What are the effects of underwater "windmills"? Most early examples of tidal energy generators used barrages (damns) to generate electricity (La Rance, Kislaya Guba, Annapolis Royal). Meanwhile, a lot of newer projects use rotary blades in the tidal flow (like windmill blades), resulting in an installation that looks like an underwater wind farm. Examples of such projects are: FORCE (Fundy Ocean Research Center for Energy), SeaGen, TidalStream...

While some have claimed that the environmental effects of these new systems are much less severe, the fact is that tidal current technology is relatively new and has been applied in only a few sites. The environmental impacts seem to be based on hypothesis, modeling and simplified lab experiments, but my questions are:

What are these environmental effects?
How much are the local currents affected by these structures?

As you have noted, this technology is new, and so far only small numbers of experimental tidal energy converters (TECs) have been deployed. For this reason, little has been possible in the way of measurement, and so as you note, all estimates are based on models or other means of prediction.

To answer the second question first - how much the currents are affected depends how much of the energy you remove. Since the whole point of TECs is to remove energy from the flow, there must be some effect on currents. Garrett & Cummins (2005) built a simple analytic model of a channel between two large basins (which reflects many tidal energy scenarios) and showed that if all other considerations (e.g. navigation, engineering practicality) are ignored then the maximum power that may be extracted from the channel is obtained when its flow rate is reduced by approximately two thirds. However, this scenario is unlikely to ever be obtained, and the real limit on energy extraction at a given site (if not an economic limit) will probably be determined by what level of environmental effects are deemed to be acceptable. The relationship between power extracted and effect on the flow is not a simple one, and in most cases a significant proportion of the available power could be obtained with a relatively small change to the local currents.

A number of modelling studies have been made of more realistic scenarios for early deployment (e.g. Admadien et al 2012). These typically predict local changes to current speeds of up to 30%, which fade after a few km. Typically there is a decrease in the flow speed in line with the TECs, and an increase to either side of the farm/array, as some of the flow diverts around the added impedence. Effects on residual velocity (i.e. that which is left in the long term when the tidal cycles are averaged out over a period of time), which is relevant to sediment transport processes, are predicted up to at least 15km away.

Some baroclinic modelling (Yang & Wang 2013) has suggested increased mixing, and thus decreased stratification, as a result of the turbulence introduced by TECs. It is also conceivable that in other scenarios, stratification might be increased as a result of reduced flow speeds.

Physical effects, then, are likely to include direct effects on current speed, sediment, and stratification.

The obvious possible biological effect is from collisions. This is not my field, but as I understand it no effect is likely on small fish populations from collisions, although individuals may be affected. Collision risk for large animals (e.g. sharks and marine mammals)  and for diving birds is a topic of active research, and is likely (especially for mammals) to depend on their behaviour around the devices. No large animal collisions have been reported on any of the prototypes undergoing testing so far.

A good review of possible effects on benthic organisms is provided by Shields et al (2011). These may include,

Direct disruption of seabed habitats by physical interference, e.g. from moorings
Disruption of ecological niches: Some organisms have evolved to survive in areas where others cannot - e.g. high current speed environments. Changes in seabed conditions, e.g. from greater or lesser current speeds, may cause them to be out-competed by other species that can then settle there.
Similarly, changes to sediment distribution represent changes to seabed habitats.
Alteration of flow patterns could have implications for species with a dispersive juvenile stage (e.g. larvae that rely on currents to spread) or those that rely on current flow for nutrient or waste transport.


To what extent has the Earth's surface been geologically mapped? To what extent has the Earth's surface been geologically mapped?

For instance, what fraction has been surveyed at 1:10 000, 1:100 000 or 1:1 000 000 scale?

Edit: By 'surface' I mean land surface, not covered by water.
By 'geologically mapped' I mean 'described by a conventional geological map which shows the exposed surface lithologies and their structures'.

As far as I know there are no official data on this. All I can do is give a rough estimate, based upon work and personal interest in about 30 countries, over 40 years.

We can say with certainty that everywhere has been mapped at the 1: million scale although, at this 'reconnaissance scale', at least half of the land surface is only sketchily known, with sparse ground-truth in areas covered by ice, dense forest or drifting sand. In such areas the best detail is probably from geophysics rather than from actual surface mapping.
For example, gravity anomaly by satellite, or aeromagnetics.

At 1:100,000 scale most areas (I'm guessing 80 to 90%) are mapped wherever there is reasonable bedrock exposure. Certainly in Europe, the America's and most of the world's mountain chains you would be hard-pressed to find areas that haven't been mapped at this scale, although the maps are not all available ('commercial, in-confidence', etc.).
The only exception I can think of is parts of the less accessible Himalayas, where the scales of around 1/2 million seem to be the best on offer. That's not to say that there isn't much scope for further mapping at 1:100,000 scale. Rock exposure changes over time, and many of the existing maps could use a more detailed upgrade, especially in respect of microfossil zonation, facies zonation and structural analysis.

1:10,000 scale or even finer detail is only for the 'very interesting' bits, such as ore deposits, mining tenements, reservoirs and engineering site investigations. There are thousands, if not millions of such maps, most of which are not publicly available. Your guess is as good as mine about what percentage of land is covered by such maps, but I would guess <<1%.

PS I forgot to add that most geology students and academics undertake geological mapping for thesis or teaching purposes. Again, they are often not published, and one has to search quite hard for a copy. These scales are usually somewhere between 1:10,000 and 1:100,000


Donation Options for Fossil and Mineral Collection The father of a friend of mine passed away recently. He left behind an extensive collection of fossils and minerals. They include a saber tooth tiger skull, mastodon tooth, shark teeth, tons of trilobites, ammonites and others. Some are in good enough shape to be museum quality and some are probably quite valuable.

His father expressed an interest in making sure that the collection was made available hands-on to those who wanted to view or study them rather than be behind glass. I know that some museums provide hands-on access to some items in their collection.

My friend has been in contact with some museums but could use some help in identifying schools or other programs where they would be used as his father wanted. We are in Indiana in the US. Any suggestions?

I work for an university collection and wo often deal with donations. If you contact a museum or university collection, be prepared for the following questions:

How many fossils are approximately included in the collection?
How were they acquired? Were they bought on fossil fairs? Were they self collected?
What's the weight of the collection? Sounds weird, but this is important for storage calculation.
Is there a list including all specimen?
Are all objects properly identified and labeled? This includes the following information: Find locality, Genus, if possible species, geological time including finer stratigraphy. Examples: Diplodocus carnegie, Upper Jurassic, Morrison Formation, Wyoming or Kallokibotion bajazidi, Upper Cretaceous, Maastrichtian, Sanpetru, Romania. The more information, the better!

Why are these questions so important? If you want the objects to be "hands-on" for example for students studying these objects, they have to be properly identified. No student or researcher can work with "trilobite of unknown age and source". All collections provide hands-on research for those who request it. We do the same. I have a lot of guests coming to our collection wanting to see Triassic ichtyhosaurs, golden ammonites from Opalinus Clay or anything else. In this cases, I prepare all fossils for hands-on access.

In your case, I would recommend the Paleontological Collection of the Indiana University. The collection is quite famous, often visited, and has a very good recommendation all over the world.

See here for their webpage


What is the mean amount (in grams) of dissolved carbon in a ton of sea water? What is the mean amount (in grams) of dissolved carbon in a ton of sea water ?

The value I want is the concentration value in say the first two metres of surface water (A distribution of this concentration by depth would also help)
If I take say the 10% surface area where the CO2 concentration is the highest on the globe, what does it change about the result ?

Please feel free to answer even if your answer is not the complete one but just part of it.
Thanks

This graph at Wikipedia/Wikimedia is part of the answer :

source:

en.wikipedia.org/wiki/Oceanic_carbon_cycle#/media/File:WOA05_GLODAP_pd_DIC_AYool.png
en.wikipedia.org/wiki/File:WOA05_GLODAP_pd_DIC_AYool.png

2.15 mol/m3 at the surface in 1990 seems like a approximative value, which makes approximately 27 g.

Now, the two questions left are:

How did this value evolve since 1990?
Does this concentration change a lot with depth?


Which plant produces the most oxygen? Which plant is the most efficient in making oxygen for it's weight? I want to think it is the greenest plant with more leaves and least                                                                                trunk in full sun?

I just read an article saying that algae produces more oxygen than all the plants in the world!!
Edit: https://www.nps.gov/romo/learn/nature/algae.htm       First paragraph last lines


How to calculate w-wind? My model (AGCM) does not compute w-wind (the vertical component of the wind velocity). However, I do have the following available variables from AGCM output:
u-wind, v-wind, geopotential, omega and some other variables (e.g.,
temperature, specific humidity) at 23 pressure levels.

where u-wind and v-wind are the horizontal components of the wind velocity, geopotential is the potential of Earth's gravity, and omega is the solution to the omega equation.
I would like to compute w-wind by using u-wind, v-wind, geopotential, omega. As suggested by JeopardyTempest in the answer below, I can compute w-wind using the equation w = −ω/ρg  . How reliable it would be if I don't use w = −ωRT/pg?

Omega, ω is closely related to w in meteorology.
It can be moved towards w using the chain (this reminder from Watkins at SJSU helped)...

ω = dp/dt = (dp/dz)(dz/dt) = (dp/dz)w

[where p is pressure, t is time, z is height, and w is vertical motion in height coordinates]

Often in general meteorology you can estimate dp/dz using the hydrostatic approximation, which assumes gravity and buoyancy approximately balance.  This allows the equations:

dp/dz = −ρg

[where ρ is air density and g is gravitational acceleration]

And so would mean you can continue

ω = (dp/dz)w = −ρgw

which results in w = −ω/ρg.

Note that this is not always valid.  As Holton's An Introduction to Dynamic Meteorology, Volume 1 suggests on page 20:

  This condition of hydrostatic balance provides an excellent
  approximation for the vertical dependence of the pressure field in the
  real atmosphere.  Only for intense small-scale systems such as squall
  lines and tornadoes is it necessary to consider departures from
  hydrostatic balance.

