since I want a qualitative answer wether or not such a slow infiltration is plausible at all. | Yes. In fact, there are sand-dunes in Antarctica [1:15] . | {
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14,492 | Where I live the soil is red. Is there a map or chart where you can see the average color of the dirt according to geographical location? What would the color be if all of the dirt on Earth was added equally to a pallet? I understand that composition of minerals determines dirt color but what makes dirt its color is not the question I am asking. Kata Tjuta, Northern Territory, Australia Sagada, Mountain Province, Philippines https://eugeneexplorer.wordpress.com/2016/11/22/blue-soil-hills/ Gentry County, Missouri, United States http://www.airphotona.com/image.asp?imageid=11944 | This gif, prepared by the United States Department of Agriculture - Natural Resources Conservation Services (USDA-NRCS) soil scientists at the National Soil Survey Center, has soil colors based on the Munsell Color System for the United States at different depths: The soil colors nearest the surface are darker due to more organic matter and are lighter at depth with varying colors by region. Source: http://munsell.com/color-blog/soil-colors-national-parks-anniversary/ This link also has soil colors of select United States National Parks. For example: | {
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14,587 | Lots of people have explained it over many sites but I still can not confidently say that I know what it means when they say Hong Kong experienced 3mm of rainfall last Friday. Does it mean that 3mm per square meter was experienced in a specific area, or does it mean that the total amount of rain had a volume such that if it was spread over all of Hong Kong the height would be 3mm. Or is it referred to in terms of per square meter? What is the time period? Is it measured per day, per hour, per minute? EDIT : The question marked as a duplicate may refer to some aspects of the question but does not effectively ask them. The title of the question refers to something different and the only answer to that question is what I am asking an explanation of. | It's confusing to measure liquid in units of length instead of volume , isn't it? Here's how it works. "One millimeter of rain" is actually one cubic millimeter per square millimeter . On average, over the area you're talking about, each square millimeter has received one cubic millimeter of rain. If you divide n mm 3 by 1 mm 2 , you get -- n mm! The field of Dimensional Analysis deals with questions like this. If there was "1 mm of rain", how much rain did a square meter receive? 1000 mm * 1000 mm * 1 mm = 1 000 000 mm 3 = 1 liter. Pour one liter of water into a square container one meter on a side, and it forms a layer one millimeter deep. To calculate "how much rain fell on Hong Kong" (expressed as total volume ), you'd take the area of Hong Kong, and multiply it by 1 mm. That's usually not what people want to know, though -- they want to know how wet things got, and the volume-per-area number is most convenient for that. | {
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14,706 | That honestly doesn't make any sense to me. How could heat pass through a gas one way but not the other? Its not like our upper atmostphere has a bunch of doors that can only open one way. To me, that seems to fly in the face of everything I've been taught about how the physical universe works. If heat can pass through a substance one way, there's no reason why it can't go through the other way. Except for 'greenhouse gases' apparently. The closest thing I know of that works this way is one-way mirrors. But that doesn't have the same effect. The reason you can see through one side and not the other is because most light is reflected, but not all. Thus if you're in a dark room, the light coming in from the outside will overwhelm what little light is being reflected from within the room. But if you're on the outside, virtually no light is coming through from the dark side and thus you can only see what is being reflected. In short, it makes no sense to me and flies in the face of everything I've been taught. Now, I'm not a climate change denier. Actually, this is something I just now thought about. I've never really bothered to question it before. | In a nutshell: The radiation that enters is shortwave radiation from the sun. Solar radiation is dominated by visible (as well as UV and near infrared) radiation with a wavelength mostly between 0.2 µm and 2 µm. This wavelength is determined by the temperature of the Sun, in the order of 6000 K. For visible radiation (roughly between 0.4 µm and 0.7 µm), all the gases in the atmosphere are (almost completely) transparent. Clouds do scatter this radiation so you can't see the Sun directly when it's cloudy, but they still let plenty of light through, so it's still light on a cloudy day (just less bright), although it does feel cooler. The radiation that leaves is longwave radiation , also known as terrestrial radiation or (incorrectly) as thermal radiation. For Earth, this is dominated by radiation with a wavelength of mostly between 4 µm and 40 µm. This wavelength is determined by the temperature of the Earth, at around 290 K. At these wavelengths, greenhouse gases are mostly opaque. They absorb the radiation. The greenhouse gases reradiate the absorbed heat, but in both directions (back to Earth AND up to higher layers in the atmosphere), and at a lower temperature. The upward longwave radiation gets in turn absorbed by higher-up layers of greenhouse gases, et cetera, such that ultimately the layers of greenhouse gases that do radiate into space tend to be high up in the air (say at 240 K). Thus, their overall effect is to radiate less heat into space than the Earth surface would in the absence of those gases. The most important greenhouse gases are water vapour, carbon dioxide, and methane. Source: gisgeography.com Source: science of doom | {
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16,000 | What is the location on Earth that is closest to the Sun? I've seen this question asked many times, and answered in varied and contradictory ways: The most common answer is “the summit of Chimborazo volcano in Ecuador”. This volcano is the point on Earth's surface that is furthest from the center of Earth , and that is then equated to being the closest to the Sun.
This is very commonly spoken of around the Chimborazo volcano area and among the people involved with tourism there ( here is an example of this answer ) Others argue that it is Cayambe volcano in Ecuador, it being the highest point along the equatorial line ( answer example ). Others say Mount Everest in Nepal/China because it is the highest point on Earth ( example ). And others argue it is Sairecabur volcano in Chile/Bolivia, because it is the highest point at the latitude which is closest to the Sun on January 5th, when the perihelion happens (i.e. the point in Earth's orbit that is closest to the sun) ( example ). A fifth answer, with the same logic as the previous, is Licancabur volcano in Chile/Bolivia, which quite as close to the latitude of the perihelion but is significantly higher than Sairecabur. What is the correct answer and why? What place on Earth is closest to the Sun? | This is an interesting question, but it lacks a key factor that is crucial to the answer: TIME . The point on Earth closest to the Sun varies through time , so the question can be asked about any moment in time, or over periods of time. Let's analyze the factors involved. At any given moment in time, the point on Earth's surface that is closest to the Sun is what is called the “ subsolar point ”. This point corresponds to the point on the surface that intersects the imaginary line that connects the center of the Earth to the center of the Sun. In other words, the subsolar point correspond to the point on Earth surface where the sunlight hits the Earth perpendicular to the ground, therefore, a vertical object would project no shadow. (image from Wikipedia: subsolar point ) The longitude of the subsolar point corresponds to that of the meridian experiencing solar noon . Over Greenwich (longitude 0°) that happens at the actual noon, and as the Earth rotates 15° every hour, that will happen one our later (at 13:00 h UTC) at longitude 15° W, two hours later (at 14:00 h UTC) at longitude 30° W, and so on. In general terms, you can use the following formula for the subsolar point longitude ( $\text{SSP}_{\text{long}}$ ). $\text{SSP}_{\text{long}} = \left(\text{UTC} -12\right)*15°$ This is a simplified formula, but accurate enough for our purpose. Let's take as an example the following date July 20, 1969, at 20:17 UTC In that moment, the longitude of the subsolar point was 124° 15' West: $(20+(17/60)-12)*15°=124.25°=124°15'$ Finding the latitude of the subsolar point is a bit more complicated, we need to know the declination of the Sun. Declination is the equivalent of latitude for celestial coordinates. For that, use a formula , a table , or a online calculator like the NOAA Solar Position Calculator . Just enter the date, and even that the location doesn't matter here, we need to select “Enter Lat/Long -->” to be allowed to enter the offset to UTC as 0, otherwise the time won't be interpreted as UTC time. From there we can find that the solar declination for our example date is 20.58° (20° 34') which corresponds to the latitude of the subsolar point: 20° 34' North. Therefore, on July 20, 1969, at 20:17 UTC, the subsolar point was at 20° 34' N, 124° 15' W, which is somewhere between Mexico and Hawaii. That was the point on Earth closest to the Sun at that moment. Now, what would happen if there were a very tall mountain close to the subsolar point? Would that mountain be closer to the Sun? The answer is: probably. It depends on how far and how much higher it is relative to the subsolar point. We can do a quick calculation based on the following diagram (in this approximation we assume that Earth is spherical, that the sun is infinitely far away and other simplifications) From there we have $r-r'=\Delta H$ $D = r ~ \theta$ ( $\theta$ in radians) $\frac{r'}{r}=\cos(\theta)$ After some algebra you can write that the extra height $\Delta H$ needed to be as close to the Sun as the subsolar point is $\Delta H = r \left(1-\cos\left(\frac{D}{r}\right)\right)$ Where $D$ is the distance and $r$ is Earth's radius (in this case makes sense to use the equatorial radius of 6378.1 km) If we plot this equation we get the following (the vertical axis is logarithmic) We can see that around 10 km away from the subsolar point, ~10 meters are enough to be closer than it to the Sun. ~30 meters at 20 km, ~800 meters at 100 km, ~3,000 m at 200 km, and if you go further than 340 km, not even Mount Everest will get you closer to the Sun. So, the closest point to the Sun will be whatever geographical feature that maximizes the value $\text{Altitude}-\Delta H$ , where $\text{Altitude}$ is the altitude of the geographical feature. Let's call that point “ proxisolar ” point. I just made up that name, but it will be handy for the following discussion. Now that we understand the basis to establish what is the closest point to the Sun at a given moment, we can tackle the question that probably most people meant when asking this question: What is the point on Earth that gets closest to the Sun over a year? The most important fact to keep in mind, is that the variations of the distance between the Earth and the Sun over the year dwarf any topographical feature