Question:
We know (think?) that Earth has three Hadley cells per hemisphere, but from observing gas giants such as Jupiter, we see that they have many more cells. According to a link from a comment in this question, Titan might have one cell going from north to south poles.

What factors affect the number of cells a planet has, and how? Or to put it another way, given a hypothetical planet with atmosphere, what facts would you need to know in order to guess at how many cells it has?

Explanation: this post want to find out the factor determines the number of Hadley cells. The anwser use some formula to infer the number is determined by the planet's rotation rate and the latitudinal distribution of heating from the parent star.

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Well for one, this will surely be the speed of planetary rotation and the wind
speeds your atmosphere can generate in North-South-direction.
In my understanding those cells form as N-S winds flow and become deflected
from this direction. So the number of cells will be determined by the winds
that start either at the equator or the pole and the distance that they can
cover before being deflected into completely zonal direction.

To have a rough idea about this distance I tried to estimate this (in a very
quick, dirty and most likely extremely wrong way) by assuming the simplified
Navier-Stokes equation in planetary coordinates on synoptic scales:
$$\partial_t \vec v = 2 \Omega \sin\Theta (\vec v \times \vec z),$$
where  $\Theta$  is the planetary latitude,  $\Omega=\Omega(\Theta)$  the
local planetary rotation frequency at the given latitude and  $v_0$  the local
wind speed, leading to a harmonic equation for the meridional speed
$$\partial_{tt} v_{\phi} = -(2\Omega \sin\Theta)^2 v_{\phi},$$  which with the
usual arguments of scale-analysis gives us a timescale  $\tau_{\phi}$  for the
periodic movement involved in solutions of that equation, of
$$\tau_{\phi} \sim \frac{1}{2\Omega \sin\Theta}$$
corresponding to a distance  $$ d \sim \frac{v_0}{2\Omega \sin\Theta_0}$$
covered by the vortex. Of course, I cheated here by assuming the initial speed
$v_0 = \text{const}$  and the initial Coriolis-factor constant with  $\Theta =
\Theta_0$  .
As now such a "structure" (solution of the above eqs.) will have a size of
$2d$  in the meridional direction, having space of  $\pi R_{\text{planet}}$
our number of Hadley cells per hemisphere becomes simply  $$N = \frac{\pi
R_{\text{planet}} \Omega \sin\Theta_0}{v_0}$$
Where we'd need to estimate this number starting at the poles, as the
Coriolis-parameter vanishes at the equator.

I will later (if I don't forget it) pluck in some numbers and see how bad this
estimate is, or perhaps someone can try that. Anyway, I'm open for
improvements of those arguments from anyone who knows something, but a quick
look into Holton, _An Introduction to Dynamic Meteorology_ , unfortunately
didn't reveal much.

But to also address your question a little bit further: Of course, we'd need
to have some climatic model to be able to estimate  $v_0$  and know the speed
of planetary rotation. Independent of how bad my formula is, the initial
arguments of deflection vs. 'climbing the meridian' should hold for the
formation of a Hadley cell.
Returning winds from higher atmospheric layers, surface friction in the
atmospheric boundary layer will also play a role for an exact calculation.

* * *

Addendum: [ Rhines (1975)
](http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=373502&fileId=S0022112075001504)
has however found a relation  $N \sim R_{planet} \sqrt{\frac{\Omega}{v_0}}$  ,
by considering how turbulent eddies will break up energy-limited by zonal
jets. Also turbulent eddies can provide a good mechanism for justifying a
locally constant  $v_0$  , that I simply assumed in my calculation above.

On the other hand I just noticed a trivial error in my above calculation and
cleared that. The fraction that is N was the other way round. Sorry for that.

* * *

I wanted to note one other thing that seems important to me (and is perhaps
also to anyone on this planet..):
While this rule of thumb-calculations can work out, there is more physics that
play a role in shaping the exact morphology of circulation-cells on a planet.
Namely the radiative equilibrium in the atmosphere, which links the exact
positions of those cells to a changing climate.

As a [ study
](http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.38.html) suggests by
measuring Tropical to Polar-Jet distances and Tropopause heights, those cells
can expand and reshape the environment people live in.


Question:
We see tropical cyclones (going by different names e.g. hurricane, typhoon, cyclone) all over the tropics, but it seems that there are never any storms in the southern Atlantic. See this map of tropical cyclone activity and note the lack of activity in the south Atlantic and also in the south Pacific until you near Australia.

Where are all the tropical cyclones in the southern Atlantic basin?

Explanation: The post asked about why there's no hurricanes in the southern Atlantic basin. The reason anwser gave is that it's about naming conventions based on location

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There have been tropical storms and hurricanes in the south Atlantic, with,
according to NOAA's webpage [ Subject: G6) Why doesn't the South Atlantic
Ocean experience tropical cyclones?
](http://www.aoml.noaa.gov/hrd/tcfaq/G6.html) , with a hurricane forming in
the south Atlantic making landfall in Brazil in 2004 and a strong tropical
depression/weak tropical storm that formed off the coast of Congo in 1991 -
but these are exceedingly rare.