That's a loose idea.  It's been a while, but basically as I remember it, hydrostatic is strongly valid when vertical acceleration (and thus vertical motion when looking near the surface) is much smaller in scale compared to the horizontal motions/accelerations.  So indeed, this is not true in tornadoes and in convection.  So you get very correct values for strong thunderstorms.  But it will help you diagnose regions of rising motion.

You can look a bit more towards breaking it down into more advanced detail and the mathematics of hydrostatic and how it fails in Doswell and Markowski's paper on buoyancy.

But, with such limited model output, not sure that you're looking to get into all of that detail, as I don't believe you'll find any extra help going those avenues with the variables you have.

With just slightly more information, most importantly temperature, you could use the ideal gas law:

 p = ρRT

[where R is the ideal gas constant of the gas under consideration and T is temperature in Kelvin]

This would allow you replace the unknown density, and you'd then end up with:

w = −ωRT/pg

Which you could then either further approximate R to be that for dry air = Rd = 287 J/Kkg, or even work up the adjustment your moisture content variable causes (though don't think doing so would offer much useful improvement as you've already got greater errors due to the failings of the hydrostatic approximation).

Since you don't have temperature, it seems your only remaining option would be to plug in the density approximation for the level of interest from the approximated "Standard Atmosphere" here (use the table where ρ is in units of kg/m^3).

It has been a long while since I've done this stuff, but I don't see any better options.  Unless somehow there's a way to work from the geopotential, maybe apply something like mass continuity in pressure coordinates, and get to something more complex to return something more correct for w.  But if there is, it outpaces me!

So, long story short, it appears that your best hope is w = −ωRT/pg, but it won't be 100% reliable, particularly in strong convection.


Using NASA's radiance calculator to make a graph to find the where the peak radiation emission are? I am trying to graphically show the radiance emissions of both the Sun and Earth. I am using NASA's radiance calculator then going to graph it on paper.

Here is what I typed in:

Low wavelength: 0.1μm

high wavelength: 100.0μm

Plot 1:

Temp: 5778k <- for the sun's avg temp

Emissivity: 1.00 <- was unsure what this was for

Plot 2:

Temp: 274.03 <- for the Earth's avg temp

Emissivity:

x-axis scale: log

y-axis scale: log

Here is what that all looks like:

As far as making a plot of the emitted energy of the Sun and the emitted energy of Earth on the same graph does this look correct?
How might one annotate/label the curves to show where the peak emission occurs?

Should the x or y axis be dialed down they are between 0.1 to 100.0 and then the y-axis sale 0.01 to 10.000

I appears that the Suns peak emission is between 0.32 - 10.00microns and the Earth is 10.00microns.

Thank you

The graph looks correct, and after having a look at the NASA/USGS Radiance calculator, it seem that curves can't be annotated. However, below the plot there is a button to download the source data of the plot as a comma separated file (CSV), that you can open in Excel or any spreadsheet manager program and plot it with annotations and all the customization you want.

For the wavelength of the peak in emission you don't have to look further than the box below the graph titled "Wavelength of Maximum Radiance", which for the Sun should say something like 0.52 micrometers as in the screenshot below:


Snowing when the actual temperature is around $\small\sf9^o$ Celsius In Paris, actually, it is snowing smoothly and the temperature, according to many sources, is around $\small\sf{9^o}$ Celsius.

I had always the feeling that snow is possible when temperature approaches $\small\sf0^o$ or below $\small\sf0^o$ Celsius (apparently, it is a huge prejudice)

How is that possible?

The clouds in which snow originates must of course be below the freezing point 0°C = 32°F.

Two factors allow snow to reach the surface when the near-ground air temperature is above freezing.

The time delay --- snowflakes may not have time to melt before reaching the surface if the surface temperature is not much above freezing. Strong downdraft air motion and larger snowflakes helps at this, bringing snowflakes down faster.
Once snowflakes fall below the clouds, they are in air at
less than 100% relative humidity. Sublimation --- evaporation
from solid to vapor --- of some snow on the outer edges of a
snowflake can then occur. This removes energy from the snowflake
overall, so it can help keep the rest of the snowflake a few degrees
cooler than the surrounding air.

The highest near-surface (at the official 5 foot height of instrument shelters) air temperature at which I have ever read of snowflakes reaching the ground is 10°C = 50°F.


Calculating exoatmosphereic flux and figuring out amount of SW Earth will absorb at an albedo of 0.3? Am working through two problems which I am not sure if I am handling correctly.   Here they are:

  a. If the Earth decreases its distance from the sun by 0.03
  Astronomical Units (AU), what will be the resulting exoatmospheric
  flux?
  Recall that at present the exoatmospheric flux = 1,370 $Wm^{-2}$

My solution (use inverse square law):

Inverse-square law:
$S = s_0(r_0/r)^2$

$S = 1370 \; (1\;AU/0.97\;AU)^2 = 1412.37\;W/m^{-2}$

  b. Assuming   an  albedo  of  0.3, how much incoming shortwave energy will
  the   Earth absorb on average in $Wm^{-1}$?
  Recall    that    Energy  absorbed    =   $S/4(1-A) Wm^{-2}$

This confuses me, the difference between $Wm^{-1}$ and $Wm^{-2}$.  What's the differences between Watts per meter to the first and watts per meter squared? Not sure how to combine the two to make the math/metrics come out right.

Going off what I have for part a:

Energy absorbed = $S/4(1-A)Wm^{-2}$
Energy absorbed = $1412.37/4(1-0.3)Wm^{-2} = 247.17$ (is this in $Wm^{-2}$ or $Wm^{-1}$?)

Thank you

Agree with Ash, I think it's a mistake.  Dimensionally:

$$\frac{S}{4}(1−A)$$ is going to have the units of $S$ has because $A$, albedo, is a unitless ratio.

Insolation comes in over an area, so is measured in $W/m^2$ as you've got.  The only way I can see you'd get units of $W/m$ is by multiplying by a distance somewhere, perhaps by integrating its values over a latitudinal distance or something... but that's not really insolation (or exoatmosphereic flux as you call it!) anymore.

No, all values of S should be $W/m^2$.


Is the seafloor of the Gulf of Mexico and Caribbean Sea mostly sedimentary rock? This comes from a related question about predicting oil (or not) in the deep sea, not just the continental shelf. Apparently the deep seafloor is igneous, which lowers its chances of holding oilfields.

Yet the Gulf of Mexico, the deep part, does have at least one oilfield being exploited. This leads me to ask if there's anything unique about the Gulf and Caribbean Sea relative to the "normal" deep seafloor, whatever that is.

Yes and no. Caribbean Sea and Gulf of Mexico have sedimentary materials and rocks at the top, but seabed sensu stricto is composed by sediments and not by rocks.

Oceanic Crust is composed by basalts and gabbros, both igneous rocks as you notice, and sedimentary materials above. We know that because of drillings and fossilized ophiolites:

  Source: earth.s.kanazawa

The Gulf of Mexico is not an exception and has sediments at the top, as said in this paper:

  "Most sediment originates from the adjacent land, primarily via fluvial transport. Direct precipitation of calcium carbonate and evaporate minerals takes place primarily on the Florida and Yucatán platforms and some coastal lagoons. The deep environments tend to be dominated by mud in a combination of terrigenous and biogenic sediments." (Davids Jr, R.A.,2017) 1

Caribbean Sea is neither an exception, as remarks Encyclopedia Britannica:

  "The ancient sediments overlying the seafloor of the Caribbean, as well as of the Gulf of Mexico, are about a half mile (about one kilometre) in thickness, with the upper strata representing sediments from the Mesozoic and Cenozoic eras (from about 252 million years ago to the present) and the lower strata presumably representing sediments of the Paleozoic and Mesozoic eras (from about 541 to 66 million years ago)" (Menzies,R.J. et al, 2019) 2

This article says there is one kilometre of sedimentary materials, enough to diagenesis to take place and so, there are sedimentary rocks under the unconsolided sediments.

At open oceans diagenesis may have still do not have place and there is only the ophiolitic sequence with thin sediments, but, as you can read in this related answer, Caribean Sea have old igneous rocks with a big stack of sediments above that have litified to sedimentary rocks (coming from continental rivers and carbonaceous platforms), becoming the biggest oil source zone on Earth after Arabian Gulf.

1) Davis R.A. (2017) "Sediments of the Gulf of Mexico." In: Ward C. (eds) Habitats and Biota of the Gulf of Mexico: Before the Deepwater Horizon Oil Spill. Springer, New York, NY

2) Menzies, R.J., Ogden, J.C. (2019) "Caribbean Sea." In: Encyclopedia Britannica.


Siberia (Beringia) during last glacier period Today, Northern Russia is considered quite cold. It is not populated because it has low biological productivity: low light and cold. It can be below -50 at winter and 10 C at summer peak. It is low populated because there is basically nothing to eat. Poor reindeers dig some moss from below the snow, which grows there a little during short summer period.

I am telling all of that, because I cannot even imagine what the hell Yakutiya would have been when all north of the continent was 1 km below the ice! Siberia stays permafrost even now, when you have warm summer days +5 Celsius. Certainly, it had to be frozen at glacier time permanently. Meantime, it is curious how people managed to cross the Bering Land Bridge during that latest glacier. How was that possible at that time, was it a Polar Voyage?

Now, I read that Yakutiya was a wild life refugee during the last ice age. It had much more rich wild life than now, comparative to what we have in Africa today. People appeared there 12 000 years ago, when half of the Great Glacier was melted

and slaughtered mammoth. The mega-fauna extinction collapsed Siberian ecosystem, say scientists who undertake an attempt to recover it. They tell that mammoth grazed grass. Did they procure it from below the snow, like reindeer do? Ok, Beringia says that

  During the ice ages, Beringia, like most of Siberia and all of north
  and northeast China, was not glaciated because snowfall was very
  light. It was a grassland steppe, including the land bridge, that
  stretched for hundreds of kilometres into the continents on either
  side.

Fine. But, the fact that all ice was concentrated over Europe and Canada does not mean that Beringia surface was not permanently frozen. If there is so cold that whole Europe is under 1 km of Ice, the cold Beringia must be frozen either, despite all ice is over Europe. We can have low temperatures without ice, right? And, if it was terribly cold in Europe, it must be at least that cold in Beringia, right? So, what could poor animals eat there, in the frozen desert?