and even the diameter of the Earth itself. Earth’s distance from the Sun (center-to-center) varies from 147,098,074 km at perihelion (closest) to 152,097,701 km at aphelion (most distant). Therefore, the difference is 5 million kilometers! . The perihelion happens around January 4th, when the solar declination is about -23°, therefore, the latitude of the subsolar point is around 23° South. That rules out Chimborazo, Cayambe and Everest, because they are too far to be the “proxisolar” point. In contrast, Sairecabur (5,971 m at 22.72° S) and Licancabur (5,916m at 22.83° S) are reasonable contestants. The problem is that the perihelion happens on different days of the year and at different times of the day every year, so the point that gets closest to the Sun on a given year is just the one that happen to be the “proxisolar point” at the time of the Perihelion. People who argues that Sairecabur or Licancabur are the points that get closer to the Sun, are implicitly assuming that the distance Earth-Sun doesn't vary much during the day of the perihelion. Therefore, the extra elevation of these mountains allows them to get closer to the Sun during that day. Unfortunately, that assumption is completely wrong. Let's see why: An approximation of the distance Earth-Sun can be obtained from the following formula $d = \frac{a(1-e^2)}{1+e \cos\left(\text{days}\frac{360}{365.25}\right)}$ Where $a$ is the semi-major axis of Earth's orbit, $e$ is the eccentricity, and $\text{days}$ is the number of days elapsed since the perihelion. To see the simplifications behind this equation look here (note that the conversion factor 360/365.25 is erroneously inverted in that link, thanks @PM2Ring for spotting that). If you solve the above equation for the perihelion and for one day before/after it, you will get that the difference is 358 km, and for half a day you get 89 km. Therefore, if the subsolar point happens to be on the opposite side of the Earth than, let's say, Licancabur volcano, this volcano would need to be 89 km higher that the subsolar point to get closer than it to the Sun that year. 89 kilometers! Therefore, we can discard the idea that a given mountain could be the point that gets closer to the Sun on EVERY year . If we plot the above equation with distances relative to the perihelion we get the following (using $a$ and $e$ from here ) Here we can see, that if the perihelion happens a bit more than 3 hours before or after the solar noon at Licancabur, the ~6,000 m of elevation advantage would not be enough to get closer to the Sun than the subsolar point at the perihelion, even if such point is at sea level. Note that three hours corresponds to 45° in Longitude, which at that approximate latitude corresponds to approximately 4,600 km. Therefore, it can be argued that Licancabur is the point on Earth that has more chances to be the closest to the Sun in an arbitrary year. But in a given year, it might or might not be the closest depending on where the subsolar point is at the moment of the perihelion. Finally, it is important to note that the distance Earth-Sun at the perihelion varies widely from year to year. If you look at this table of perihelions between years 2001 and 2100 , you will see that perihelions often vary by several thousand of kilometers . Therefore, for example between years 2001 and 2100, the closest perihelion by far is the perihelion of next year (2020), and it will happen when the subsolar point is in the middle of the Indian ocean, about 12,700 km away from Licancabur and Sairecabur volcanoes. Therefore, the point that will be closest to the Sun this century will be one in the middle of the Indian ocean about 320 km south of Rodrigues Island . Said this, the question of which point on Earth will get closest to the Sun depends on the period of time on which it is considered. For each year, each century and any other arbitrary period of time, the answer will be different. | {
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16,078 | I'm discussing climate change with a friend who is a climate change denier, basically the argument is that looking at this graph, is not clear that something unusual is going on, so the sea level rise is just something that was there, all natural not caused by human activity. Could you please explain this ? | The problem is the increase in the rate of sea level rise. I pulled out some approximate numbers from the figure you presented: Can you see now how the sea level is rising much faster today than a century ago? Sea level rise, as well as climate change are normal things on Earth history. However, most times they happen at a very slow rate, allowing ecosystems and other processes to adapt to the change. For example, if the climate changes over several hundreds of years, animals and even tree populations can "move" towards the side of their distribution where climate is still good for them. But if the climate changes in 50 years, all the trees can die before they had time to grow in the areas where they could thrive in the new climate. In the case of sea level rise. The delta of a river for example, stays in equilibrium with the sea level because of the accumulation of sediments carried by the river. If the sea level rises slowly, the sediments can fill the delta and keep it roughly at sea level. But if the see level rises too quick for the sedimentation to keep up, it will be flooded by the sea, killing all the animals and people that live on such fertile environments. Analogously, coastal infrastructure have a given lifetime. Let's say 50 years. If the sea level doesn't change much over that period (8.5 cm at the 1880-1940 rate), there is no problem. Once the infrastructure gets replaced, the new building will be set a bit higher.
However, in the next 50 years the sea level could rise 50 cm, or even more (it would be 19 cm if we assume the last rate from the figure won't increase any further), and that is a big deal. That could mean that much coastal infrastructure will be flooded, and maybe destroyed during storms. In places like Bangladesh there are hundred of millions of people that could be displaced due to sea level rise if the rate keeps increasing. People that will also need to find a new home. Coastal infrastructure loss, coastal erosion and immigration could be some of the worst expressions of fast sea level rise. ADDITION Given the interest risen by this question I'm adding here some data beyond what is presented by the OP, and brought to my attention by @Bobson in the comments. It comes from the paper Recent global sea level acceleration started over 200 years ago? and shows a sea level reconstruction going back to 1700, and it shows also the changes in rate over that period. This is summarized in their figure 3: Here you can clearly see how, with some ups and downs, the rate of sea level rise have been increasing over the last few centuries. And notably the current rate, about 20 years after the end of this plot is already out of the scale, and around 3.2 mm/year as pointed also by other answers. I would highlight the following from their abstract: Sea level rose by 6 cm during the 19th century and 19 cm in the 20th
century.Superimposed on the long-term acceleration are quasi-periodic
fluctuations with a period of about 60 years. If the conditions that
established the acceleration continue,then sea level will rise 34 cm
over the 21st century. Longtime constants in oceanic heat content and
increased ice sheet melting imply that the latest
Intergovernmental Panel on Climate Change (IPCC) estimates of sea
level are probably too low. Regarding to whether this is caused by humans or not, I rather stay out of that argument and point that our best science and models suggest that lowering $\text{CO}_2$ emissions can make a significant impact in slowing down sea level rise in the upcoming centuries, so we should ACT NOW, and stop arguing whether it was or not our fault in the first place . | {
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16,272 | I've read many papers about Grand Solar Minimums and Glassberg Minimums, known to science for a very long time, and studied and monitored with many public funds. History shows solar activity is the main driver of our climate, and additional contributors like orbital parameters and geomagnetic field intensity are also still connected to how much energy the Earth gets from the Sun. The present trend towards a Grand Solar Minimum was predicted since the '70s and we already see signs of it in solar activity and in our climate, yet it seems to have consistently been ignored by climate experts, by our governments, and by the media. Why is this the case? Solar activity reconstructed from tree rings and Carbon-14 data: (Some pertinent studies regarding such Minimums include the iconic paper J.A.Eddy, 1976: The Maunder Minimum and this 2014 paper by Valentina Zharkova on predicting the next solar cycles , among many others.) | The IPCC do mentions solar minimums and maximums, as part of extremely careful treatment they do of the reconstructions and predictions for the changes in solar irradiance. The Assessment Report 5, Working Group 1, Chapter 8 , have a whole section (~4 pages) dealing with solar irradiances. If the IPCC don't mention grand solar maximums/minumums that often, is because the research in the area have shown that solar minimums and maximums have smaller impact in climate than other factors. Therefore, such factors deserve more attention, like Greenhouse gases or land use changes. These factors deserve more attention both because they have a greater impact on climate and because we can do something about them. When it comes to solar activity we can't do much anyway. The work of the IPCC is to combine the research of the whole scientific community, their conclusions are not based in the result of a single study but in the combination of pretty much all of them. Some studies could be contradictory, so instead of cherry-picking what suits them, they compare and combine all the studies to have the most reliable answers and a good idea of the uncertainties. If predictions of multiple models differ a lot it means uncertainties are large. If they all give the same result, we can be a bit more confident about it. For example, the figure 8.11 compares the reconstruction of solar irradiance between years 1750 and 2000 from six studies: You can see how the Dalton minimum shows up around 1810-1820, but the total solar irradiance change is rather small (less than 1 W for over a total of 1360 W). Here some excerpts from the IPCC reports that are relevant to this question, and where solar minimums are mention and considered in the wider context of climatic forcing (text between square brackets were added by me for clarification): Page 662: Satellite observations of total solar irradiance (TSI) changes from
1978 to 2011 show that the most recent solar cycle minimum was lower
than the prior two. This very likely led to a small negative RF [Radiative Forcing] of
–0.04 (–0.08 to 0.00) W m $^{–2}$ between 1986 and 2008. The best
estimate of RF due to TSI changes representative
for the 1750 to 2011 period is 0.05 (to 0.10) W m $^{–2}$ . This is
substantially smaller than the AR4 estimate due to the addition of the
latest solar cycle and inconsistencies in how solar RF has been
estimated in earlier IPCC assessments. There is very low confidence
concerning future solar forcing estimates, but there is high
confidence that the TSI RF variations will be much smaller than the
projected increased forcing due to GHG during the forthcoming decades.