The reason why these storms generally don't occur in the south Atlantic,
according to the Penn State webpage [ Upper-level Lows
](https://courseware.e-education.psu.edu/public/meteo/upperlevel_lows.html) as
being:

> There are two primary reasons why tropical cyclones are rare in the south
> Atlantic basin. First, vertical wind shear between 850 mb and 200 mb is
> typically greater than 10 meters per second (check out the long-term average
> of vertical wind shear between 850 mb and 200 mb). To make matters worse,
> westerly shear dominates over latitudes where tropical cyclones would be
> most likely to form. Second, easterly waves from Africa do not form south of
> the equator (the MLAEJ is a northern hemispheric singularity.

Further, from the NASA News page [ The Nameless Hurricane
](http://science.nasa.gov/science-news/science-at-nasa/2004/02apr_hurricane/)
, they provide an extension to the explanation with

> Vertical wind shears in the south Atlantic are too strong for hurricanes,"
> Hood explains. Winds in the upper troposphere (about 10 km high) are 20+ mph
> faster than winds at the ocean surface. This difference, or shear, rips
> storms apart before they intensify too much

An article [ The first South Atlantic hurricane: Unprecedented blocking, low
shear and climate change
](http://onlinelibrary.wiley.com/doi/10.1029/2005GL023390/full) (Pezza and
Simmonds, 2005) suggest that the implications of the southern hemisphere
hurricane presents

> evidence to suggest that Catarina could be linked to climate change in the
> SH circulation, and other possible future South Atlantic hurricanes could be
> more likely to occur under global warming conditions.

_Catarina refers to the southern hemisphere hurricane_

_SH = Southern Hemisphere_

edited to add an [ NASA Earth Observatory
](http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=12935) satellite
image of the hurricane:

![enter image description here](https://i.sstatic.net/Z3EMC.jpg)


Question:
How long does it take the magnetic field to move once the poles start to flip? What environmentally would change? Does the core of the Earth flip?

The magnetic poles are moving now. When will it start to move faster and how would a compass behave?

Explanation: The post wants to know what happens when the North and South poles filp. The anwser indicates that pole reversal will ease the magnetic field. The Earth's magnetic field temporarily weakens, allowing more cosmic radiation and solar wind to reach the Earth.There may be some low-level radiation effects on living organisms, but likely not enough to cause a mass extinction event.Apart from increased radiation, there is limited direct impact on human activities.After the reversal is complete, the Earth's magnetic field will re-establish itself, but with the magnetic north and south poles switched.

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First of all I will try to explain what a geomagnetic pole reversal is.

A magnetic pole reversal happens when the magnetic field weakens until it can
no longer sustain itself. This can take 1000-3000 Years or more to complete
the time it takes is very variable.

After years with a weak and sometimes variable magnetic field. The magnetic
field re emerges in its original, or in a changed direction. So it is not the
poles moving over the surface of earth.

Link [ https://en.wikipedia.org/wiki/Geomagnetic_reversal
](https://en.wikipedia.org/wiki/Geomagnetic_reversal)

What happens during a geomagnetic pole reversal.

During a pole reversal earths magnetic field is weak so our planet is less
protected from cosmic radiation and from the solar wind.

Link [ https://en.wikipedia.org/wiki/Earth%27s_magnetic_field
](https://en.wikipedia.org/wiki/Earth%27s_magnetic_field)

This increase in cosmic radiation does not fit any of the known extinction
events that have happened in Earth's history. This does not mean nothing at
all will happen to life during this time it only means the dangers are low.

If you want to follow how the cosmic radiation changes you can do this on the
last part of this page [ http://spaceweather.com/ ](http://spaceweather.com/)

Link [ https://en.wikipedia.org/wiki/Cosmic_ray
](https://en.wikipedia.org/wiki/Cosmic_ray)

I mentioned in my comment Earth's magnetic north pole is really in the south.
To understand how this can be true in a magnetic compass the magnetic north
pole is repelled by Earth's north pole so it does point in the opposite
direction (a bad explanation but it is how it works).


Question: I have never understood why earth's inner core is solid. Considering that the inner core is made of an iron-nickel alloy (melting point around 1350 C to 1600 C) and the temperature of the inner core is approximately 5430 C (about the temperature of the surface of the sun). Since Earth's core is nearly 3-4 times the melting point of iron-nickel alloys how can it possibly be solid?

Explanation: This post tries to find out why the inner core of earth is solid. According to the temperature and pressure in the core,

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Earth's inner core is solid even though the temperature is so high because the
pressure is also very high. According to [ the Wikipedia article
](http://en.wikipedia.org/wiki/Inner_core) on the Earth's inner core, the
temperature at the center is $5,700\ \text{K}$ and the pressure is estimated
to be $330$ to $360\ \text{GPa}$ ($\sim3\cdot10^{6}\ \text{atm}$).

The phase diagram shown below (taken from [ this paper
](http://www.agu.org/books/gd/v031/GD031p0083/GD031p0083.pdf) ) shows the
liquid/solid transition, where fcc and hcp are two different crystalline forms
of solid iron. You can see clearly from the slope of the line going off toward
the upper right that iron should be solid at this temperature and pressure.