To summarize, my question is:

How is this nonsense possible that the 'freezer' of our planet was flourishing, whereas the rest of the Planet was starving in frozen state during last glacier period?

PS! I was suggested to look at the climate zones during Last Glacier. I have seen it but I cannot believe that northern Canada and Greenland was thriving forest whereas almost whole US was under permanent ice sheet. Similarly, we see that Tundra (current Eastern Siberia state) ruled in permafrozen Beijing whereas Eastern Siberia was enjoing same thriving steppe Forest that Alaska did

That is unbelievable and that is what I basically ask to explain. I guess that I have just mismapped the colors. There are too few colors. On the other hand, if anomaly did exist, I want to hear that explicitly, desirably with explanation. Is it really large animals who maintained the warmth at the very north (see "Park" above) before people exterminated them or what?

PS2 I have revised the climate zones map and realized that we had Polar Desert in Siberia and Alaska rather than Forest Steppe. This makes more sense. But how did we had the large animals and thriving greenlands there anyway? Polar Desert is incompatible with green land, obviously. Polar Desert at zones map seems also to contradict the Beringia article, attributing Beringia to grassland steppe during Ice Age.

This paper basically summarizes the thinking behind the highly-productive mammoth step theory:

Zimov, S.A., Zimov, N.S., Tikhonov, A.N., Chapin, F.S., 2012. Mammoth steppe: a high-productivity phenomenon. Quaternary Science Reviews 57, 26–45. https://doi.org/10.1016/j.quascirev.2012.10.005

The idea is basically that a large herbivore density (comparable to modern day intact African Savannah ecosystems) kept soil moisture low and reduced permafrost temperatures. This was favoring gramminoids (grass like plants) that are much more productive than the present day prevailing mosses. Plant productivity in these regions is usually limited by nitrogen (other ecosystems are carbon limited). The higher than today herbivore population provided a lot of nitrogen input from dung and urine propelled the productivity of plants, which in turn allowed for more herbivores. Low temperatures and shorter summers also limited damage to gramminoids, thus they could grow faster in the next season. The ecosystem had several positive feedback mechanisms sustaining high productivity.

When humans arrived in the region, they hunted all the herbivores. The system collapsed and lost its productivity.


What is the height above ground of NCEP/NCAR Reanalysis 1 variables? I am confused regarding the height above ground of this data set. http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html

I have to reprocess this data set and need to know this piece of information.

The height coordinate varies and is described for the various variables on the site you link.  For example:

  Levels:

  Surface or near the surface (.995 sigma level), or entire atmosphere (eatm)

and

  Levels:

  17 Pressure levels (mb): 1000,925,850,700,600,500,400,300,250,200,150,100,70,50,30,20,10
  Some variables have less: omega (to 100mb) and Humidities (to 300mb).

For the pressure levels coordinate the height above the ground is variable and dependent on atmospheric conditions.  The sigma levels are terrain following and flatten out with height and will vary with local topography. (see this for more info)

You can estimate height using the hypsometric equation, or use the geopotential field from the data to determine the height at specific grid locations (it will vary horizontally).  For processing, I would recommend using the geopotential data since it is available for each of the vertical coordinate possibilities in this dataset.


Was the filling of the Three Gorges Dam's impact on the Earth's rotation rate detectable? I'm a big fan of Vsauce and the video How Earth Moves is just one example of science related to the Earth available there (there's plenty more).
But the statement in this video starting at 01:39 strikes me as a little surprising.

You know how a figure skater, spinning in place, can slow down their (rotational) speed by extending their arms out; by moving some of their body mass away from the middle of their body? Well the same thing can happen to Earth.
The Three Gorges Dam did exactly what a figure skater does when they move their arms away from their center. It transferred thirty nine trillion kilograms of water one hundred and seventy five meters above sea level.
NASA calculated that that massive amount of water moved, caused Earth’s rotation to slow down, so that every day of your life since that dam was finished, has been longer by 0.06 microseconds. (emphasis added)

If I calculated correctly, that would be a "slip" of about 1 centimeter per year at the equator.
There are many causes of change to the Earth's distribution of mass, it's moment of inertia, and therefore the length of a day. Tidal forces from the Moon can contribute substantially to changes in Earth's rotation as well, and there could be short term geological effects as well.
Is this roughly 0.06 microsecond/day shift well below the "noise" level, where in this case I'm using "noise" to refer to effects that are smaller than the error that can be modeled? So for example exchanges between sea ice and sea water can be measured and modeled to some level of accuracy, and the Moon's tidal effects can be measured and modeled to great precision, so even if the Three Gorges Dam effect turns out to be small compared to those, it could still be detected as long as its effect is larger than the error that those effects can be accurately calculated.

below: Screen shots from the Vsauce YouTube video Water.

The .06 microseconds figure doesn't sound accurate

The moment of inertia for a solid sphere is $$I = \frac{2mr^2}{5},$$ where $m$ is the mass of the sphere and $r$ is its radius. Applied to the Earth, this gives us $$I = \frac{2\cdot5.972\times10^{24} \text{ kg} (6371000 \text{ m})^2}{5} = 9.70\times10^{37} \text{ km-m}^2.$$

A simplified estimate of moment of inertia for the water in the three gorges dam is $mr^2$. The change in moment of inertia from raising that water by 175m can be calculated by $$\begin{align}\Delta I &= m_{water}\left[(r_{earth}+175)^2-r_{earth}^2\right] \\ &= 39\times10^{12}\left(6371175^2-6371000^2\right) \\&= 8.69\times10^{22} \text{ km-m}^2.\end{align}$$

Assuming that the angular momentum ($L$) of the Earth stays constant, then since $L=I\omega$, $$I_{before}\omega_{before} = I_{after}\omega_{after},$$ or $$\omega_{after}=\omega_{before}\frac{I_{before}}{I_{after}}.$$ The angular velocity, $\omega$, of the Earth (before) is $7.292\times10^{-5} \text{ radians/s}$. The discrepancy between the before and after moment of inertia is so insignificant that I would be wary of floating point error on this calculation, but using Python2 I calculate the delta in rotational speed to be $6.776\times10^{-20} \text{radians/s}$. For a rotation of the Earth this means the rotation will take about 80 picoseconds longer, or 0.00008 milliseconds.

Conclusion

The statements made by VSauce don't match up with Physics 101. I am not inclined to believe this claim unless there is some other explanation or compelling evidence. Given that the magnitude of the change in rotational speed is three orders of magnitude less than he claims, and that 80 picoseconds is about 1/4 of the time it would take your computer to add 2 integers, I think it is safe to say that this is not measurable.


How did helium deposits form? I always wondered, because helium gas is so lightweight, how did it get into the deposits like the ones that people tap? Is it formed underground, or was it trapped?

Helium (He) is formed by radioactive decay of uranium (U) and thorium (Th) to lead (Pb).

There's another name for the nucleus of a 4He atom: It's an alpha particle. Alpha decay is one of the pathways by which radioactive elements eventually decay into non-radioactive elements. For example, 238U decays to 234Th via alpha decay. There are three key isotopes that lead to the generation of helium: 238U, 235U, and 232Th. Each of these is a "primordial" radioactive isotope. they were present when the Earth formed, and their half-lives are sufficiently long so that a good fraction of that original content has not yet decayed.

So how much helium does the decay of these isotopes produce?

238U eventually decays to 206Pb via the uranium decay series (aka radium series). This decay process ultimately produces eight alpha particles.
235U eventually decays to 207Pb via the actinium decay series. This decay process. This decay process ultimately produces seven alpha particles.
232Th eventually decays to 208Pb via the thorium decay series. This process ultimately produces six alpha particles.

After the alpha decay, the parent nuclide suddenly has an excess of two electrons orbiting it. The alpha particle has a deficit of two electrons. The alpha particle quickly grabs those two excess electrons and becomes a helium atom.

Most of the helium deposits in the Earth are thought to be the result of alpha decay. For example, see Helium Facts: Where Helium is Found.


Do I need a permit for putting retrievable sensors in ocean (Atlantic)? The project would be to place about a dozen sensors in "swarm" fashion and have them collect data from the Atlantic ocean.

The sensors would be about the size of a softball and would be retrieved days later after data is collected.

I've been looking at regulations and permits needed for this but only find construction based permits for building homes or factories near the ocean.

Is there a permit I need to pull for this or a body who regulates this kind of research?

And would a different permit be needed if the sensors weren't meant/or unable to be retrieved?

It depends where you live as laws verry from country to country. Just ask your local lawyer or law enforcement official. It may also depend on what data you are recording


Plotting a mineral stability diagram I'm trying to figure out how to plot the stability of 4 minerals — $\ce{CaCO3}$, $\ce{CaSO4}$, $\ce{CaMg(CO3)2}$, and $\ce{CaMg3(CO3)4}$ — with axes of $\mathrm{ln}\,a_{\ce{Mg^{+2}}}$ vs $\mathrm{ln}\,a_{\ce{SO_4^{-2}}}$  where $\mathrm{ln}\,a$ is the natural log of the activities of each ionic species. The activity of carbonate is a given.

I found solubility $K$ for each dissolution, based on the Gibbs free energy, $R$, and $T = 25^\circ \mathrm{C}$, then tried to set up equations to plot the following:

$\mathrm{ln}\,K_{\ce{sp,\,CaCO3}} = \mathrm{ln}\,a_{\ce{Ca}} + \mathrm{ln}\,a_{\ce{CO_3}}$
$\mathrm{ln}\,K_{\ce{sp,\,CaSO4}} = \mathrm{ln}\,a_{\ce{Ca}} + \mathrm{ln}\,a_{\ce{SO_4}}$
$\mathrm{ln}\,K_{\ce{sp,\,CaMg(CO_3)_2}} = \mathrm{ln}\,a_{\ce{Ca}} + \mathrm{ln}\,a_{\ce{Mg}} + 2\,\mathrm{ln}\,a_{\ce{CO_3}}$
$\mathrm{ln}\,K_{\ce{sp,\,CaMg_3(CO_3)_4}} = \mathrm{ln}\,a_{\ce{Ca}} + 3\,\mathrm{ln}\,a_{\ce{Mg}} + 4\,\mathrm{ln}\,a_{\ce{CO_3}}$

Based on the thermo data each $\mathrm{ln}\,K$ is a constant (since $T$, $P$ are constant), since the activity of carbonate is given it seems like I should be able to solve equation 1 for the activity of calcium, but then each equation is a point not a line.