{8.4.1, Figures 8.10, 8.11} Page 690 (explicit mentions of grand solar minimums): 8.4.1.3 Attempts to Estimate Future Centennial Trends of Total Solar Irradiance Cosmogenic isotope and sunspot data (Rigozo et al., 2001;
Solanki and Krivova, 2004; Abreu et al., 2008) reveal that currently
the Sun is in a grand activity maximum [a.k.a. grand solar maximum ] that began about 1920 (20th
century grand maximum). However, SC [solar cycle] 23 showed an activity decline not
previously seen in the satellite era (McComas et al., 2008; Smith and
Balogh, 2008; Russell et al., 2010). Most current estimations suggest
that the forthcoming solar cycles will have lower TSI [Total Solar Irradiation] than those for
the past 30 years (Abreu et al., 2008; Lockwood et al., 2009; Rigozo
et al., 2010; Russell et al., 2010). Also there are indications that
the mean magnetic field in sunspots may be diminishing on decadal
level. A linear expansion of the current trend may indicate that of
the order of half the sunspot activity may disappear by about 2015
(Penn and Livingston, 2006). These studies only suggest that the Sun
may have left the 20th century grand [solar] maximum and not that it is
entering another grand [solar] minimum . But other works propose a grand [solar]
minimum during the 21st century, estimating an RF within a range of
-0.16 to 0.12 W m $^{–2}$ between this future minimum and the present-day TSI (Jones et al., 2012). However, much more evidence is needed and at
present there is very low confidence concerning future solar forcing
estimates. Nevertheless, even if there is such decrease in the solar
activity, there is a high confidence that the TSI RF variations will
be much smaller in magnitude than the projected increased forcing due
to GHG (see Section 12.3.1). Summarizing, the IPCC consider solar irradiance variations, the timing of solar minimums and maximums. However, it consider also many other factors that also affect Earth's energy budget. Then, they do predictions based on the combined effect of all these factors. In contrast, some people tend to focus on just one factor (as solar activity) and erroneously assume that it will dominate over all the others. For the particular case of solar activity, as you can see in the cites above. The IPCC acknowledge the possible occurrence of a solar minimum in the future, but combining all the models they conclude that there is a high confidence that its effects will be much smaller in magnitude than the projected increased forcing due greenhouse gases . Part of the reason your question might not be well received is because it starts from an assumption that is false: "climate experts from the UN/IPCC never mention Grand Solar Minimum". They do mention it, and if you follow the references in the IPCC you will find plenty of discussion about Dalton, Maunder and older Grand Solar Minimums. And part of that discussion is to estimate the real impact that those events can have in Earth's climate. Then the IPCC get those estimates and figure out how they interplay with the many other factors that conjugate to determine current and future Earth's climate. | {
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17,244 | The picture below shows an imaginary line on the globe which crosses the Pacific Ocean and works as a rough separator of the Eastern and Western hemispheres. What is this line called in English? I'm trying to find the history behind its funny shape. | It's the international date line and marks the boundary between the time zones that are +12 and -12 hours from UTC / Greenwich. It should follow the +/-180 degree meridian line, but zigs and zags to include territories or islands within a "day" thus the Aleutians islands are in the same time zone as the Hawaiian islands. | {
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17,411 | North-hemisphere ice-shelf melts on summer and grows on winter. I would expect appreciable changes on sea-level between seasons, but sea-level looks equal on winter than on summer. Why doesn't sea level show seasonality? | Sea level has a strong seasonal signal. The annual variability is less than the daily changes associated with tidal forcing in most locations, but still can be on the order of 5-10 cm (maximum values about 15 cm). The causes of the seasonal fluctuations are mostly associated with seasonal changes in wind intensity and patterns, changes in temperature that relate to thermal expansion , and in salinity (haline contraction) and river discharge fluctuations. The annual sea level cycle is only partially related to ice melt and this effect tends to be quite local. The largest sea level seasonal cycles are associated with areas in the vicinity of large rivers with strong seasonal cycles (e.g., Bay of Bengal). Also, there is a lot of spatial variability in the seasonal cycle with the northern hemisphere having a larger signal (likely caused by the stronger seasonality in wind patterns). An example of the seasonal cycle can be seen in the monthly data from the Permanent Service for Mean Sea Level (PSMSL) for Woods Hole, MA (USA) (in m offset to avoid negative values in the PSMSL database). The monthly data shows strong seasonal variability and also a clear trend. As the data is monthly averaged, the tidal oscillations are filtered out. Most sea level rise graphs tend to use annual data and thus the seasonal information is not included. | {
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17,611 | Can living in a region that is rich in (rare) earth metal that has an inherently strong magnetic field provide any protection from cosmic radiation? Where are the strongest magnetic field other than the north or south pole and is there a map of these areas? Related: How would our weather change in the event of a magnetic pole shift? | Absolutely not. First of all, "rare earth magnet ore", meaning the ores of metals like neodymium (Nd) and samarium (Sm), is not magnetic at all. It only becomes a magnet once you make a magnet out of it. For example, one such magnet is Nd 2 Fe 14 B and it only becomes a magnet after neodymium is combined with iron and boron. Naturally occuring neodymium ore is usually in the form of minerals such as bastnäsite (a rare earth carbonate) or monazite (rare earth phosphate) that are not magnetic. It gets even worse! Even if by some force of magic the rare earth ore would protect you from cosmic radiation, you now have two problems: Most rare earth ore also has significant amounts of the radioactive thorium and uranium. You do not want to live in these areas, as any "protection" from cosmic radiation will be more than compensated for by the background radiation of the place you are now living in. These are not nice places to live in. The three operating or recently operating rare earth extraction operations (aka mines) are in the desert. Bayan Obo in China, Mount Weld in Australia, and Mountain Pass in the United States. | {
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18,354 | Why are some gases Greenhouse gases while some are not? I did search this on the net but didn't get any clearcut/credible answers. What exactly is the property that is common among Ozone, Water Vapour, CFCs, and Methane that makes them all greenhouse gases? | A molecular gas is a good infrared absorber if it has several atoms (not just 2, like O 2 and N 2 ) or if it is hetero-nuclear (e.g. CO and NO). These type of molecular arrangements allow more infrared energy to be absorbed because there are more vibrational states that are possible. Yes, ammonia fits that description, but it is not long-lived in the atmosphere and it is not widespread. So, even though ammonia is a good infrared absorber , it is not an important greenhouse gas. Ammonia in the atmosphere is very reactive, and will form to make nitrates and sulfates while in solution (e.g. in cloud droplets). Thus, the atmospheric lifetime of ammonia is short-lived, whereas most important greenhouse gases are long-lived and not reactive. For global warming discussion, you would likely associate ammonia with a net cooling effect since the increased formation of particulate (clouds/haze) increases the albedo of the Earth (which scatters light back to space). | {
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18,634 | I am wondering if there are any theories about the formation of oases, and I am also curious about why an oasis can even last for a very long period of time. I have heard that fresh water exists on the surface of the desert because of a difference in elevation of the desert, causing the underground water to pop up. Is it the reason why oases exists? | Oasis are places where aquifers are connected to the surface. The source of the water in the aquifer however can be hundreds of miles away in areas that do get significant rainfall. The trick is geologic strat have different properties; some allow water to flow easily, others are very water tight, a water accessible layer covered by a water proof layer creates a aquifer. Faults or erosion can breach this water seal allowing to water to the surface, as can human wells. Recharge areas are places at higher elevation where water can enter the aquifer and flow downhill, they can be hundreds of miles away and are often in the mountains which often get much higher rainfall. A picture is worth a thousand words. | {
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18,767 | If I understand the information from this link correctly, Instrumental temperature record between 1950 to 1980 there were negative temperature anomalies in the years of those decades. decade difference (°C) difference (°F) 1950–1959 −0.02 -0.0360 1960–1969 −0.014 −0.0252 1970–1979 −0.001 −0.0018 This seems odd considering massive oil consumption started in 1880 IIRC, and by 1980 over 350 billions of oil barrels were already consumed (much more probably, since the data before 1950 isn't considered because there weren't reliable records). Why was there a negative temperature anomaly between 1950 to 1980? | This phenomenon is known as global dimming . It was due to the particles and aerosols mostly released by combustion of fossil fuels such as diesel. Those particles block the radiation from the sun, so they have a cooling effect. For some decades this effect counterbalanced the warming effect of greenhouse gases, although it is no longer the case at a global scale (see for instance Wild et al. 2007 ). Particles emission has been reduced thanks to better engines and new regulations, which stopped their masking effect on global warming. Which is a good thing since those particles have a serious impact on health. | {