![enter image description here](https://i.sstatic.net/w82eb.png)


Question: The enlightening image below is of a lightning strike slowed down at 10,000 frames per second. It can be seen that the most intense flash produced from the lightening occurs in the direction from the ground up. Why does this final "ground-up" strike occur and why is it so much brighter and faster than the initial part of strike heading towards the ground?

Explanation: The post wants to know why the most powerful lightening strike comes from the ground-up. The reason is the existence of the opposite chargers.

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> **Does lightning strike from the sky down, or the ground up?**
>
> The answer is both. Cloud-to-ground lightning comes from the sky down, but
> the part you see comes from the ground up. A typical cloud-to-ground flash
> lowers a path of negative electricity (that we cannot see) towards the
> ground in a series of spurts. Objects on the ground generally have a
> positive charge. Since opposites attract, an upward streamer is sent out
> from the object about to be struck. When these two paths meet, a return
> stroke zips back up to the sky. It is the return stroke that produces the
> visible flash, but it all happens so fast - in about one-millionth of a
> second - so the human eye doesn't see the actual formation of the stroke.
>
> **Source:** [ National Severe Storms Laboratory
> ](https://www.nssl.noaa.gov/education/svrwx101/lightning/faq/)

The reason is that when cloud-to-ground strike approaches the ground, the
presence of opposite charges on the ground enhances the strength of the
electric field and the "downward leader" strike creates bridge for the "return
stroke"; this [ per the wiki page for Lightning
](http://en.wikipedia.org/wiki/Lightning#Upward_streamers) .

* * *

**Cloud to cloud and Intra-Cloud Lightning**

![enter image description here](https://i.sstatic.net/vBIiD.jpg)

Might be worth also noting that cloud-to-ground is not as common as **[ Cloud
to cloud (CC) and Intra-Cloud (IC
](http://en.wikipedia.org/wiki/Lightning#Cloud_to_cloud_.28CC.29_and_Intra-
Cloud_.28IC.29) ) ** :

> Lightning discharges may occur between areas of cloud without contacting the
> ground. When it occurs between two separate clouds it is known as inter-
> cloud lightning, and when it occurs between areas of differing electric
> potential within a single cloud it is known as intra-cloud lightning. Intra-
> cloud lightning is the most frequently occurring type.

**Ground-to-Cloud**

![enter image description here](https://i.sstatic.net/RM2Xe.jpg)

Appears that ground-to-cloud is possible, though normally only a result of a
man-made object creating "unnatural" electric potential, and is the least
common type of lightning.


Question: As far as I understand it is perfectly valid for air to have 100% humidity. At that point, all water can still exist in form of vapor, non-condensed.Does it immediately start to rain if humidity is >100%?If so, why do we have slight rain and heavy rain if any 0.1% above 100% drops out immediately? That should always be only a small amount of rain and rainstorms could not be explained.If not, what is the limit of humidity if not 100% and why can it exceed 100%?

Explanation: The post wants to know the relationship between whether it rains or not and humidity. But humidity alone is not a sufficient condition for rain to start. High humidity means there is a lot of water vapor in the air, but rain requires several other factors:

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Short answer: humidity is not a proxy for rain starting and no, it does not
start raining automatically when 100% humidity is reached (haze or clouds can
form though). The onset of rain is dependent on many things including
humidity, but a specific value of humidity is not a sufficient condition for
rain.

* * *

Water vapor is a gas and invisible. The amount of water vapor in the air can
be expressed as relative humidity (RH) which is the ratio of water vapor
pressure ($e$) and saturation water vapor pressure ($e_s$). Saturation vapor
pressure is the partial pressure of vapor when evaporation and condensation
rates are equal, represented by RH=100%. When RH > 100% net condensation
occurs, but water has its own ideas.

In a mixture of pure dry air and water vapor, water will not condense until
around 400% RH. Reasons for this are a bit complicated but it has to do with
very small droplets being more likely to evaporate as their curvature is very
large ( [ Kelvin effect ](https://en.wikipedia.org/wiki/Kelvin_equation) ,
saturation vapor pressure is higher over curved surfaces than flat ones).
Luckily for us, our atmosphere is not pure air but has small particulates
suspended in it (aerosols). Some of these aerosols are classed as cloud
condensation nuclei (CCN) and enable droplet formation at lower relative
humidities. These work by forming a solute in water increasing the energy
needed to break bonds and evaporate the water ( [ Raoult's_law
](https://en.wikipedia.org/wiki/Raoult%27s_law) )

The combined interaction of these are described by [ Köhler theory
](https://en.wikipedia.org/wiki/K%C3%B6hler_theory) and describe droplet
growth in terms of drop size, solute and supersaturation (RH-100%). In a
nutshell, there is a critical drop size below which drop size decreases for
decreasing supersaturation and above which drop size _increases_ for
decreasing supersaturation. The critical supersaturation is the
supersaturation needed to attain the critical drop size, and is generally
small (e.g. 0.3% supersaturation).