I'm confused as to how each of the equations is dependent on $\ce{SO_4}$ at all (which is supposed to be my y axis), and how to relate everything. I tried to treat activity $\ce{Ca}$ as variable, then solve for it in terms of activity $\ce{SO_4}$, to then make equations 3, 4 dependent on $\ce{SO_4}$, and $\ce{Mg}$, but then I had no idea how to treat equations 1 and 2. Also the values I got for that were really big, the lines were on $y=35+3x$ and $y=18+x$.

This question is based on a question for a textbook, which I'm trying to use to study for an upcoming exam (1 week), so any help would be greatly appreciated.

Remember that you are concerned about stability fields. The lines on your stability diagram are the places where two minerals are in equilibrium. One one side one mineral will be more stable, on the other side the other one will be.

Let's talk about $\ce{Ca}$ first. The reactions all have the same $\ce{Ca}$ so it's activity isn't a factor in their relative stability. If you write a combined reaction for any mineral to another, it will cancel out. Try it. Calcite stability has nothing to do with $\ce{Mg}$ or $\ce{SO4}$ it's stability will only depend on $\ce{CO3}$ and $\ce{Ca}$. But wait, you know $\mathrm{ln}\,a_{\ce{CO3}}$ so you can calculate $\mathrm{ln}\,a_\ce{Ca}$ in equilibrium with calcite. So you can use that to help figure out where calcite is the most stable.

You only have one equation where sulfate is included. What direction will that stability boundary take on your diagram? Say you want to divide your diagram space into areas of calcite vs gypsum stability, how could you combine the two solubility equations to come up with the conversion of one mineral to the other? What dissolved species activities enter into the equation (hint: you know one of the two). Given that you can figure out the other one and plot that on you diagram.

Can you follow a similar process for calcite and dolomite (hint: yes)? What about calcite and huntite? Is there any part of the diagram where huntite is more stable than dolomite (or vice versa)?

Now figure out the stability relationship between dolomite and gypsum. What angle will the line take on your plot? Figure out where it will occur by combining reactions. This is a bit trickier but the $\mathrm{ln}\,a$ values will depend on the $\mathrm{ln}\,K$ for the combined reaction.

Now erase any lines in a field where those two minerals are less stable than anything else.

Good luck and let us know if you get stuck. I didn't want to just give you the answer because I'd have to actually work it out.


In the USA, are pesticides regulated differently for animal vs human consumption? I'm looking into growing edible oyster mushrooms. I'm considering using whole-oat horse feed in the growing process. I realize that additional steps are taken to ensure food is fit for human consumption, but for my purposes, I'd be fine with having a few dead bugs in the mix. However, I am wondering if there could be additional chemicals in horse oats over human oats. In the USA, are pesticides regulated differently for animal vs human consumption? Anything else I should be concerned about?

I'm not familiar with pesticide legislation in the United States, but I do know that all oats are grown with human consumption in mind. Only the lower grades are used for animal feed. In the rare event that high grade oats become contaminated with insect pests, they would also be consigned to animal feed. Animals are less fastidious than humans, and don't mind a few beetles in their oats. It is highly improbable that your animal feed would be contaminated with pesticides, except to the extent that all non-organic farm produce has minute traces of it.

Most mushroom growers in UK use horse manure or stable muck as a substrate for growing their crop. The manure has to be suitably matured and not fresh. I would imagine it is cheaper than animal grade oats.


Is Mars' mantle homogeneous or heterogeneous and what might this say about mantle convection? A supervisor of mine made an off-hand comment about the fact that Mars used to have plate tectonics, and there is evidence that Mars used to have plate tectonics, however now it does not. The theory that she mentioned was that Mars is either heterogenoeous or homogeneous, however I have forgotten which one she said! Now I can't find anything online which says which one it is! Can anyone help me out here?

Also, the follow on from this was that this led to the ceasing of mantle convection which ended the possibilities of plate tectonics? Could somebody more knowledgable than I perhaps outline a theory of why this is?

In addition if mantle convection has ceased, than when do we think it stopped being active?

The mantle of Mars is heterogeneous. While there are many things that are unknown about the Martian mantle and the entire internal structure of Mars, this is not one of them. The source of this finding comes from the analysis of meteorites originating from Mars.

Analysis shows that 4.5 billion years ago - when Mars was forming along with the rest of the solar system - two distinct "reservoirs" formed in the mantle - the "Enriched Reservoir" (ER) and the "Depleted Reservoir" (DR). The ER had high ratios of lanthanum/ytterbium and $^{87}$Sr/$^{86}$Sr, and a low ratio of samarium/neodymium. The DR had the opposite ratios.

Eventually, meteorites known as shergottites formed, mixtures of both of these reservoirs. A graph (which I'd insert here, but I'm unsure of the copyright on it) entitled "Heterogeneous Martian Mantle" illustrates how these meteorites have a wide range of composition compared to rocks from Earth. This provides compelling evidence for a heterogeneous Martian mantle.

At the moment, I can't find enough information to answer the follow-up part, though the convection certainly did stop.

Sources:

PSRD article 1

PSRD article 2


Why are most peninsulas oriented north-south? Weird question, bear with me.
Most large peninsula on Earth are oriented southwards (and most of the rest northwards), much fewer east or west. Some examples:
Florida, Californa peninsula, Yucatan, South America, Antarctic peninsula, India, Kamtjatka, Korea, Malaysia, Scandinavia, Italy, Greece, Greenland. Contrary examples would be east Siberia, Kola, Turkey. (Is it just me, or have this question ever been raised before in history?)

Is this just coincidence, or could conceivably some tectonic, gravitational or rotational forces help cause this phenomena? Maybe because the tectonic ridges in the Atlantic and the Pacific are north-south oriented and stretch almost from pole to pole? And could that in turn be caused by the dynamics of the inner Earth?

Elevation maps of Venus are more fragmented than the continents of Earth. And Mars is half highland half lowland. Neither has plate tectonics like Earth.

Starting from the bottom of your question I would not compare planets with plate tectonics with planets that don't have this process. During rifting of a tectonic plate a triple junction will be the most favourable way of faulting the crust. After that two arms of the rift are favoured and the third one becomes a failed rift (aulacogen). Because of this process linear features on a planet will be very emphasized.

The current geometry of mid ocean ridges and subduction zones does fall into a strong north-south pattern (Exceptions are Antarctica and the Aleutian trench). This might also be due to the fact that the plates form a system that in all its complexity still strives for lower stress and effective heat transport. In this case a parallel system of mid-ocean-ridges and subduction zones might be more favourable than a totally random alignment of those features.

The dynamics of the inner Earth are of course not independent of the plates and it is difficult to assign causality. Nonetheless you could again think of a complex system that seeks for efficiency (pragmatism if you will). With subduction plates being plate-shaped, circulation above and beneath them will probably be more stable in a cylindrical fashion. So the heat distribution on the upwelling side of the circulation should also create linear features. In any case we just happen to live in a time where the system went for a North-South oriented minimum.

Continuing this thought process we can also find an explanation why features in this tectonic-north-south part of the world are oriented in this way. Many north-south striking islands have north-south striking back-arc-basins that separate them from their main land masses. And peninsulas are sometimes the arms of failed rifts or the tapering of back-arc-basins. Outside of this prevalent orientation we see features like Nowaja Semlja with very different orientations.

But be cautious with this theory. Assigning each feature on Earth to a north-south and east-west orientation gives you a 50 % chance that a randomly aligned feature will confirm your theory. Even four directions still give you 25 % chance. So the more directions you consider the harder it is to prove the theory. I am not aware of anybody doing a global peninsula inventory, so it would be interesting to see this.


How would a Mars rover identify a microbialite fossil? I recently read of a company in Canada using robotic technology to study freshwater microbialites. Their claim is that greater understanding of terrestrial microbialites could help in the search for extraterrestrial microbialite fossils (if they exist) on Mars.

If a rover encountered a microbialite fossil on Mars, how would it identify the rock as being a fossil?

I'm basing this answer off an article: Kevin Lepot, Karim Benzerara, Gordon E. Brown Jr., Pascal Philippot (2008). 'Microbially influenced formation of 2.7 billion-year-old stromatolites'. Nature Geoscience Vol.1 No.2, pp.118–121. DOI: http://dx.doi.org/10.1038/ngeo107

Instruments that are desirable (and are actually used) to study stromatolites/freshwater microbialites:

Raman microspectroscope
Confocal laser scanning microscope
Scanning transmission X-ray microscope
Transmission electron microscope (TEM), including high-resolution TEM
Near-edge X-ray absorption fine structure spectroscope
The usual sets of fine analytical chemistry and microcutting tools

As you might suspect, this may require rather massive, power-hungry and bulky rovers. There is a solution and it's called sample return. Although the tools and instruments are constantly evolving and undergoing miniaturization, a skilled and attentive lab researcher will find out more back here on Earth than a rover on Mars.

Another consideration in favor of sample return is the verifiability of possibly sensational results - the sample will be curated and subdivided between universities and labs worldwide, applying diverse techniques and not relying on a single set of ultra-costly but possibly systematically biased hardware.


Why high tides occur simultaneously on opposite sides of the Earth? Most explanations for high tides say high tides come from water being attracted by the moon (2/3) and the sun (1/3). Attraction occurring in the direction of the moon is visible on the side close to the moon below:

Source: This question
However why is water also moved away on the opposite side of earth?
And moreover, why is this high tide on the opposite side also occurring when moon and sun are in conjunction and therefore nothing can attract water on the far side?

Source

First of all, tides are not as simple as the "two-bulge" simplification. In reality, the diagram shown is misleading. The two bulges appear assuming an ocean of constant depth covers the entire surface of Earth. Clearly that is not the case and in the diagram you can see the continents. Considering the different sizes of the basins and the distinct frictional characteristics in each location, the resulting tidal effect is much more complex. The difference in phase and amplitude is shown here and it clearly shows that the the tide varies for the same longitude. That wouldn't be the case in the simple explanation above.