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18,854 | I went to Mexico and the pyramids are covered by vegetation. It's only after archeologists clean them that you can actually see the pyramids. Before that they looked like a natural hill, with soil, trees, ... How is that possible? I can imagine grass or small weeds growing here and there in between the stones with seeds being disseminated by wind or birds. But how did soil get up there? Whatever 'lucky parts' of soil that got up there should be washed off with gravity, wind, rain, no? | I am an archaeologist and I specialize in the ancient Maya. Here's how it happens: 1: The vast majority of ancient Maya buildings are built using a "core and veneer" technique. The bulk of the building's volume is earth and stone rubble, faced with a veneer of nicely-shaped limestone blocks that are themselves covered by a layer of lime plaster (stucco). The geology of the Yucatan peninsula is almost entirely limestone, so they built with what they had available. 2: Most pyramids have multiple construction phases, with each one adding a new layer of rubble/earthen core and cut stone and stucco veneer. The building gets bigger with each renovation. 3: Following abandonment, strong seasonal rains quickly erode the lime stucco on much of the outermost layer. This eroded stucco is chemically and structurally identical to the eroded bedrock that underlies natural soils in the Maya region, meaning that plants can readily grow on it. 4: The limestone used for cut stone veneers is itself really porous and prone to crack, pit, and erode in most parts of the Maya lowlands. I am constantly seeing small trees growing out of ostensibly solid blocks of limestone--their roots can still get plenty of purchase. As such, trees can easily take hold in the space between blocks and in small imperfections in the stones themselves. 5: As the trees grow (and especially as their roots push or pry out loosened veneer stones), the cut stones from the veneer give way, exposing the earthen/rubble core underneath. 6: Other posters are correct that some topsoil builds up as a result of wind-transported dust and decaying plant matter, but this is a very, very small amount, especially on the steeper upper portion of the building (remember, the buildings become less steep overall as they erode, because what had been on the upper part tumbles down and piles up around the base, reducing the gradient). Instead, most of the "soil" was actually put there by the Maya as part of the final construction phase, covered by a layer of modern humus. 7: It follows that in many parts of the Maya world, what you see when you look at a cleared/restored pyramid is not really what it looked like in its final stage. Instead, it's usually a mix of the last and second-to-last construction phases, both having been subject to varying degrees of erosion, with the last phase usually only preserved toward the bottom and at the very top. | {
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18,920 | Why doesnt mankind "collect" thermal energy (there has to be some way: thermal couplings, detour over chemical energy, whatever) and after it has been concentrated at one point, turn it to electrical energy (like steam turbines do) or at least radiate it to space? | This is due to thermodynamics, the three laws of which can be summarized as 1) You can't win; 2) You can't even break even; 3) You can't leave the game. The crucial point here is that heat engines don't actually work on heat, they work on temperature differences. So you can't really "collect" heat and turn it into other forms of energy, because you need a colder place to transfer the heat to in order to convert the heat to say electricity. Which is why power plants are usually situated by oceans, lakes, or rivers, in order to use the water as the cold side of the generator. (And ones that aren't have large cooling towers, in order to use the air.) When you do move heat around, say with the heat pumps used for home heating and cooling, you're always using some extra energy to "pump" the heat from one place or another. If you're heating, you move some heat from the ground outside to your house, but the net result is that the system of ground+house gets a bit warmer, because the electricity used for the pump becomes heat. WRT sending the heat back into space, that's actually the cause of global warming. Atmospheric CO2 acts as an insulating blanket, preventing some of the sun's heat from being radiated back out into space. By increasing the amount of CO2 in the atmosphere, we've increased the thickness of the blanket, so the Earth gets warmer. | {
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18,964 | My question refers to the current process of climate change. CO 2 is rising, which leads to the greenhouse effect, which raises temperatures. This leads to more wildfires, which reduces number of trees, increasing CO 2 and reducing CO 2 capacity absorption. Ice caps start to melt, which reduces sunlight reflection (less snow), trapping more heat on atmosphere. Water rises, taking over land and trees, further enhancing the CO 2 absorption capacity. Oceans acidify, lowering their CO 2 absorption capacity too. Etc etc. It seems the process of climate change is a "vicious circle", with a lot of feedback loops reinforcing the trends. Is this the case? Are there counteracting forces that go against this circle? Related questions which imo do not provide an answer to this one: here , here . | There are indeed a lot of positive feedback mechanisms, i.e. a warm climate leads to a warmer climate. From this Wikipedia article , they are: Carbon cycle feedbacks Cloud feedback Gas release Ice-albedo feedback Water vapor feedback However, there are also a few negative feedbacks (same source): Blackbody radiation Carbon cycle Lapse rate Impacts on humans Now the question is: what is the net budget between positive and negative feedbacks? To assess this, climatologists use some metrics, the main ones being "transient climate response" (TCR) and "equilibrium climate sensitivity" (ECS). From Knutti et al. (2017) : TCR is defined as the global mean surface warming at the time of
doubling of CO $_2$ in an idealized 1% yr $^{−1}$ CO $_2$ increase
experiment, but is more generally quantifying warming in response to a
changing forcing prior to the deep ocean being in equilibrium with the
forcing. Based on state-of-the-art climate models, and instrumentally
recorded warming in response to CO $_2$ and other anthropogenic and
natural forcings, the Intergovernmental Panel on Climate Change's
Fifth Assessment Report (IPCC AR5) assessed that the transient climate
response is 'likely' (>66% probability) to be in the range of 1 °C to
2.5 °C. By contrast, the equilibrium climate sensitivity (ECS) is defined as
the warming response to doubling CO $_2$ in the atmosphere relative to
pre-industrial climate, after the climate reached its new equilibrium,
taking into account changes in water vapour, lapse rate, clouds and
surface albedo. [...] The estimated range of ECS has not changed much
despite massive research efforts. The IPCC assessed that it is
'likely' to be in the range of 1.5 °C to 4.5 °C. Which basically means that the climate will get warmer in the future, until it will eventually reach some kind of equilibrium. | {
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19,134 | Or does the mantle and crust above you counteract the increase at one point and it actually decreases? | The below figure, taken from Wikipedia shows a model of the free fall acceleration, i.e., 'gravity'. The left-most point corresponds to the center of the Earth; then further right at $6.3\cdot1000$ km you are at the Earth's surface; and then further out you move into space. You can follow the blue line for PREM to get an idea of the average (expected) gravity. As you see, the gravity actually increases slightly within the Earth (reaching a maximum at the core-mantle boundary), but tapers down within the core. To make this kind of calculations, you must think of the Earth like an onion: made up of many concentric spheres. Whenever you move a bit deeper into the Earth, you strip off all the layers you've crossed. As you get closer to the center of the Earth, there are fewer and fewer layers, and eventually, there's nothing left at the center! The reason why gravity goes up ever so slightly within the Earth is that you get close to the much denser core material. If the density of the Earth were constant (per the green 'constant density' line), the gravity would just decrease linearly. See the other answer and the discussion below for some more details on the math and procedures required to make these calculations. | {
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19,191 | Waves are mostly caused by Friction of wind on surface of water . Wind blows from sea to land in day and land to sea in night due to pressure and temperature difference. So it is intuitive why there are waves towards shore in day, but even in night there are waves towards shore. Sometimes even stronger than waves during the day. Why is that? | Ocean waves (and also in mediterranean type seas and larger lakes, but on a smaller scale) are generated by two processes: locally generated waves ("wind waves"), which follow the direction of the wind; waves generated further out in the sea (i.e. "swell waves"), which do not necessarily follow the direction of the wind. During the night, you are probably seeing swell waves. Of course, some wave energy is generated also in the opposite direction by the wind blowing offshore, but one can only "see" these waves further out in the sea if the wind is strong enough (see the definition of fetch ). Also check the Figure [1] below for clarification on how swell is generated (the longer period wave energy travels faster than the energy of the shorter period waves, so the initial "random" wave field disintegrates into regular swell). Shallow water wave processes like diffraction and refraction will curve the paths of the waves that start reaching the coast and "feeling the bottom". The waves appear to be heading almost perpendicular when they reach the coast, even if 1 km out to sea they're moving almost parallel to the long distance coast. How the waves appear at the coast to the observer depends on the direction of the local wind. Generally, if the local wind blows in the same direction as the waves (i.e. onshore wind), the waves appear "mushy" as the wind helps to break the waves (see example image ). If the local wind blows in the opposite direction of the incoming waves (i.e. offshore wind), the waves maintain their shape due to opposing wind and they break later (see example image ). These conditions are favourable to surfers. [1] Holthuijsen, L. H. Waves in oceanic and coastal waters. Cambridge University press, 2010. | {