Droplets below the critical size are 'haze drops' and these make up the haze
you see on very humid days. Drops that reach the critical size can continue to
grow to become cloud drops. The condensed water is carried in the air but is
no longer water vapor and is not part of relative humidity (but does
contribute to the parcel density)

So... when does it rain?

It rains when water vapor is in the presence of CCN, driven to a
supersaturation causing growth to the critical drop size (on the order of
$\mu$m) and continuing to grow to cloud drops and further to the much bigger
drop sizes that make up drizzle (100-300 $\mu$m)and rain drops(mm), a process
that takes around 40 minutes. Drops will grow until the updraft can no longer
support their mass and then they fall from the cloud as rain.

Your question asks at what humidity does it rain, but what surface humidity
determines is how high the cloud bases are. When the dew point depression (the
difference between temperature and dew point) is high, the cloud bases will be
higher than when the dew point depression is small. As air rises it cools, and
at some point 100% RH is attained. If there is forcing for vertical ascent,
parcels can rise to this height and then to a height where they freely convect
due to decreased parcel density caused by the release of energy during
condensation (see: [ CAPE
](https://en.wikipedia.org/wiki/Convective_available_potential_energy) ).

So far to have rain we've needed water vapor (but not at 100% at the surface),
aerosols to aid condensation (CCN) and a way to cool the air to reach 100% RH
via lifting. It is these three things -- moisture, aerosols and cooling, that
we need for a rain storm. We can have 100% RH days that are just hazy or foggy
that do not rain and we can have days with mextremely little RH (e.g. deserts)
that result in little rainstorms or large severe storms. We also have storms
we call 'elevated convection' that are completely disconnected from surface
conditions and when these storms cause rain is not related to surface humidity
at all.

If you are looking for a magic trigger for rain, your closest bet will be
looking at temperature, dew point and the height parcels need to attain to
freely convect ( [ LFC
](https://en.wikipedia.org/wiki/Level_of_free_convection) ). If there is
forcing for parcels to get that high and instability above, then rain is a
good bet. Forcing for lift can be anything from convergence along a boundary
(sea breeze, cold front, outflow from another storm), orographic lifting
(mountains, hills), thermally or dynamically forced.

* * *

To address your specific concerns:

> Water vapor is the gaseous state of water and is invisible
>

>> Invisible to me would mean that clouds are excluded from humidity.

Correct, clouds are not part of humidity, they are suspended liquid water
drops, usually condensed onto a solute of some kind.

> Absolute humidity is the total mass of water vapor present in a given volume
> of air.
>

>> That again makes me think a cloud must be included.

Water vapor contributes to humidity but water vapor does not include liquid
water. Cloud water, ice, snow, rain, grapple, hail all contribute to the total
mass of a volume of air, but are not humidity.

> The humidity is affected by winds and by rainfall.
>

>> Rainfall certainly decreases humidity, but it is not stated at what
percentage it starts to rain.

Humidity will increase during a rainstorm as rain and puddles evaporate.
Temperature will decrease toward the wet bulb temperature. As noted in the
answer, there isn't a magic number of %humidity that causes rain to start.


Question:
Forgive my ignorance of the subject but I was always wondered about the exact reason of this phenomenon.

Vernal equinox happens around March 20, whereas autumnal equinox happens around September 22, so wherever you are in Northern Hemisphere, the length of the day, and consequently the amount of solar energy that reaches the place should be almost the same.

However the average temperatures differ widely, for example Toronto has average temperatures of 2°C to 6°C on March 20th, and 14°C to 19°C on September 22nd. So around 12°C difference [link].

So obviously there is some sort of temperature inertia, as temperatures seem to experience a delay in responding to changes in day length.

What is the main reason for it? Is it effect of sea ice or snow-covered land albedo? Energy stored in oceans? Energy absorbed by melting snow and ice?

Explanation: The post wants to know the reason behind the fact that the March is colder in the Nothern Hemisphere. The phenomenon is called season lag. Basically temperatures lag behind the peak in solar radiation by several weeks because it takes time for the land, atmosphere, and especially the oceans to absorb and release the accumulated heat energy due to their different thermal properties and capacities.

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**The phenomenon is called[ seasonal lag
](http://en.wikipedia.org/wiki/Seasonal_lag) . **

There's [ a more extensive answer elsewhere on this site
](https://earthscience.stackexchange.com/questions/2490/equinoxes-and-
solstices-start-of-the-season-or-mid-season/2603#2603) but the basic idea is
that temperature lags behind insolation by several weeks, because it takes
time to change the mean temperatures of the land, the atmospehere, and
especially oceans change their mean temperature. This diagram tries to show
the lag, along with various ways of reckoning the seasons:

![enter image description here](https://i.sstatic.net/gWNb5.png)


Question:
Intuitively, it makes perfect sense to think that the coldest day of the year would be the day that gets the least sunshine, the winter solstice. In the northern hemisphere, this occurs a few days before Christmas. But as anyone who lives in places that gets snow can tell you, the most bitter parts of winter are in January and February, not December.