Source Wikipedia.
Looking at this tidal animation from TPXO is also illustrative.

The simple "two-buldge" explanation would result in a pure two peak daily tide. That is certainly not the case in places like the Gulf of Mexico.
As mentioned in Camilo Rada's answer, the bulges are a consequence of the tidal force. This  apparent force result from the difference in strength in the gravitational field. The result is that Earth's body is stretched toward and away from the center of mass of the Earth-Moon system. The water thus adjust to this difference in geopotential giving rise to the tides.
A more intuitive explanation is given in Project Earth Science: Physical Oceanography

The explanation of the two-bulge tide comes from the fact that the Moon and Earth form a two-body system that rotates about an axis located within Earth.
The bulge of water on the side of Earth that faces the Moon is easily explained. It is due to the gravitational attraction between the Moon and Earth, including the water on Earth. This attraction pulls water toward the Moon and creates a “bulge” on the surface of Earth. The bulge on the other side of Earth is due to inertia. Inertia is the tendency of an object at rest to stay at rest and the tendency of a body in motion to continue its motion in a straight line.
There is an inertial tendency resulting from the rotation of the Earth-Moon system for objects (water among them) to move away from both sides of Earth—the side facing toward the Moon and the side facing away. The model demonstrates that the effect of things moving away from Earth is much greater on the side facing away from the Moon.
Many textbooks and other sources use the concept of “centrifugal force”— which is actually a preconception—to explain the effects of inertia. According to this preconception, there is a force that acts on all objects that are in circular motion, and this force pushes or pulls the object out from the circle. There is no such force. The preconception arises from our own experience with circular motion.

The gravitational forces of Earth, Sun and Moon cause a bulge of water on the nearest side and an equal bulge on the other side. Thus, in this simple scenario, the tide is composed of two bulges of water (four, in fact), traveling around the world as the world spins. When Moon and Sun aligned, their respective bulges add together to form "spring tides" every two weeks. When the Moon and Sun are at right angles, we encounter "neap tides", as the bulge of the sun adds to the low lunar tide, resulting in higher low tides but lower high tides.
The limitations of this model are:

It cannot explain that there are places without tides, with one daily high, and most with two tidal highs each day.
Tidal height is not maximal at the Equator (and minimal at the poles) as the simplification suggests.
High tide is not associated with the position of the Moon. It occurs at different times of the lunar cycle depending on the location.
If continents are included, the tidal wave would reflect off the continental shelf as it reaches a continent. A tidal wave of almost equal magnitude will be propagating in the opposite direction, which is not observed.
The tidal waves required for this model would have to travel at much faster speeds that are possible in reality.

In reality, the tides instead of running east to west as Earth rotates, tidal waves propagate around in circles around islands, and certain points in the sea, called tidal nodes or amphidromic points. These nodes can be seen in the first figure from this answer.
Thus, the tidal patterns in the ocean are a set of rotating standing waves. These waves have periods that represent the natural resonance periods of the ocean basins. These waves can be considered modes of "vibration" and can be decomposed using a Fourier decomposition. That is the source of the different tidal constituents that are used currently for tidal prediction.


What geological processes affect the speed of Kilauea lava flows? The recent lava flows from the current eruption from the Kilauea volcano in Hawaii have generated a considerable amount of interest and concern for property.

In the article slow-motion disaster as lava nears main Hawaiian road, USA Today suggests that the variable speed of the lava flow has caused considerable concern and uncertainty:

  "It's like slow torture," said Paul Utes, who owns the Black Rock Cafe in Pahoa. "It speeds up, it slows down, it speeds up, it slows down. It's not like any other event where it comes and goes ... and you can move on."

  Predicting the future speed of the lava flow simply isn't possible,
  geologists say, because the advance rate is extremely variable and
  depends on how much lava the volcano produces.

Despite that evasion, it seems there must be more to the story...
What geological processes affect lava flow speed from the Kilauea volcano?

The speed of the lava flow from Kilauea depends on a few things:

The amount of activity at the source of lava.  This flow you are talking about is coming from a vent that is 20 miles from the Kilauea caldera.  So, it is more like a leak, than a true eruption.  There's not a lot of force behind these types of events and thus not a lot of lava coming out.
The viscosity of the lava.  This is basalt, so it can move pretty quick, for lava.
The terrain.  The source of the lava is not near the summit, but rather far down the mountain at only 700 ft elevation.   The surrounding terrain has a very shallow slope, which doesn't allow the lava to move downhill quickly.
The cooling rate of the lava.  It rains a lot in Pahoa.  Especially in late October.  This helps to cool and harden the lava, which slows the movement.
Barriers and surface characteristics.  There are a lot of natural contours in the land, trees, and brush in the Pahoa area.  This helps to slow down the movement.

Having grown up on the Big Island, I can say from experience that these flows that come out of side vents are often slow enough to walk right up to and play with.  They don't move fast at all.  The reason it's such a big news event is because it nearly covered a main highway, which would've cut off a lot of people from the main town, Hilo.

The fast-moving lava flows usually come from the main caldera, where the amount of lava erupting is far greater.  Also, Kilauea is really just a pimple on the side of Mauna Loa.  So, it is a bit of a release valve for the big volcano.  Shouldn't be too long before Mauna Loa erupts again and then you will see a lot of fast moving lava from a 13,000+ ft location.


Is Mount Everest currently becoming taller or shorter every year? Is Mount Everest becoming taller or shorter every year? By how much?

I would like an explanation in terms of tectonic plate movement if possible.

Source: IamKatieHoffman.com

According to one survey using GPS mounted on a plateau below the summit, Mount Everest is increasing its height approximately 4mm each year.

As for plate techtonics, this site describes it pretty simply:

  Much like a car crashing head-on with a truck, crumpling occurred when the two continents met and the result was the creation of the Himalayan Mountains. Although the northward drift of India slowed dramatically with initial collision, the two continents have continued to converge as India slides under Asia.


Would a volcanic winter trigger an oceanic "spring"? There is large amounts of observations supporting the effects of volcanic eruptions on climate: A long term subtle warming effect due to $\small\mathsf{CO_2}$ and a short term, but more intense cooling effect due ash and sulfur gases.
There is also much evidence supporting long and dramatic volcano triggered
 cooling events dubbed "volcanic winters", like the one after the 1815 eruption of Mount Tambora, or the Toba catastrophe 74,000 years ago, perhaps lasting up to a decade.

For more recent volcanic eruptions, there is evidence of algae blooms produced by the input of nutrients from volcanic eruptions (1, 2). Perhaps an expected outcome given that the ecosystems at the surface of the ocean are nutrient limited, therefore wasting most of the available energy due to the lack of iron, nitrogen and phosphorous mainly.

For all the above, I would be inclined to think that a large volcanic winter would encompass a period of prosperity for oceanic life. Where a small decrease in solar irradiation is more than compensated by a large increase in nutrients availability. However, I haven't hear of evidence of such oceanic boom.

Are there observations suggesting or disproving such oceanic booms during volcanic winter events?

PS: I started pondering this question while thinking about this article, on which they found that a coastal community in South Africa didn't seem to have been affected by the Toba "catastrophe", but on the contrary, they seem to have benefited from it.

I can't answer the question in detail but maybe take some of the tension, addressing the so called bottleneck hypothesis.

It was never really supported by archaeology, the mentioned article just adds to the evidence that human variety didn't suffer that greatly from the Toba eruption as the bottleneck hypothesis might suggest and that there was no decade long volcanic winter.

Example from lake Malawi after the eruption:
https://www.sciencedirect.com/science/article/abs/pii/S0047248417302750?via%3Dihub

Southe Africa:
https://www.nature.com/articles/nature25967

India:
https://science.sciencemag.org/content/317/5834/114

And to my knowledge no extinction signal in Neandertal population (they were few anyway) during oxygen isotope stage 4 in Europe.

Some think that it is time to put the bottleneck hypothesis ad acta.

There is evidence that eruptions "fuel" phytoplankton, but not necessarily connected to volcanic winters.


SST and air temperature differences I'm trying to understand what causes the sea-surface temperature (in the open ocean and coastal areas) and the air temperature immediately above it to be different. I know very little about the subject, as I have not taken a class on boundary layers. I would think that in the winter, air is colder than the ocean (and vice-versa in the summer) because the ocean has a higher heat capacity than air. Am I right? What other factors contribute to the differences between SST and local air temperature, and are there notable exceptions to what I say above?

If there are relevant papers or texts on this topic, please let me know...

You're right. Water has a much higher specific heat capacity than air, so will change temperature much more slowly. This means that,

the sea surface tends not to reach the same extremes as the air (hence the seasonal effect that you mentioned)
the sea tends to lag behind the air in seasonal changes
the air readily heats and cools with day and night, while the sea does this much less; so which one is warmer can vary on a diurnal as well as a seasonal basis.

All this can then be greatly complicated by sea and air currents, especially vertical circulation in the ocean - but you have the right idea.


How much of one day can be considered nighttime, on average? What is the split between night and day on Earth on average?

To further clarify, assume we are on the equator, I want to know how long a time, as a percentage, you could consider to be nighttime on Earth, with the points in time separating night and day being within sunrise and sunset.

I tried to answer for myself with many a time spent on Google, but I came up lacking so I am here to ask for help from you intelligent people that roam these parts of the internet.

I hope this question fits in Earth Science, I couldn't think of a better place to ask.

To further clarify, assume we are on the equator, I want to know how long a time, as a percentage, you could consider to be nighttime on Earth, with the points in time separating night and day being within sunrise and sunset.

I personally wouldn't consider that the dividing line between night and day. I consider night to be the period between dusk and dawn rather than sunset and sunrise. One doesn't need to stop playing a ball game on an unlit field just because the Sun has set. There's still another twenty to forty minutes of light from the blue sky. Dusk and dawn last the shortest in the tropics, with civil twilight (e.g., the period when one can continue playing a ball game on an unlit field) generally considered to be the 24 minute period prior to sunrise / after sunset in the tropics. With this, "nighttime" comprises about 47% of the time in the tropics.