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19,487 | Is there a name for the great circle where latitude and longitude are equal? I have attempted a google search but only the equator and the prime meridian are defined in the sources I can find. ( It is of relevance in developing a map application which keeps track of latitude and longitude ). | The curve where latitude and longitude are equal is not a great circle. But as joe khool writes in his excellent answer , it's called the curve of Viviani ! It's easy to see that the curve is not a great circle, because, using naïve spherical coordinates (in radians) $(\phi,\lambda)$ with $\lambda$ being longitude and $\phi$ being latitude (zero at equator), this curve passes through $(0,0)$ , and also through $(\pi/2,\pi/2)$ which is the north pole ( $(\pi/2,\lambda)$ is the north pole for any $\lambda$ ), But it also passes through, say, $(1,1)$ which is not on the great circle the between previous two points. In fact the curve you get looks like this: Note. I plotted this by defining Cartesian coordinates in the obvious way: $$
\begin{align}
x &= R\cos\phi\cos\lambda\\
y &= R\cos\phi\sin\lambda\\
z &= R\sin\phi
\end{align}$$ and then plotting $(x,y,z)$ for $\phi = \lambda$ and $\lambda\in[-\pi/2,\pi/2]$ . An earlier version of this answer plotted $(x,y,z)$ for $\phi = \lambda$ and $\lambda\in[-\pi,\pi]$ . This means that $\phi$ takes values which are not in $[-\pi/2,\pi/2]$ of course. I had assumed that these points would end up around the back of the planet: that you'd get a kind of 'S' which wraps around the planet, but in fact it ends up around the front of it again: This surprised me! | {
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20,021 | Do you know any example of a river that does not flow into sea or lake? For example, if water rises from the underground spring and forms a river but water vaporises in hot climate before it reaches any larger body of water. Or if there is a valley with very moderate precipitation and many little creeks flow out of the valley and the water from them is absorbed by the soil. | The Okavango River is a good example of this. It drains into a swampy delta in the middle of the Kalahari desert: The Okavango Delta, CC BY Justin Hall. | {
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20,110 | I saw this graph about the global temperature, it goes back for 2000 years. How is it possible to measure temperature 2000 years back with such a precision of like ~0.1 C? The image from Reddit post I don't know the source of original data. What tools are used to measure it? | How it's possible to measure temperature 2000 years ago? Sans the technology used by Bill and Ted ("Bill and Ted's Excellent Adventure"), it obviously is not possible to directly measure the temperature from yesterday, let alone 2000 years ago, or longer. What is used are "proxies", things that can be measured today that serve as stand-ins for things such as temperature in the past. One example of a proxy is the amount of oxygen-18 in the ice in Greenland and Antarctica. Water with two hydrogen atoms and one oxygen-16 atom has a boiling point of about 100° C. Water with the oxygen-16 atom replaced by an oxygen-18 atom has a slightly higher boiling point. This means that heavy oxygen water evaporates less readily but precipitates more readily than does normal water. This in turn means that the fraction of oxygen 18 versus oxygen 16 in the water and air bubbles in the ancient ice in Greenland and Antarctica are indicative of the climate at the time that that ice and those air bubbles formed. Despite being a half a world apart, the ratios of oxygen-18 to oxygen-16 over time are highly consistent between Greenland and Antarctica. The consistency of these measures at half a world apart are almost universally taken as being a proxy for something else, and that something else is climate. There are many other proxies for past climate. There are other isotopes that serve as proxies. The amounts and kinds of pollen in mountain glaciers and ice sheets form yet another kind of proxy. The kinds of plants that do grow in some locales and how fast they do grow is highly temperature dependent. Like ice, buried muds in the oceans also show variations in various proxies. Climate scientists put these proxy measurements together to arrive at the temperatures from 2000 years ago, and even further into the past. | {
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20,117 | In my study, I found that in volcanoes when the magma is going up it formed different types of rocks. There are basic, acidic and ultrabasic. My question is why isn't there ultra acid igneous rocks formed? | How it's possible to measure temperature 2000 years ago? Sans the technology used by Bill and Ted ("Bill and Ted's Excellent Adventure"), it obviously is not possible to directly measure the temperature from yesterday, let alone 2000 years ago, or longer. What is used are "proxies", things that can be measured today that serve as stand-ins for things such as temperature in the past. One example of a proxy is the amount of oxygen-18 in the ice in Greenland and Antarctica. Water with two hydrogen atoms and one oxygen-16 atom has a boiling point of about 100° C. Water with the oxygen-16 atom replaced by an oxygen-18 atom has a slightly higher boiling point. This means that heavy oxygen water evaporates less readily but precipitates more readily than does normal water. This in turn means that the fraction of oxygen 18 versus oxygen 16 in the water and air bubbles in the ancient ice in Greenland and Antarctica are indicative of the climate at the time that that ice and those air bubbles formed. Despite being a half a world apart, the ratios of oxygen-18 to oxygen-16 over time are highly consistent between Greenland and Antarctica. The consistency of these measures at half a world apart are almost universally taken as being a proxy for something else, and that something else is climate. There are many other proxies for past climate. There are other isotopes that serve as proxies. The amounts and kinds of pollen in mountain glaciers and ice sheets form yet another kind of proxy. The kinds of plants that do grow in some locales and how fast they do grow is highly temperature dependent. Like ice, buried muds in the oceans also show variations in various proxies. Climate scientists put these proxy measurements together to arrive at the temperatures from 2000 years ago, and even further into the past. | {
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20,242 | Why is it that the volcanoes found in the Tharsis Montes region near the Martian equator, (one of which is Olympus Mons) so much larger than those found on Earth. In comparison, Hawaii's Mauna Loa, the tallest volcano on Earth, only rises 10 km above the sea floor. Olympus Mons rises three times higher than Earth's highest mountain peak, Mount Everest. What makes these volcanoes rise to such enormous heights in Mars, when comparing to those found on Earth and the rest of the Solar System? | This is mostly due to the fact that Mars does not have plate tectonics. Therefore the plate stays above the hotspot without moving, allowing magma to rise and pile up at the same place for millions and millions of years. Above the Hawaii hotspot, the oceanic plate is moving, so volcanism tends to drift away with time (actually the volcanism happens at the exact same place from a mantle point a view, but its surface expression moves with the plate). It's why rocks of the Hawaiian-Emperor seamount chain are older in the West and younger in the East. This is true even at the scale of Hawaii island itself, where Kohala and Mauna Kea are extinct, while volcanism $-$ or rather the plate $-$ has shifted to Mauna Loa, Kīlauea and Kamaʻehuakanaloa. Imagine if all the magma comprising these islands had piled up at the same place, it could have built a gigantic volcano like Olympus Mons! Well... not really. There is another parameter to account for: a theoretical limit to how high a mountain can possibly get, because of compressive strength of rock (or glacial erosion in some theories). See for instance answers in these questions: How high can a mountain possibly get? Why is Mauna Kea taller than the maximum height possible on Earth? On Earth the limit is ~10 km. So even if magma kept piling up at the same place, the resulting mountain would start to laterally spread or collapse. But on Mars gravitational acceleration is lower, making the limit much higher. | {
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20,570 | I originally thought that you can only find certain things like iron, copper, and other things in certain places in the world and deep in mines but I saw in a few YouTube videos that you can find iron in rocks just laying on the ground. I searched for where copper comes from and it seems that, even though a lot of it comes from Chile, it can be found everywhere in smaller amounts. Is this true? Could I walk out into the woods right now anywhere in the world and find copper and iron? | Metals occur on every continent . The issue is whether or not they occur in sufficiently large deposits for them to be mined economically. The difference between a mineral deposit and an ore body is economics: can it be mined for a profit? One of the issues with mineral resources is that once they've been mined, they're gone. Cornwall and Devon , in the UK, were once a significant producer of tin and copper. Spain is major source of metals in Europe. Some of its mines were started by the ancient Romans. Poland too, has a active mining industry, as does Sweden . Chile and Peru are major sources of copper, but copper is also mined in the US, Zambia, Australia and Russia. Africa has a well established mining industry that mines many metals: gold, copper, iron, lead, zinc and platinum. Metal mining in Australia and Canada provide both countries with significant sources of export income. China has iron ore deposits that grade between 25 and 40 percent iron, but it prefers to buy higher grade iron ore, grading 65 to 70 percent iron, from elsewhere, mostly from Brazil, Australia and South Africa, because it is cheaper to produce steel from higher grade ore. However, mineable quantities of some metals, such as lithium, platinum and rare earth metals tend to be concentrated in specific locations. | {