Why does it get so much colder when sunlight (and the warmth it should be bringing with it) is increasing?

Explanation: The post wants to know that why the weather gets colder after the solstice, i.e., the less sunlight day of the year. The reason behind it is the pheomenon of seasonal lag

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Primarily because of **inertia** . This phenomenon is called [ seasonal lag
](https://en.wikipedia.org/wiki/Seasonal_lag) .

It is true that the December solstice is the moment that the northern
hemisphere gets the lowest total amount of insolation. Conversely, the June
solstice corresponds to the moment of maximum insolation. The oceans, and to a
lesser degree the land, absorb a lot of heat. Due to the large heat capacity
of water and the large amounts of water, the seasonal lag can be considerable.
In general, mid-latitude areas near large bodies of water have the largest
seasonal lag, to the extent that the warmest month of the year in most of
western Europe is August.

It is illustrated very well by this _Warmest day of the Year_ map that NOAA
NCDC (now NCEI) produced for the contiguous United States:

[ ![NCDC/NCEI map of warmest day of the year](https://i.sstatic.net/b4A5P.jpg)
](https://i.sstatic.net/b4A5P.jpg)

As expected, areas with a humid climate, such as southern Texas and nearby
areas, tend to have the warmest day of the year in August. The west coast is
even more extreme, with some places having the warmest day in September.
Conversely, desert areas in the southwest (southern Arizona, New Mexico,
western Texas) have the warmest day of the year in June. (For the Big Bend
area in Texas, it's even in the _first_ half of June, which has different
reasons; I'm no expert on the climate of the region, but according to [ a
comment by David Hammen
](https://earthscience.stackexchange.com/questions/7233/why-does-winter-get-
colder-after-the-solstice/7234?noredirect=1#comment59315_7234) , it's _due to
the North American Monsoon. July and August are much cloudier (and
occasionally, much more rainy) than is June in the Big Bend area_ ). There is
a corresponding map for coldest day of the year:

[ ![Coldest day of the year](https://i.sstatic.net/46JC8.jpg)
](https://i.sstatic.net/46JC8.jpg)
(Source: [ NCEI ](https://www.ncdc.noaa.gov/news/when-to-expect-coldest-day-
of-year) )

The detailed structure is somewhat different, which has to due with local
climate and circulation, but the overall trend related to humidity and
proximity to ocean shows up in both. We can also see the mountains, which I
would guess to be a snow albedo feedback effect (ever notice how nights get
colder when the ground is snow-covered?). At NCEI you will also find [ similar
maps for the rest of the USA ](https://www.ncdc.noaa.gov/news/when-to-expect-
coldest-day-of-year) as well as [ updated information focussing on the
1991-2020 timeframe ](https://www.ncei.noaa.gov/news/when-expect-warmest-day-
year) .


Question:
My son wants to replicate some experiments and try to grow plants in Martian soil for his A-level science project. I know NASA have managed to produce soil that mimics Martian soil, however I also know how expensive it is.

My question is, what is the closest proxy to real Martian soil that I can create using readily available supplies from builders merchants, garden suppliers, chemists, and supermarkets?

Explanation: The post wants to know the ingredients to make martian soil. The anwser gave it a list of ingredients and their percentage in composing it.

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the other posters are correct - true Martian soil contains perchlorates, high
levels of iron, and can be highly toxic. What you want to look for is called
"Mars Regolith Stimulant". There are a few websites that have recipes.

[ https://reprage.com/post/home-made-simulant-mars-dirt
](https://reprage.com/post/home-made-simulant-mars-dirt)

> The five most abundant ingredients, account for almost 90% of the dirt taken
> from Mars samples.
>
>   * SiO2 - 49.5%
>   * Fe2O3 - 17.9%
>   * Al2O3 - 7.2%
>   * MgO - 7.7%
>   * CaO - 6.7%
>

>
> That seems like a good starting point. If I pad those out to reach 100% and
> use the results as a weight ratio, it should make a decent first batch of
> home-made Martian soil. Luckily most of this stuff can be found in hardware
> and health food stores.
>
> SiO2
>
> Silicon dioxide, yeah sand. You can go budget and get a bag of propagation
> sand (it won’t be 100% silica). If you want a bit more precision you can
> hunt around for educational or scientific samples that contain less
> impurities. You can get 2.7 kilograms for about \$16
>
> Fe2O3
>
> Iron oxide, is red cement colouring and is often advertised as red oxide or
> similar. You can get 2.2 kilograms for about \$20
>
> Al2O3
>
> Aluminium oxide, is used as an abrasive. It gets stuck to sandpaper and is
> used in sandblasting. It was a bit difficult obtaining smaller amounts in
> Australia (places wanted to sell me 20kg bags). You can get 340 grams for
> about \$10.
>
> MgO
>
> Magnesium oxide, is a dietary supplement found in health food stores. You
> can get 225 grams for about \$10.
>
> CaO
>
> Calcium oxide, Now this one is tricky. I couldn’t easily buy calcium oxide.
> It seems that calcium oxide reacts with CO2 in the air and gets converted
> into calcium carbonate. But you can buy calcium carbonate (CaCO3) as a
> dietary supplement. This can then be turned into calcium oxide by ‘lime-
> burning’, just heat it up in a kiln to above 825°C. You can get 340 grams of
> calcium carbonate for about \$10

and others that lets you buy your own (I won't link to them because of
advertising, but the Martian garden is one such site). I think you could get
close with the above recipe.