The question however asks about the period between sunset and sunrise. Even on an airless planet, "nighttime" by this definition will comprise a bit less than half the time at the equator because sunrise and sunset occur when the top of the Sun appears above / drops below the horizon. Since the Sun's angular diameter is about 32 arc minutes, on average, this alone means that daytime is about two minutes longer than nighttime+twilight at the equator. We don't live on an airless planet. Atmospheric refraction means that when we see the very top of the Sun at the horizon, the Sun is geometrically well below the horizon.

This, coupled with the Sun's angular size, makes daytime last about 12 hours and 6 to 8 minutes at the equator, with very little variation over the course of a year. Taking 12 hours and 7 minutes as the average, daytime represents 50.5% of the time while twilight plus nighttime (or just night, per the question) represents the other 49.5% of the time.


In the 800 kyr $\ce{CO_2}$ record, how detectable would a short-term peak be? It is stated in https://cdiac.ess-dive.lbl.gov/trends/co2/ice_core_co2.html that the temporal uncertainty for the glacier $\small\mathsf{CO_2}$ records is less than $\small\mathsf{5~\%}$ at any given moment. That means ‒ for example ‒ ca. $\small\mathsf{5,\!000}$ years at $\small\mathsf{100,\!000}$ years ago.  Could there have been peaks in the $\small\mathsf{CO_2}$ record that rose and fell back so quickly we wouldn't be able to detect them? Is there some paper or perhaps a monography that lists all the possible events in Earth's history that could have occurred in the last $\small\mathsf{800,\!000}$ or more years, which would lead to a likewise fast $\small\mathsf{CO_2}$ increase as today's, but also showing that, on the other hand, any such events would leave a long-lasting trace in the $\small\mathsf{CO_2}$ that should be easily detectable in the record?

I've found out that part of the answer is already in answers to this question.

$\small\mathsf{CO_2}$ does not fall back quickly:

  graph shows the IPCC 2007 $\small\mathsf{CO_2}$ liftime from a computer model run that shows what happens to a single instantaneous pulse of $\small\mathsf{CO_2}$ emitted to the atmosphere. One thousand years after the $\small\mathsf{CO_2}$ is emitted to the atmosphere 20% is still there according to this model.
  only zero carbon

The decline shown is mostly due to $\small\mathsf{CO_2}$ dissolving in the oceans.


Does the Composition of Rock Near the Epicenter Affect the Effects of an Earthquake? Aside from the earthquake triggering a quick clay landslide, is there anything else notable?

The soil and rock type are one factor that determines the amplification of the shaking.  In general, the softer the underlying material is the greater the shaking.  This table, from the USGS, presents the velocities of 5 soil types in the San Francisco Bay area and their shear velocities.

  Soil type A   Vs > 1500 m/sec Includes unweathered intrusive igneous
  rock. Occurs infrequently in the bay area. We consider it with type B
  (both A and B are represented by the color blue on the map). Soil
  types A and B do not contribute greatly to shaking amplification.

  Soil type B   1500 m/sec > Vs > 750 m/sec Includes volcanics, most
  Mesozoic bedrock, and some Franciscan bedrock. (Mesozoic rocks are
  between 245 and 64 million years old. The Franciscan Complex is a
  Mesozoic unit that is common in the Bay Area.)

  Soil Type C   750 m/sec > Vs > 350 m/sec  Includes some Quaternary (less
  than 1.8 million years old) sands, sandstones and mudstones, some
  Upper Tertiary (1.8 to 24 million years old) sandstones, mudstones and
  limestone, some Lower Tertiary (24 to 64 million years old) mudstones
  and sandstones, and Franciscan melange and serpentinite.

  Soil Type D   350 m/sec > Vs > 200 m/sec  Includes some Quaternary muds,
  sands, gravels, silts and mud. Significant amplification of shaking by
  these soils is generally expected.

  Soil Type E   200 m/sec > Vs  Includes water-saturated mud and artificial
  fill. The strongest amplification of shaking due is expected for this
  soil type.

The slower the shear wave velocity, the greater the shaking.

Source: https://earthquake.usgs.gov/hazards/urban/sfbay/soiltype/

Additionally, the composition of the rock that the seismic waves are travelling through is a major factor in determining the speed of the waves.  Two rock properties that determine the speed of the waves are reflection and refraction:

Reflection

  When a wave encounters a change in material properties (seismic
  velocities and or density) its energy is split into reflected and
  refracted waves.

Refraction

Source: http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_and_interior.html

There are many factors involved in determining the magnitude of an earthquake with underlying sediments and rock being one of the major factors.  This is not an exhaustive list of all those factors but hopefully it points you in the right direction for an answer to your question.


Do archaeologists deliberately leave parts of sites untouched so that someone can come back later? Thinking about all the new technologies and techniques that are available to archaeologists now compared to even 50 years ago, it occurred to me that there might be considerable benefit to leaving parts of a site untouched so that future archaeologists can use what you discover, plus new T&T to discover even more. This is the flip side of contaminating samples and disturbing sites, in a way, but it's as much an extension of the "dig, think, then think some more before digging" approach.

Against that I can see how "publish or perish" and similar pressures create incentives to grab as much as possible as quickly as possible. Not everyone can afford the time and money to focus years on the first thing they find on a site.

Do archaeologists deliberately leave things untouched for later?

Yes.

From this article in National Geographic:

  The last step before digging is to divide the site into a grid to keep
  track of the location of each find. Then archaeologists choose sample
  squares from the grid to dig. This allows the archaeological team to
  form a complete study of the area. They also leave some plots on the
  grid untouched. Archaeologists like to preserve portions of their dig
  sites for future scientists to study—scientists who may have better
  tools and techniques than are available today.

(emphasis mine)


Why do Tsunamis travel slower than sound? Tsunamis, in the deep ocean, travel at around 800 kilometers per hour.

The speed of sound under water is about 5300 kilometers per hour.

Both of these waves are pressure waves, operating in the same medium. Why is one so much faster than the other?

Tsunamis and sound waves are different types of wave - one is a transverse wave and the other is a longitudinal one. Let's look at the factors that influence the speed of each one.
Tsunami - transverse wave in shallow water
A transverse wave is one of the type that we think of from day to day - where the direction of oscillation is perpendicular to the direction of travel. The speed that a transverse wave travels at depends on different factors depending on the depth of the water. For this purpose, "shallow water" is usually defined as existing where depth < wavelength/20. The wavelength of a tsunami is very large - of the order of hundreds of kilometres - so for a tsunami, any part of the world's oceans counts as "shallow water".
In shallow water, the speed of a transverse wave can be described by,
$$V = \sqrt{gD}$$
where $V$ is the wave's speed, $D$ is the depth, and $g$ is the acceleration due to gravity (9.81 m/s2). In the case of a tsunami in the deep ocean, then, if we assume a depth of 4 km we can estimate a speed of 198 m/s, or 713 kph. That's a back-of-an-envelope calculation, but it's sufficiently similar to the 800kph that you quoted in the question that I'm happy with it.
Sound - longitudinal wave
In a longitudinal wave, the direction of the oscillation is parallel to the direction of travel - i.e. it's an oscillation in the density of the material. We don't see many of these in everyday life, but one good example is the wave that moves down a slinky if you jerk the ends towards or away from each other.

(Image source)
Sound, in a liquid or a gas, is an example of a longitudinal wave. The speed of a longitudinal wave depends on the stiffness and the density of the material that it travels through, in the following way:
$$c = \sqrt{\frac{K}{\rho}}$$
where $c$ is the speed of the wave, $K$ is the bulk modulus of the fluid, and $\rho$ is its density. Note that there is no dependence on the depth of the fluid (in this case the sea) - sound one metre below the surface of a salt water swimming pool would move at roughly the same speed as sound one meter below the surface of an ocean.
Summary
Sound waves and tsunami waves propagate through different mechanisms, and thus different things influence their speeds.


Are Richter-magnitude 10 earthquakes possible? 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?

Adding a little bit of practical data to the above answer about the Gutenberg-Richter relationship, here is a plot of the per-year cumulative probability of earthquakes in a particular province in Japan, based on observed frequencies over many hundreds of years:

Source: http://topo.earth.chiba-u.jp/afr/backnumber/no24/24%E5%8F%B702kumamoto.pdf

The relationship is plausibly log-linear (in accordance with G-R); if you accept that the relationship will hold up to higher magnitudes, you would estimate the probability of a M10 event in this location at once every 30,000 years.

To get an estimate for "anywhere in the world" you would need the cumulative data for all. A good place to start is the USGS website - they have a handy table with data since 1900.

Taking these data and plotting them on a log-linear graph, then extrapolating a linear fit, gives the following chart:

This is rather scary because it suggests that there is about a 1:100 chance of a M10 earthquake anywhere in the world, in any given year. Note that I plotted the data for magnitude x to x.9 at the location of x - which tends to underestimate things a little bit. Note also that at the extreme end of very large earthquakes (8 and above), the data looks like it might be deviating from the straight line - but there are not enough data points to draw any firm conclusions about the shape.

There are several other caveats. First - the assumption that the model can be extrapolated can be challenged: a particular fault may not be constructed in such a way as to store the energy required for an M10 event, as it will always release the energy before it gets there (and there may be a "stress shadow" effect, which says that after a big earthquake, the chance of another big one is temporarily reduced because stresses have been relieved; this is why this model can only be used "over a long period", and does not accurately reflect the risk of earthquake in the next five years).

Still - one percent.


What is the technical term for spatially aggregated measures? Is there an adjective to describe data that has areal weight, like area, biomass, population, etc. — data that has to be aggregated by sum for example — vs. data that doesn't, such as landcover classification? Categorical for the latter, perhaps, but what for the former?

To illustrate aggregation by sum: if you had rasters of biomass and elevation on 200 m $\times$ 200 m cells, and you wanted to re-scale it to 400 m $\times$ 400 m, you'd give each new, larger cell the average elevation on the 4 smaller cells that comprise it, but the sum of the 4 biomass values.

Count data.

Quantitative data that varies discretely and arbitrarily along some scale can be called count data.