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21,161 | The Puget Sound region is wedged between the Pacific and the Cascades...you'd think with all that rain (and snowmelt in the warmer months), there would be catastrophic flash flooding in lowland areas, of the magnitude seen in Oklahoma ( 1 , 2 , 3 ), Texas ( 1 , 2 , 3 , 4 ) or Tennessee ( 1 , 2 , 3 ). On the contrary, western WA is actually subject to droughts and wildfires , not something you'd expect from such a wet region with rainforests. | Regarding climate, it does not rain in Seattle as much as people think; Seattle is in the snow shadow of the Olympic Mountains. It doesn't rain much in summer at all. Seattle gets rather dry in July and August. Regarding flooding, the Skagit and Snohomish rivers north of Seattle flood regularly experience flooding. Some of the land in the flood plain is used for farming, and there are scattered households in the 100 year flood plain. Regarding drought, drought severity is relative to the amount of precipitation and soil moisture levels typical for a specific locale. This means that even a rainforest can suffer drought. Because it's a relative concept, the rainfall during a drought in a rainforest would be a rainfall bonanza in arid climates. Regarding forest fires, the Seattle area occasionally undergoes offshore flow. This occurs when the air pressure is lower in the Seattle area than it is east of the Cascades. This makes winds blow from the east, and if there's a forest fire east of the Cascades (which has a rather dry climate), that smoke can be carried westward toward the Seattle area by the offshore flow. That said, wildfires have occurred on the western slopes of the Cascades, and sometimes even in the temperate rainforest to the west of the Seattle area. | {
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21,241 | In the morning, Google shows the temperature from four or five hours ago (around 2:00 a.m.). But then I update the report, and I get the temperature slightly lower than before. Why is the temperature at 2:00 a.m. higher than the temperature at 7:00 a.m.? | The Earth is always radiating heat to the space. But in the day the Sun delivers some heat. The net heat flux is then defined as the sum of those two factors. If the energy delivered by the Sun is bigger than the cooling rate, the Earth is net warming (positive net flux – we can imagine it like heat is travelling "to us"), as opposed to the opposite case (cooling; negative net flux – heat is travelling "away"). In the day, the Sun warms the ground until the evening. The Sun's heating rate is higher than the cooling rate, so the temperature is rising until it gets to a point where the heating rate is same as cooling rate. This happens in the evening, so the temperature is steady at that point. But the Sun goes even lower, so the net flux becomes negative. In the night there is only cooling of Earth, so the temperature is falling steadily until the Sun is high enough that it balances the cooling. This happens at a point of a minimum temperature. Of course, this is valid for most of the days, but we can have some other effects that can change the time of the minimum temperature (clouds, fronts or advection, for example). On graph: So, the temperature is falling over night after the sunset, but rises again after the sunrise. Thus, the temperature is at its lowest point in the morning. Appendix for all you loving calculations: Note: Simplified to toy model, no atmosphere The cooling rate of the Earth is approximatelly given by the Stefan-Boltzmann equation: $$j_E=\sigma\cdot T^4=5.670 \cdot 10^{-8} \frac{W}{m^2 K^4} \cdot (288.15 K)^4 = 390 \frac{W}{m^2}$$ The maximum heating rate of Sun in the zenith is $j_{\text{S max}}=1361 \frac{W}{m^2}$ . So, the heating rate of Sun at altitude $\alpha$ is: $$j_S=j_{\text{S max}}\cdot \sin{\alpha}=1361 \frac{W}{m^2}\cdot \sin{\alpha}$$ When is the heating rate equal to zero? $$0=j_S-j_E=1361 \frac{W}{m^2}\cdot \sin{\alpha} - 390 \frac{W}{m^2}$$ $$1361 \frac{W}{m^2}\cdot \sin{\alpha} = 390 \frac{W}{m^2}$$ $$\alpha = 17 °$$ So, with our calculations, the minimum temperature is at the time when the altitude is equal to 17°. | {
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21,268 | The Democratic Republic of the Congo's Katanga Province contains almost 40% of the world's reserves of cobalt [1]. Why are deposits concentrated so strongly in such a small portion of the earth? I would have thought that this is due to similar reasons to: Why do gold deposits form only in certain areas of the earth? But the distribution of cobalt seems far more uneven than that of gold. Do the geological processes causing this uneven distribution significantly differ from those that cause the uneven distribution of gold? [1]: British geological survey, 2009 | Part 1 The Democratic Republic of the Congo's Katanga Province contains
almost 40% of the world's reserves of cobalt [1]. Why are deposits concentrated so strongly in such a small portion of
the earth? Cobalt isn't as unevenly distributed as it seems. It is correct that most of the world's cobalt reserves are in DRC, but most of the cobalt resources are not in DRC. And this is a very important point. Resources are known and estimated quantities of economically extractable materials. Reserves are resources for which detailed plans for extraction have been made. This means that to turn a resource into a reserve you need to have plans for the extraction and refining plants, have environmental approval, have the workforce figured out, have authorisation from the owners of the land, and more. As you can guess, making a resource into a reserve is a time consuming and expensive process. The reason why most of the Earth's reserves are in DRC is simply because it's easier and cheaper. The infrastructure already exists, and the expertise exists. At current technology levels and cobalt prices, the investment in defining a reserve from a resource elsewhere is simply not economical. This does not mean that DRC has most of the cobalt. It only means that getting cobalt out of the ground and making it into a product is cheaper in DRC than it is in other countries. Part 2 Do the geological processes causing this uneven distribution significantly differ from those that cause the uneven distribution of gold? The reasons are the same, in principle. Every element behaves differently and will separate from other elements and concentrate because of various geological processes. Cobalt tends to follow copper and nickel, and in most cases it is mined as a by-product of those elements. That's why cobalt is extracted from the African Copper belt, and why it's extracted from nickel deposits in Australia. Not much point going into the chemistry and thermodynamics of why this happens, but the point is that elements which are concentrated by rare geological processes will be found in fewer places, and elements that are concentrated by common geological processes will be found in more places. | {
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22,478 | Currently it is winter in Antarctica. According to news I read, Antarctica has set a new record high temperature, above 18 °C. How did this temperature records occur? | You just read the news too fast. WMO announced that, after evaluation by a committee, they have validated the 18.3°C temperature recorded in February (i.e., in summer) last year: GENEVA, 1 July 2021 (WMO) - The World Meteorological Organization
(WMO) has recognized a new record high temperature for the Antarctic
continent of 18.3° Celsius on 6 February 2020 at the Esperanza station
(Argentina). | {
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22,663 | Apologies for asking a silly question like this. But I want to offer some points which I could not counter, as follows: Ice is less dense than water, that is why it floats. For the same unit mass, if density is lowered, the volume increases, thus ice ends up having more volume/consuming more space. AFAIK, the polar ice on Earth does not originate from other planets. Above points conclude that, if polar glaciers continue to melt (very unfortunate) the resulting water is denser than the ice so it would occupy less volume per unit mass and as the polar glaciers were originally from Earth, there is no new volume addition which can explain the rising sea levels. So how does glacial melting cause global water levels to rise? Are there other factors which amplify this, like maybe global polymer waste dumping into the oceans? | Arctic ice, around the north pole floats on top of water . When it melts it does not add to sea level rises and likewise for other ice on water , as illustrated in this video and this video . Ice on land is a different matter. Ice on Greenland, Antarctic land & glaciers around the world will add to sea level rise because any melt water will eventually end up in the oceans. It's like adding water to a partially filled glass of water. The thickness of the Greenland Ice Sheet is more than 2 km and it is of the order of 2900 km long and 1000 km wide. The amount of ice in Greenland is 2,850,000 cubic kilometres. If this alone were to melt it would increase sea levels by 7 m. The island of Greenland cannot contain all the melt water and it will end up in the sea. The Antarctic ice sheet contains 26.5 million cubic kilometres. | {