In any scenario, I would strongly recommend supervising your child when they
are working with the stimulant. It should be handled with gloves and a
respirator mask. Ingestion is very dangerous, and it might not be the worst
idea to check the number of your local poison control center. While this may
seem like overkill, it would make a great section of your child's science
project. "Methodology", "Safety", and "Standards & Practices" are all very
important parts of working in a lab and in engineering.

Best of luck!


Question:
In the United States, the upcoming autumnal equinox is marked on most calendars as the "first day of autumn." Similarly the solstices are commonly called the "first day of summer" and "first day of winter." However in most other languages -- and even in older discussions in English -- the solstices have names like "Midsummer's Day" and "Midwinter's Day."

Is there some sound reason (e.g. typical temperatures, asymmetrical sunrise/sunset times, thermal inertia, etc.) to consider a solstice as the first day of a new season, rather than the middle?

Also (perhaps less on-topic) is there any record of when the change in nomenclature took place in the U.S.? Are there similar discrepancies in other English-speaking countries or in other cultures?

Explanation: The post is confused about the naming of some seasonal checkpoints, doubting that they may not precisely describe the real season is. The anwser gave a expaination that it may due to a seasonal lag, and gave some ways to reckon seasons.

$\begingroup$

There are three main ways to reckon [ seasons
](http://en.wikipedia.org/wiki/Season) :

  * **Solar.** The 3 months with the greatest insolation are designated summer, so the solstices and equinoxes fall in the middle of the season. In the Celtic calendar, summer started on 1 May; in the traditional Chinese calendar it is [ 5-7 May ](https://en.wikipedia.org/wiki/Lixia) . The cultural 'Midsummer' and 'Midwinter' festivals reflect this.
  * **Astronomical.** The solstices and equinoxes are the starts of the seasons, with summer starting on about 21 June ( [ it varies ](http://en.wikipedia.org/wiki/Solstice) ). Its use dates back at least to the Romans, and it's used in most western cultures today (apparently not in Russia, Australia, or New Zealand).
  * **Meteorological.** This is what professional meteorologists use (e.g. [ UK Met Office ](http://www.metoffice.gov.uk/climate/uk/summaries/2014/summer) ), and is based on the prevailing weather on land at mid-latitudes. Seasons are 3 months long and summer starts on 1 June.

I've tried to illustrate the relationships with insolation and temperature
here:

![Different ways to reckon seasons](https://i.sstatic.net/gWNb5.png)

There are some other ways too:

  * **Ecological.** Scientists who study the behaviour of organisms (hibernation, blooming, etc.) adapt to the local climate, [ sometimes using 6 seasons ](http://seasons2011.blogspot.ca/2011/06/ecological-seasons.html) in temperature zones, or only 2 in polar and tropical ones.
  * **Agricultural.** This would centre around [ the growing season ](http://en.wikipedia.org/wiki/Growing_season) and therefore, in North America and Europe at least, around frost.
  * **Cultural.** What people think of as 'summer', and what they do outdoors (say), generally seems to line up with local weather patterns. In my own experience, there's no need for these seasons to even be 3 month long; When I lived in Calgary, summer was July and August (hiking), and winter was December to March (skiing). [ Here's another example ](http://www.bom.gov.au/iwk/yawuru/index.shtml) of a 6-season system, and [ a 3-season system ](http://www.bom.gov.au/iwk/maung/index.shtml) , from the Aboriginal people of Australia, all based on weather.

Why do systems with later season starting dates prevail today? Perhaps because
at mid-latitudes, the [ seasonal lag
](http://en.wikipedia.org/wiki/Seasonal_lag) means that the start of seasonal
weather is weeks later than the start of the 'insolation' period. In a system
with no [ heat capacity ](http://en.wikipedia.org/wiki/Heat_capacity) , there
would be no lag. In systems with high heat capacity, like the marine
environment, the lag may be several months (Ibid.). Here's what the lag looks
like in three mid-latitude cities:

![Seasonal lag. Licensed CC-BY-SA by EOEarth](https://i.sstatic.net/jOEgw.png)

The exact same effect happens on a [ diurnal
](http://en.wikipedia.org/wiki/Diurnal_temperature_variation) (daily) basis
too — the warmest part of the day is often not midday (or 1 pm in summer). As
with the seasons, there are lots of other factors too, but the principle is
the same.

These aren't mutually exclusive ways of looking at it — there's clearly lots
of overlap here. Cultural notions of season are surely rooted in astronomy,
weather, and agriculture.


Question: Why is the sea level in Hudson Bay decreasing so much? Hudson Bay is pretty far up north, much closer to glaciers. Would it make sense for it to recede at this level with sources of fresh water relatively close?