Nick Chrisman (1995; Beyond Stevens: A revised approach to measurement for geographic information, presented at Auto-Carto; see this PDF for a transcript) pointed out that there are many more typologies of measurement than the 'classic' four proposed by Stevens and outlined in Wikipedia's Typologies of measurement article.

None of the measurement typologies are rigid or prescriptive, but Crisman is not alone in recognizing counts as an important statistical data type:

  Another class of geographic measurements consist of counts aggregated
  over some region in space. Counts are discrete, since there is no half
  person to count, but a count captures more mathematical structure than
  the other discrete levels (nominal and ordinal).

Mosteller and Tukey also called them counts (see Mosteller's book, Data Analysis and Regression).

It's clear to me that the concept is a good fit for, say, population, but your biomass example seems intuitively less clear-cut. The point is that it's a countable quantity, as opposed to a ratio scale, so it might just be a matter of being careful how you define the term.

This answer was edited after clarification in the comments.


Calculating error in measuring geomagnetic Anomalies using IGRF In textbooks it is mentioned that

Magnetic anomaly = Observed Geomagnetic field - Theoretical Geomagnetic field

observed value is measured using instruments like proton precession magnetometer or flux gate magnetometer etc , whereas Theoretical values are computed by IGRF formula which takes coordinates , elevation of the point as input . (https://www.ngdc.noaa.gov/geomag-web/#igrfwmm )

My doubt is regarding error in anomaly calculation . In the observed part the error is in the range 0.1 - 1 nT depending on the instrument . IGRF error for total field is around 152 nT .

(from : https://www.ngdc.noaa.gov/geomag/WMM/limit.shtml )

Therefore our total error in magnetic anomaly calculation is around 152-153 nT . In such case if our anomaly  ( observed - theoretical )  is small (say around 30 nT ) , what would be the validity of such observation as the value we are trying to estimate is smaller than the margin of error .

The key point here is that both theoretical models quoted - IGRF or WMM - are of the field from the Earth's core only, and only the largest scale parts of it at that. Only the WMM provides an error estimate with these values (as described above), and that error is a combination of "commission errors" from the approximation of the model to reality, and "omission errors" from various magnetic field sources which are not included in the model.

To calculate a magnetic anomaly (usually considered the field from the Earth's crust) you would remove all the other known field sources from the observed field:

  anomaly field = observed field - (core field + external fields)


Can human activity affect volcanoes? As part of a course on Natural Disasters, I have a group presentation in which we focus on a specific natural disaster. One of the questions we have to address is how human activity may have affected the geographic area and/or the disaster. The topic for our group is volcanoes, with a specific focus on the Mount St. Helens eruption in 1980. So my question is — is this possible?

The closest I can think of is human-induced seismic activity, due to fracking or wastewater injection, for example. However, I do not know if this could conceivably also affect in any way nearby volcanism. I also don't think there are or have been any oil-related activities around Mount St. Helens, but I really don't know for sure. So, is this possible?

I think you have drawn the 'short straw'! As disasters go, volcanoes have about the least to do with human activity. Basically, volcanoes are the product of plate tectonics, which operates on a scale vastly greater than human impacts on the planet. At least one book has been written about earthquakes, volcanoes and human impacts, but the linkage is tenuous at best. The argument goes something like this: Humans modify the water table by pumping, diverting water, building dams, etc. Also by human-induced climate change contributing to rising sea levels. Increased hydrostatic pressure in the ground 'lubricates' fault planes causing lesser but more frequent earthquakes. In and around volcanoes, there is more groundwater which increases water vapour in/near magma chambers, and hence increases the activity of volatiles, contributing to volcanic eruptions.

This is all speculative, and most geologists doubt whether these effects are significant, or even if they exist at all. I think you will have a hard task trying to provide supporting evidence for any of this.

On the other hand there is passive interaction in that many terrestrial volcanoes in wet climates produce vast quantities of volcanic ash that weathers rapidly to produce fertile soil. This fertility attracts farming communities who put themselves in harms way, being at risk from hot ash clouds, hot lava, volcanic bombs and mudflows. The latter are produced by hurricane level rainfall mobilizing the loose ash, such that mudflows (or 'lahars') kill far more people than the volcanoes themselves - we have seen many examples in the news in recent decades - in South and Central America, Indonesia, Philippines, etc.

The Mount St. Helens eruption was caused by the Juan de Fuca tectonic plate subducting under North America. It's a 100% natural process, with no human causation whatever.

There are no known cases of fracking having any impact upon volcanoes. Can you imagine any fracking company taking the legal risk of setting off a volcanic eruption? In any case, the basic geology of volcanoes and hydrocarbon bearing sediments is quite different.


Does cloud fraction of 0 for low clouds means it was raining? What information does cloud fraction provide about weather on a certain day?

Do low-level (1000-700 hPa) cloud fraction, optical thickness of all clouds and total cloud fraction provide any information about whether it rains or not?

Depending on context, cloud fraction can mean:

the fraction of the visible sky covered by clouds (total cloud fraction).  This is what an observer from below or above sees and this is what is normally reported by measurements
the fraction of a particular area and altitude covered by clouds.  This is what is typically reported by models.

The differing definitions need to be kept in mind when comparing models to measurements.

Under normal circumstances, clouds are a necessary but not a sufficient requirement for precipitation (there are some exceptions).

To answer your titular question: No, a cloud fraction of 0 for low clouds does not mean it was raining.  Nor does it necessarily mean it was not raining.  Remember: a total cloud fraction of 0 means a clear sky, a total cloud fraction of 1 means a completely covered sky.  So a low level cloud fraction of 0 means there are no low level clouds.

To answer the question you ask in the content of your post: Statistically speaking, yes.  The quantities low-level cloud fraction, cloud optical thickness, and total cloud fraction do contain information about precipitation.

If the total cloud fraction is zero, there is no precipitation.

If the cloud fraction for low clouds is zero, but for mid-level or high clouds is non-zero, it may or may not be raining.

Cloud optical thickness should correlate with precipitation occurrence and intensity.

However, these quantities do not provide complete information about precipitation.  You can't say with full certainty that a day is rainy based on these quantities alone — but you can say which days certainly aren't.


What is responsible for the underwater waterfall illusion of Mauritius Island? From an aerial view just off the island of Mauritius, there is what appears be an underwater waterfall.  The picture below illustrates this visual phenomenon:

The image can also be seen from the google maps satellite here.

What is responsible for this majestic illusion near the island?  I have heard that it may be sand moving over the edge of an underwater cliff, and if so what causes the sand to be pushed off continuously without the process coming to a stop and what is responsible for this unique geographical landscape allowing this to occur?

The cause for this phenomenon is sand and silt moving down an underwater cliff as you guessed.  The geography that makes this possible is as follows.  The island of Mauritius is located on a giant plateau that was created by seafloor spreading a few million years ago.  Most of which is under 8-150 meters of water.  Where the waterfall effect occurs there is a huge drop-off which drops down thousands of meters.  Because of the islands angle when the waves beat upon the shore and displace sand, the drawing out of the waves creates, you could say, a down pulling  effect.  This motion pulls the sand and silt down the drop-off, thus creating the underwater waterfall illusion.  Because waves constantly bring sand and weathering and erosion keep making sand and silt, is not very probable that this illusion will stop for a long time, if it stop at all.  I hope this was helpful.


How does the air-circulation in the lower stratosphere look like? I would like to simulate the trajectory of a weather balloon in an altitude of 12 to 20 km.
For this I would like to know how the velocity-field (i.e. wind-speed and direction as a function of position and time) looks like and how constant it is in the time and space domain.

Is there some publicly available data about air-circulation in the stratosphere? Or do you have an idea how to generate test-data for my simulation?

This presentation mentions "Atmospheric Motion Vectors (AMVs)", and specifically mentions a number of AMVs, which separately cover the globe (GOES AMVs ±60N, plus polar AMVs from MODIS/AVHRR/VIIRS, and a combined product from UW CIMSS) but says that are all only 3 layers. That presentation is talking about producing a more global, higher resolution product, but it's only a year old, so I'd guess the data isn't available yet.

However, the UW CIMSS AMV page says that their datasets have 13 layers, so maybe it is suited for what you need. The data is gap-filled with modelled data.


Why do snowflakes form into hexagonal structures? 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.


How to solve libnetcdf.so.12: cannot open shared object file in pyferret? My pyferret was working perfectly until I had installed netCDF4 and now it is throwing up this error

readline: /etc/inputrc: line 19: term: unknown variable name
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/cccr/msingh/anaconda3/envs/FERRET/lib/python3.7/site-
   packages/pyferret/__init__.py", line 45, in <module>
    from pyferret import libpyferret
ImportError: libnetcdf.so.13: cannot open shared object file: No such
file or directory

You need to uninstall netCDF4 as it is not compatible with pyferret library as of now.

pip uninstall netCDF4

should do.


How are 'road-like passages' between Himalayan mountains formed? If you take a look at this picture you can see what looks like almost nicely cut out 'roads' between the mountains.

What forces are responsible for this?

It's a glacier, it this case the Baltoro Glacier in Karakoram, Pakistan. The road-like pattern is formed as the glacier slowly flows towards lower altitude from a nearby ice-cap or accumulation zone. Rocks and dirt is transported both within the ice and on the surface.

With a closer look, the surface is rarely smooth, but often deep trenches, cravasses, and meltwater channels are formed as the glacier moves. On a human timescale, the glaciers crawl slowly, but from a geological perspective they are fast.

Closeup of a glacier with crevasses. (By Roylindman at en.wikipedia, CC BY-SA 3.0)

Under the glacier, a U-shaped valley is formed as the glacier erode the bedrock. Depending on the dynamics of the glacier and the erosion, the glacier might also appear clean, without sediment on the surface.

Large glacier without sediments, but also with a very uneven surface due to the movements of the glacier (Wilkes Land).


How do weather services measure wind I'm curious to know where commercial weather services like the Weather Channel and Weather Underground get their wind speed and direction measurements, to produce maps like this one (http://www.wunderground.com/maps/us/WindSpeed.html?MR=1):

A model is probably used, but how does the station measure or acquire real measurements to assimilate the data into the model? Is it by regularly placed anemometers on the ground? Or data measured by satellites?