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22,741 | According to this website , there were 3,372 active satellites in orbit at the beginning of 2021. Furthermore, SpaceX is planning to launch 12,000 satellites to provide cheap internet for everybody, among other functions. Does this huge collective quantity of objects (together with space trash) reflect away enough solar radiation to have a measurable effect? Could this affect earth's climate, mitigating climate change? | Does this huge collective quantity of objects (together with space trash) reflect away enough solar radiation to have a measurable effect? Could this affect Earth's climate, mitigating climate change? No. Suppose, instead of 12,000 tiny satellites, SpaceX was planning on launching 12,000 satellites the size of the International Space Station. The ISS, with its huge solar arrays, has a cross section of about 7,000 square meters (6,528 square meters for the arrays alone). Fun fact: This is about the size of a FIFA-sanctioned international match field. I'll bump up the size to 8250 square meters, the maximum size of a FIFA-sanctioned international match field. The ISS blocks sunlight that would otherwise hit the Earth about half of the time. I'll halve the 12,000 satellites to 6,000 to reflect this. Six thousand maximum-sized FIFA-sanctioned international match fields results in an area of 49.5 square kilometers. The Earth's cross section to sunlight is $\pi r^2$ , where $r$ is the Earth's mean radius (6371 km). This cross section area is $1.275\times10^8$ square kilometers. That's over 2.5 million times larger than those 6000 maximum-sized FIFA-sanctioned international match fields. Summary: Even 12,000 satellites larger than the ISS would have a minimal impact on the amount of sunlight hitting the Earth. The 12,000 satellites SpaceX is planning to launch are orders of magnitude smaller than the ISS. The impact is in the noise. | {
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22,974 | Why is the Arctic melting, but the Antarctic doing great? That's what the latest IPCC report says (p. 2-192 of the Physical Science Basis provides a time series), but it says nothing, unless I missed it, on the causes of such a discrepancy. It's same ice, isn't it? Why is Antarctic ice more heatproof? EDIT: I've found the answer in the IPCC report (AR6 WGI), and it's even fairly intelligible (unlike much of the rest of the report). Start reading from page 7-80, if you're interested | Not quite. The report actually shows an increase in extent , but not the volume, of sea-ice around Antarctica. That sounds good, but the data appendix in the report shows the Antarctic continental ice-sheet is shedding ice so fast during the summer months, and into the Autumn, that the ice can't melt or migrate away during the season. When the winter freeze comes the ice already in the water is making the ocean surface cooler and nucleating sea-ice growth farther off shore than it should be. We're seeing a larger but thinner sheet of ice around the Antarctic which can seem unaffected by climate change, but its ice is actually melting at a rate comparable to the Arctic. | {
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23,625 | I was talking to a friend of mine who is an environmentalist like me but not a big fan of nuclear power and she told me it was not a good option in the long run because of the decay of uranium. Of course this is a ridiculous argument because U-238's half-life is measured in billions of years, but nonetheless I couldn't tell her whether or not the world's reserves were shrinking in a geologic time scale. Looking at the many decay elements of U-238, I believe there may be a handful of isotopes that decay into it. And I'm certain there is a name for these elements I'm looking for, I just can't find them. Follow-up question: given our current known reserves of transuranic elements and again taking into account their decay products, where will the greatest uranium reserves be in, say, 10 half-lives of U-238? I can barely make sense of a nuclide chart and don't even know where to begin looking for a map of these elements. | Let's start with the easy question: given our current known reserves of transuranic elements and again taking into account their decay products, where will the greatest uranium reserves be in, say, 10 half-lives of U-238? I can barely make sense of a nuclide chart and don't even know where to begin looking for a map of these elements. 10 half lives of U-238 will pass in around 45 billion years. We have only about 5 billion years before the sun starts expanding beyond Earth's orbit. Therefore, the answer to your question is "inside the sun". Now, the main part of your question: Of course this is a ridiculous argument because U-238's half-life is
measured in billions of years You are entirely correct. We can even calculate that. Let's ask Wolfram Alpha how much uranium will have decayed after 1000 years. This is the answer: remaining fraction of number of particles | 99.9999845%
= 0.999999845 So for all practical purposes, after one thousand years (!!), the amount of uranium in the Earth does not change (other than the uranium used for power generation). I couldn't tell her whether or not the world's reserves were shrinking
in a geologic time scale First, note that the word "reserves" has a very specific meaning . You are probably asking simply about the quantity of uranium in the Earth. So yes, it is shrinking. The half-life of uranium is about 4.5 billion years, which happens to be the age of the earth. So the Earth currently has half of the uranium it had when it first formed. I believe there may be a handful of isotopes that decay into it Yes, Pu-242 will decay to U-238 with a half-life of 375000 years. Since we're more than 100 half-lives past the formation of the Earth, there is no Pu-242 left on Earth for all practical purposes. Statistically speaking, there might be a few atoms left hanging around since then, but don't put your bets on their decay for increasing the amount of U-238. | {
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23,911 | According to the theory of continental drift, South America and Africa was so closed to each other that the convex triangle of South America meets the concave hollow of Africa. Source: usgs.gov ; this image is in the public domain However, according to the Plate tectonics, there is much space (let's name it A) on the right-hand-side of South America and much space (let's name it B) on the left-hand-side of Africa. Source: noaa.gov ; this image is in the public domain If these 2 plates were near to each other so that the two lands meet, where did the space A and B go? I think there are only 2 possibilities: A was over B, B was over A. Both possibilities imply that South America and Africa can't join together to form a whole land. Do the two theories just contradict each other? (Source of the pictures: https://education.nationalgeographic.org/resource/continental-drift , https://education.nationalgeographic.org/resource/plate-boundaries ) | The boundary between the African and South American plate is a 'divergent' boundary. The two continents were joined as part of the Pangean super-continent. In the Cretaceous period a rift opened up and oceanic crust began forming along the mid-Atlantic ridge . The oceanic crust that fills the 'space' between the two continents has been created over the last 60 million years as magma rises and spreads, pushing the two continents apart. | {
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23,915 | In relation to this experiment: https://www.remineralize.org/rem_publications/action-of-microorganisms-in-basalt-powder/ It is said that applying basalt rock dust to soil can improve soil fertility especially so if a complete soil food web is present due to biological weathering by bacteria and fungi - which help to release plant nutrients into the exchangeable and soluble pool. Does basalt have a high rate of weathering and is it in the ‘discontinuous series’ or ‘continuous’ series? Goldich dissolution series | The boundary between the African and South American plate is a 'divergent' boundary. The two continents were joined as part of the Pangean super-continent. In the Cretaceous period a rift opened up and oceanic crust began forming along the mid-Atlantic ridge . The oceanic crust that fills the 'space' between the two continents has been created over the last 60 million years as magma rises and spreads, pushing the two continents apart. | {
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24,787 | Recent news articles related to this paper report the claim that Earth's solid inner core sometimes rotates backwards. The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum. The matter is discussed in this SE question , but there is also the matter of multi-decadal cycles. What is the actual assertion? Is the inner core thought to be cycling between prograde and retrograde rotation, or is it something more subtle, like a wobble, sometimes slightly leading and sometimes slightly lagging, and at what amplitude? Is it a matter of "gaining" or "losing" a few degrees over some number of years (so would only be rotating "backwards" in the frame of reference of the crust, not non-rotating space)? | The mantle rotates about 131850 degrees per year. The actual assertion is that the inner core cycles between rotating about 131851 degrees per year versus 131849 degrees per year over the course of 70 year cycle. The paper was only published yesterday, so the scientific consensus is not there yet. The scientific consensus is not there yet on work done by the same authors eight years ago. The inner core rotates in the same direction as do the mantle and crust, and rotates at almost at the same rate as the mantle and crust. The claim is that this rate varies by a tiny fraction compared to the mantle's rotation rate, plus or minus a degree or so per year compared to the mantle (but keep in mind that is one degree out of 131850 degrees). It does not switch directions. | {
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24,789 | Why is sodium chloride far and away the most abundant salt dissolved in ocean water? Its two constituent ions do have a very high frequency in the crust of the earth, but they are far from the most common. Chlorine is (according to Wikipedia ) the 21st most abundant element, and sodium 6th. I certainly understand that a combination of their solubility and reasonably high frequency would lead one to expect them to be abundant in sea water, but they are hyper abundant, completely dominating all other salt ions. Iron, for example, is twice as abundant, and potassium only a little less abundant, and fluorine more abundant than chlorine. Moreover, if the salts are deposited in the ocean through weathering of rocks and deposition via rivers, why does the salinity not simply grow and grow? I understand that some is lost due to tectonic activity, but it seems extraordinarily unlikely that these two forces should be equally balanced, and so we would see a significant change in average salinity over time. (Please note I am migrating this question from the Chemistry SE at their recommendation.) | Fluoride salts tend to be not particularly soluble in water. Chloride salts are. The same goes for salts containing sodium versus those containing calcium. Sodium chloride is ridiculously easy to dissolve. Regarding your second question, it is geological forces that keep salinity more or less constant. People formerly argued that the Earth can't be more than a few hundred million years old because otherwise the river waters running into the oceans would eventually result in an insanely high salinity. It turns out that the Earth's oceans are young (young compared to the 4.5 billion year age of the Earth). The vast majority of oceanic crust is less than 100 million years old. We see huge salt deposits sprinkled across the world because those are the dried up remnants of former seas and oceans. Salt is also drawn into the Earth at subduction zones, where it combines chemically with basalt. | {