Explanation: The post is confused why the relative sea level falls in hudson bay. The anwser reveals that it's a phenomenon of post-glacial isostatic rebound.

$\begingroup$

**The area is experiencing[ post-glacial isostatic rebound
](https://en.wikipedia.org/wiki/Post-glacial_rebound) . **

Much of Canada was covered in an extensive ice sheet in the [ last glacial
period ](https://en.wikipedia.org/wiki/Last_glacial_period) (the 'Ice Age'),
from about 110 ka until 12 ka. The ice in the Hudson Bay area was among the
last to melt:

![Ice retreat in North America](https://i.sstatic.net/FEFew.png)

A thick ice sheet depresses the crust (the lithosphere), making a small dent
in the uppermost mantle (the [ asthenosphere
](https://en.wikipedia.org/wiki/Asthenosphere) ) in the process. Well, not
that small: p 375 in [ Gornitz
](https://books.google.ca/books?id=yRMgYc-8mTIC) (2009, _Encyclopedia of
Paleoclimatology and Ancient Environments_ ) says it could be 800 m for a 3000
metre-thick ice sheet!

Since the asthenosphere is highly viscous, it takes a long time time for the
depression to 'bounce' back up. This map from Natural Resources Canada shows
the current rate:

![Isostatic rebound rate in Canada](https://i.sstatic.net/kd0gN.jpg)

Since _global_ sea-level is [ currently rising
](https://en.wikipedia.org/wiki/Current_sea_level_rise) at about 3 mm/a, a
_local_ uplift at this rate will break even. Anything more will result in
relative sea-level fall, as we see in Hudson Bay (as well as in Scandinavia,
the UK, Alaska, and elsewhere — [ this map is wonderful
](https://tidesandcurrents.noaa.gov/sltrends/sltrends.html) ).

Interesting, for geologists anyway, is the sedimentological record this
leaves. I love this example of raised beaches and a small delta experiencing [
forced regression
](https://www.sepmstrata.org/Terminology.aspx?id=forced%20regression) on the
shores of Hudson Bay:

![Forced regression of small delta](https://i.sstatic.net/5ugZd.jpg)

Last thing — you asked:

> Would it make sense for it to recede at the level that it is receding with
> sources of freshwater relatively close?

Since Hudson Bay is connected to the world's ocean, mainly through Hudson
Strait, the runoff into the Bay has no measurable effect on the water level.

**Credit** [ Ice retreat map
](https://commons.wikimedia.org/wiki/File:Retrait_des_glaces_en_Am%C3%A9rique.svg)
by TKostolany, licensed CC-BY-SA. Rebound map by NRCan, free of copyright.
Google Maps image contains own credit.


Question:
Are clouds a solid, liquid, or gas?

I have been looking online and they are often described ambiguously as a "mass". For instance, from NASA:

A cloud is a mass of water drops or ice crystals suspended in the atmosphere. Clouds form when water condenses in the sky. The condensation lets us see the water vapor. There are many different types of clouds. Clouds are an important part of Earth’s weather and climate.

Since they describe it as a vapor, that makes me think it is indeed a gas. But condensation is by definition the change of state from a gas to a liquid. So that makes it sound like it could be a liquid, or contain liquid droplets rather than "freely moving" molecules that typically make up a gas.

But at another web site, which doesn't seem all that authoritative, it says that clouds are a solid, liquid, and gas simultaneously!

A cloud is a liquid, a solid, and a gas.

That does seem intriguing. If I remember correctly, liquids are supposed to be incompressible, and clouds are likely compressible.

Explanation: The post wondered that whether the cloud is gas/liquid/solid. The anwser gives a explaination that the cloud is a mixture of them

$\begingroup$

The cloud that you see is a mixture of solids and liquids. The liquid is water
and the solids are ice, [ cloud condensation nuclei
](https://en.wikipedia.org/wiki/Cloud_condensation_nuclei) and ice
condensation nuclei (tiny particulates that water and ice condense on). The
invisible part of clouds that you cannot see is water vapor and dry air. The
majority of the cloud is just plain air in which the invisible water vapor is
mixed with and the very tiny water drops and ice particles are suspended in.

A cloud is a mixture of gas, liquid and solids.


Question:
Aside from the fraction of water stored as ice on land and temperature of the water, are there other factors that change sea level, and if so what are is the magnitudes of the these changes?

For example, by how much does sediment and soluble matter entering the ocean change sea level? What about volcanoes and tectonic activity? Is there a tendency toward hydrostatic equilibrium where the Earth is entirely covered by an ocean of uniform depth?

Explanation: The post wants to know other reasons that causes rising of sea level. The anwser lists some examples.

$\begingroup$

Yes, there are lots of other factors.

Factors affecting sea levels are no different from other natural processes:
there is a large number of coupled, non-linear effects, operating on every
time scale, and at every length scale, and across many orders of magnitude.