Weather stations and airports around the world almost always include an anemometer. Because of ground friction, the wind speed varies with height, so instrumental deployment is set at a standard height of 10 metres in open rural areas. This may require some adjustment in urban or forested areas. In fact, measuring accurate wind speeds above tree canopies is particularly difficult. There is quite an art in correctly positioning the instrumentation.
Thousands of such anemometers and direction sensors are deployed on nearly every continent, and the results are part of the routine reporting. Such reporting may be every 24, 12, or 6 hours, or even shorter periods down to and including 'continuous'. Then comes the number crunching, in which the strength and direction is gridded and plotted on what appears to be a uniform distribution of points. In reality it is very far from a uniform distribution.

With the introduction of increasing numbers of wind farms higher elevation wind measurements are becoming more important.


How do we measure (or empirically calculate) the amount of entropy in the atmosphere and oceans? Here's a paper on the entropy budget1, where moist entropy is defined in equation (8) as

  $$
s = (1-q_t)(C_{pd} \ln T - R_d \ln p_d) + q_t C_l \ln T + \frac{q_v L_v}{T} - q_v R_v \ln \mathcal{H}
$$

  In this expression, $q_v$ is the speciﬁc humidity for water vapor, $C_{pd}$ is the speciﬁc heat at constant pressure of dry air, $C_l$ is the speciﬁc heat of liquid water, $T$ is the temperature of moist air, $R_d$ and $R_v$ are the gas constants of dry air and water vapor, $p_d$ is the partial pressure of dry air, $\mathcal{H} = e/e_s$ is the relative humidity, $e$ is the water vapor pressure, and $e_s$ is the saturation vapor pressure.

I'm just curious: is entropy directly measured, or indirectly measured from other sources? If we measured it, what would be the spatial-temporal resolution of such a measurement?

1 Pauluis, Olivier, and Isaac M. Held. "Entropy Budget of an Atmosphere in Radiative–Convective Equilibrium. Part I: Maximum Work and Frictional Dissipation." Journal of the Atmospheric Sciences 59: 125-139.

There is a convention for moist air entropy built from dry air entropy being 0 at T = 273.15K and P = 101325Pa, and liquid water entropy being 0 at its triple point. See the page numbered 47 of this NIST document:

http://www.nist.gov/calibrations/upload/5241.pdf

Entropy is not measured directly.

Entropy $S$ is defined as $dS \equiv dQ_{rev}/T$

Therefore, entropy can measured by measuring heat transfer and temperature.

See Calculation of Entropy Change in Some Basic Processes for more information.


What properties must a rocky body possess in order to exhibit the magnetic dynamo? The magnetic field of the Earth is produced by the magnetic dynamo. What conditions must a rocky body possess in order to exhibit the dynamo?

Clearly, the planet's interior must not be too cold. However, Venus for example, according to Wikipedia, also lacks the dynamo although its interior is hot:

  The lack of an intrinsic magnetic field at Venus was surprising given it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo. This implies the dynamo is missing because of a lack of convection in the Venusian core.

They offer explanation in which either:

Lack of plate tectonics reduces heat gradient and prevents convention in mantle.
Inner core is solidified, missing or lacking temperature gradient.

I am interested in more complete list of conditions that are necessary, like for example in this answer about plate tectonics.

Aside from the three requirements already mentioned, electrically conductive fluid, rotation of the planet and convection of the fluid, the main additional requirement is a seed magnetic field.  There must be an initial magnetic field for the dynamo to start.

See the following reference for more information: http://astro.berkeley.edu/~gmarcy/astro160/papers/Magnetic_dynamos_physics.pdf


CMIP5 Historical experiments, what do they mean? What are the main characteristics of CMIP5 Historical experiments?

e.g.
Initial conditions? Greenhouse gases forcing?

How they differ from RCPs? And how they are similar to Observed reanalysis data?

Thanks

The CMIP5 experimental protocol is documented at https://cmip.llnl.gov/cmip5/docs/Taylor_CMIP5_design.pdf

The historical experiment for a model spans model years 1850-2005 and is initialized from a control experiment for that model (forcings from 1850 are used repeatedly).

For models that are not coupled to a modeled carbon cycle, greenhouse gas forcing are based on the observed record (including ice cores) of atmospheric concentrations. For models that are coupled to a modeled carbon cycle, atmospheric CO2 concentration is predicted, and the model is forced with anthropogenic CO2 emissions (and land use change).

RCPs are extensions into the 21st Century. There are different RCPs, based on different assumptions about future greenhouse gas concentrations and/or emissions, land use, population, etc.

Reanalysis data is from a model this is constrained to be close to observations of the atmospheric state (e.g., air temp, winds, pressure, etc.) This differs from CMIP models that run unconstrained.


Why is Earth's density gradient a step-function, rather than smooth? Why is the density of particles in Earth's atmosphere above surface-level so much less than ground and below? Why, instead, isn't Earth simply a ball of swirling gasses / liquids, which become progressively denser as you approach the core (since gravitational pull is stronger closer to the core)?

Roughly put, it's the same thing that makes a density step function when you try to combine oil and water:
Source
The components do not mix with or dissolve into each other, so gravity makes the denser material -- water -- settle to the bottom.  The density as a function of height jumps up from the oil density to the water density when you go below the interface.
In the case of Earth, there are several progressively denser phases that do not mix.  The biggest step in the density gradient is when the rock phases (olivine, spinel, perovskites, and ferropericlase which is not shown in the illustration below) give way to the much denser core.  In the illustration below, this boundary is where the bottom part of the rocky mantle, the D" layer, meets the predominantly iron core.
Source
One major difference between the interior of Earth and the oil-water combination in the first picture is that much of Earth's interior is solid, so it takes a lot of heat and pressure -- both generated by gravity -- to make the materials yield and flow to their equilibrium positions.  The same is true of other predominantly solid celestial bodies, so only relatively large and massive ones have enough gravitational power to enable separation of different density phases.  Planetary scientists call this process differentiation.


Current consensus on the Messinian Salinity Crisis In 1973, Hsü et al. explained the findings of a large evaporite deposit during the Messinian (Latest Miocene stage, ca. 6Ma) in the Mediterranean by a basin-wide desiccation event. It has been speculated later that it was due to the closing of the Gibraltar strait.
Much research has been done on the subject since then, generating controversies on whether the entire basin became totally or partly desiccated, on the actual cause of the event, on whether the Strait closed because of tectonic activity or because of sea level drop, etc.
Hence my question: What is the current consensus on the extent of that event and its causes?

Hsü, K. J., Ryan, W. B. F., Cita, M. B. 1973. Late Miocene Desiccation of the Mediterranean. Nature, 242: 240-244.

According to palaeoclimatic records modelled from sediments studied at Monte dei Corvi, in the paper Reconstructing the temperature and salinity of the Mediterranean Sea through the Late Miocene (13 Ma - 6 Ma) prior to the Messinian Salinity Crisis (Alexandrina et al. 2014), that using alkenone paleothermometry, there was a cooling trend starting at 8.1 million years ago, developing a cold period around 7 million years ago and continuing to the Messinian Salinity Crisis (MSC) as evidenced by a disruption in marine sedimentation (and changes to the surrounded flora and desertification in the Sahara).

Research presented in the paper The Onset of the Messinian Salinity Crisis along the offshore Betic Range (Ochoa et al. 2014) suggest that tectonic processes along the offshore Betic Range in southern Spain may be a trigger for the closure (see tectonic map below for the relevant area).

Image source: Wikipedia

The MSCis described in the paper Modelling global-scale climate impacts of the late Miocene Messinian Salinity Crisis (Ivanovic et al. 2014) as being the result of the influences of climatic change as observed in sedimentary strata described by Alexandrina et al. 2014 and of tectonic activity of the offshore Betic range, as detailed by Ochoa et al. 2014.  They also suggest that during the MSC, there were intermittent outflows of saline water into the Atlantic - causing disruptions to the thermohaline circulation.

Dr. Iuliana Vasiliev of the University of Utrecht, suggests that one possible influence during the MSC was interactions with the closing remnants of the Paratheys Ocean, as shown in the map below:


Is there a name for this type of ice crystal? Recently, I took the following picture:

I have read about this phenomenon and I think it is caused by water vapour that is pressed out of the ground and then freezes due to low temperature. However, I cannot recall the name of this phenomenon and I couldn't find it on the internet.

The picture was taken in the Alps around 3000 meters above sea level in August. I'm pretty sure that the white thing is ice. They're a couple of centimetres long and quite porous. I found them on top of an earthy and slaty flank of a mountain early in the morning (around 8 am).

What is this phenomenon called?

This is needle ice which forms when moist soil freezes, for example during a cold night. Freezing starts at the ground surface and as ice is formed moisture gradients develop which draws water to the freezing front and growth continues. Ideal conditions may be a sub-freezing night after a rainy day or a sub-freezing night after a day of snow melt.

According to the National Snow and Ice Data Center (NSIDC) site needle ice be up to 40 cm in length. Since the favourable conditions are not unique to soils, although most often encountered in soils, needle ice can develop from freezing of, for example, vegetation


What will be the depth that water can sink to the ground? Currently there is a pond in one acre of land which is of 4 feet depth. If the pond is filled with water it is expected that some percent of the water will be absorbed by the ground under the pond. To what depth will the water will be absorbed by the ground? The type of soil is clay type.

The reason for asking the question is, there is a shrimp pond which is infected with a virus and it has been observed that in next few days the adjacent pond is infected with the virus and like that whole area is infected which is causing huge loss for the farmers. We are looking for the option to restrict the transmit of water from the adjacent ponds by placing a plastic/HDPE boundary kind of the thing for x feet depth so that our pond water doesn't get make contact with adjacent pond's water.

I am interested to know the depth water can sink to the ground, to ask my query in a different way.. take a 4 ft dry pond and let's assume water is fully filled and the pond is kept ideal for a week. After a week how much depth the water would have sinked, can it be 20 ft or 30 ft or 40 ft or..?

One quick and dirty solution may be to lower the virus-pond level and raise the new-pond level ... so the flow should be from the new-pond to the virus-pond.

Soil being clay may mean the flow is happening below the soil ... if the virus-pond is in contact with the unit below the clay you may already have widespread contamination.