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24,817 | So I was in this place called Chamundi hills, Mysore . I was riding back at around 7pm down hill and noticed this strange behavior. About every 50-100 meters the temperature fluctuated. It was extremely cold in some areas and in the next 100 meters I could feel the warmth. The whole cycle repeated all the way (about 8-10 kilometers). Is there any scientific explanation about this? | To extend @Poutnik's invisible river analogy, the cold air can also 'pool' in small dips. On clear nights, temperature fluctuations are quite a frequent occurrence due to radiative cooling. If the air cools sufficiently to reach the dew point, then cloud can act as a 'tracer' for the air pockets. An example of this are fog patches that you potentially pass through in certain conditions. The air in the fog patches is a little colder (or higher absolute humidity, for example over a lake) than the surrounding fog-free areas. Even if the air wasn't cold enough to form cloud, the same temperature variation can occur invisibly. Here's an example from Wikimedia user Simo Räsänen : In the mountains, the invisible rivers of cold, descending air that @Poutnik mentions can sometimes be seen by tracer clouds flowing down hillsides. Colder air is denser than the surrounding air and has less friction so gravity pulls it down. The areas with cloud will be colder than the adjacent cloud-free zones. This effect can lead to strong down-slope katabatic winds . In Antarctica this effect is responsible for some of the strongest winds in the world. Here's a good example from Wikimedia user Andrew J. Kurbiko : To summarise, the key to the effect you noticed is almost certainly radiative cooling and the movement of colder denser air. Radiative cooling occurs due to any object above absolute zero giving off radiation. On a clear night this radiation passes through the Earth's atmosphere and is lost into space. This cools down the object (land) that the radiation came from. The land, in turn, cools down the air above it. On a cloudy night, the radiation is reflected by the clouds and reabsorbed by the land keeping the temperature constant. | {
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35 | For the other sciences it´s easy to point to the most important equations that ground the discipline. If I want to explain Economics to a physicist say, what are considered to be the most important equations that underly the subject which I should introduce and attempt to explain? | Instead of proposing specific equations, I will point to two concepts that lead to specific equations for specific theoretical set ups: A) Equilibrium The most fundamental and the most misunderstood concept in Economics. People look around and see constant movement -how more irrelevant can a concept be, than "equilibrium"? So the job here is to convey that Economics models the observation that things most of the time tend to "settle down" -so by characterizing this "fixed point", it gives us an anchor to understand the movements outside and around this equilibrium (which may be changing of course). It is not the case that " quantity supplied equals quantity demanded " (here is a foundational equation) $$Q_d = Q_s$$ but it is the case that supply tends to equal demand (of anything ) for reasons that any economist should be able to convincingly present to anyone interested in listening (and deep down they all have to do with finite resources). Also, by determining the conditions for equilibrium, we can understand, when we observe divergence, which conditions were violated. B) Marginal optimization under constraints In a static environment , it leads to the equation of marginal quantities/first derivatives of functions. Goods market: marginal revenue equals marginal cost . Inputs market: marginal revenue product equals marginal reward (rent, wage). Etc. (I left "utility maximization" out of the picture on purpose, because, here first one would have to present what this "utility index" is all about, and how crazy we are ( not ), by trying to model human "enjoyment" through the concept of utility). Perhaps you could cover it all under the umbrella "marginal benefit equal marginal cost" as other questions suggested: $$MB = MC$$ Economists live in marginal optimization and most consider it self-evident. But if you try to explain it to an outsider, there is a respectable probability that he will object or remain unconvinced, instead usually proposing "average optimization" as "more realistic", since "people do not calculate derivatives" (we don't argue that they do, only that their thought processes can be modeled as if they were). So one has to get his story straight about marginal optimization, with convincing examples, and a discussion about "why not average optimization". In an intertemporal setting , it leads to the discounted trade-off between "the present and the future", again "at the margin" -starting with the "Euler equation in consumption" , which in its discrete deterministic version reads $$u'(c_{t})=\beta(1+r_{t+1})u'(c_{t+1})$$ ...and one cannot avoid the theme of utility, after all: $u'()$ is marginal utility from consumption, $0<\beta<1$ is a discount rate and $r_{t+1}$ is the interest rate ( don't consult wikipedia article on Euler's equation in consumption, the concept behind it is much more generally applicable and foundational than the specific application that the wikipedia article discusses). Interestingly, although dynamic economics are more technically demanding, I find this more intuitively appealing since people seem to understand way better "what you save today will determine what you will consume tomorrow", than "your wage rate will be the marginal revenue product of all labor employed". | {
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225 | The intuitive criterion by Cho and Kreps is a refinement to minimise the set of perfect Bayesian equilibria in signalling games. What would a simple and intuitive example to explain this criterion be? Assume any undergrad student should be easily able to appreciate the refinement through the example. | A concise, completely informal way of putting it is this: The intuitive criterion rules-out any out-of-equilibrium beliefs that can only be correct if some player did something stupid. Below is a slightly more long-winded explanation with an informal example. In many signalling games (that is, games in which one player—the sender—can communicate information to another—the receiver), there are often a lot of implausible equilibria. This happens because the Perfect Bayesian solution concept does not specify what the receiver's beliefs must be when the sender deviates; we can therefore support a lot of equilibria simply by saying that if the sender deviates from those equilibria then he will be "punished" with very bad beliefs. Such punishment will usually be enough to make the sender play a strategy that would otherwise not be a best response. For example, in Spence's classic job market signalling paper there is an equilibrium in which high-ability individuals invest in education (learning is easy for them) whilst low-ability individuals do not (because they find it too costly to do so). Education is then a signal of ability. We might ask: is there also an equilibrium of this game in which nobody chooses to get an education and no information is transmitted to the receiver? The answer is 'yes'. We can support such an equilibrium by saying that a deviation in which a sender is educated causes the receiver to adopt the belief that the sender is certainly low-ability. If education has the effect of signalling low-ability then, of course, everyone is happy to play along with the putative equilibrium and not get educated. It is also clear that this equilibrium is not very plausible: the receiver knows that it is less costly for a high-ability agent to get an education than a low-ability one, so it doesn't make much sense for him to think of an education as signalling low-ability. The intuitive criterion rules out this kind of equilibrium by requiring beliefs to be "reasonable" in the following sense: Suppose the receiver observes a deviation from the equilibrium. The receiver should not believe that the sender is of type $t_{\text{bad}}$ if both of the following are true: the deviation would result in
type $t_{\text{bad}}$ being worse off then if he has stuck to the equilibrium for any beliefs. there is some type $t_{\text{good}}$ who is better off by playing the deviation than by sticking to the equilibrium for some belief other than $t_{\text{bad}}$ . Returning to the education signalling model: Suppose that the equilibrium is that nobody gets an education and that the receiver believes that a deviation to getting education signals low ability. Anticipating these beliefs, a low ability worker is made worse-off by deviating because he not only incurs the cost of the education but is then thought of as a bad type as a result. Thus, condition 1. is satisfied. Can we find some alternative belief such that the high-ability worker would like to deviate to getting education? The answer is yes: if the receiver believes that education signals high ability then this deviation is indeed profitable for the high-type. Thus, condition 2 is also satisfied. Since both conditions are satisfied, the intuitive criterion rules-out the implausible pooling equilibrium. | {
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