The Wikipedia page [ Current sea level rise
](http://en.wikipedia.org/wiki/Current_sea_level_rise) lists many of the known
processes. And I wrote a blog post, [ Scales of sea-level change
](http://www.agilegeoscience.com/journal/2011/4/11/scales-of-sea-level-
change.html) , a couple of years ago with a long list, mostly drawn from Emery
& Aubrey (1991). Here's the table from it:

![Factors affecting global sea levels](https://i.sstatic.net/hqb0B.png)

**Reference** Emery, K & D Aubrey (1991). _Sea-Levels, Land Levels and Tide
Gauges._ Springer-Verlag, New York, 237p.


Question:
We all know that as waves approach the shallow shores, the waves begin to form a characteristic shape. The upper portion of these breaking waves appears to curl forward and downwards over the bottom segment of the wave, before breaking into "white wash". The image below illustrates what this characteristic shape looks like:

So why do waves form this characteristic breaking shape as they approach the shallow shores?

Explanation: The post wants to know why the sea wave is of a certain shape. The phenomenon is wave shoaling, which can be inferred by dispersion relationship.

$\begingroup$

The physical process you describe is known as [ wave shoaling
](http://en.wikipedia.org/wiki/Wave_shoaling) .

At the basic level, waves propagating into shallow water become shorter and
higher, and consequently, steeper. In shallow water, the water particles near
the crest move **forward** faster than those below them. Similarly, the
particles near the trough move **backward** faster than those above them. This
causes strong shearing of the near-surface body of water, eventually forming a
plunging breaker, or a surf wave. For small-slope (linear) and inviscid (no
friction) waves, the above is a consequence of the bottom boundary condition
for the water velocity to be zero at the sea floor.

There are two fundamental and related properties of water waves that
contribute to shoaling. One is the wave frequency $\omega$ remaining constant
as the depth $d$ decreases. Think of this as the conservation of wave crests
at any fixed point. However, the wavenumber $k$ (wavelength $\lambda$) must
increase (decrease) with decreasing depth, as per the [ dispersion
relationship ](http://en.wikipedia.org/wiki/Dispersion_%28water_waves%29) of
water waves (neglecting the effects of viscosity):

$$\omega^{2} = gk\tanh{(kd)}$$

where $g$ is gravitational acceleration. In shallow water, the dispersion
relationship reduces to:

$$\omega = \sqrt{gd}k $$

and phase speed $C_p$ and group velocity $C_g$ are both proportional to the
square root of the water depth:

$$ C_p = \dfrac{\omega}{k} = \sqrt{gd} $$

$$ C_g = \dfrac{\partial\omega}{\partial k} = \sqrt{gd} $$

Individual wave crests propagate with phase speed $C_p$. Wave groups and wave
energy propagate with group velocity $C_g$. Thus, waves entering shallow water
become **shorter** and **slower** . The second important property leading to
shoaling is the conservation of wave energy flux, which is proportional to the
group velocity and wave energy:

$$\dfrac{\partial(C_{g}E)}{\partial x}=0$$

Because the group velocity decreases, the wave energy (read: height) must
increase (locally). This causes the waves to grow in height as they enter
shallow water. As pointed out by @IsopycnalOscillation in a comment above, the
separation of water particles from the wave crests happen because the
individual orbital velocities at the top of the crest exceed by far the phase
speed of the wave.

Although bottom friction is significant and non-negligible in shallow water,
it does not cause shoaling. Mathematically, wave shoaling can occur for
completely inviscid and small-slope (linear) waves that propagate into water
of decreasing depth.


Question:
Messing around on Google Earth recently I noticed a number of striations in the Eastern Pacific. These appear in an East-West orientation and seem to start on the North and South American continental shelves, and extend for roughly half the Pacific Ocean. For example one of these striations start in Santa Rosa Island off California and ends at Hawaii. These striations also appear to be roughly equally spaced at 8 degree intervals. The North and South American striations are angled with respect to each other and seem to converge at roughly Tahiti.

What causes these? I'm a fascinated novice.

Explanation: The post wants to knwo what the ridges in the pacific ocaean are. They are fraction zones actually

$\begingroup$

**They are[ fracture zones ](https://en.wikipedia.org/wiki/Fracture_zone) . **

I've annotated your image with the names of these very long, tectonically
important features. They even have names, such as the [ Mendocino Fracture
Zone ](https://en.wikipedia.org/wiki/Mendocino_Fracture_Zone) :

[ ![Fracture zones in the northeastern
Pacific](https://i.sstatic.net/ceZqX.jpg) ](https://i.sstatic.net/ceZqX.jpg)

I also labelled some survey tracks (rather subtle, very straight, very regular
in width), which are data artifacts — these are the things you noticed
radiating from the vicinity of Los Angeles.

There are yet other types of linear feature on the sea floor:

  * [ Transform faults ](https://en.wikipedia.org/wiki/Transform_fault) are strike-slip faults connecting segments of spreading centres in the oceanic crust.
  * The [ spreading centres ](https://en.wikipedia.org/wiki/Mid-ocean_ridge) themselves.
  * Roughly linear archipelagos like Hawaii.
  * The long, curved trenches at [ convergent plate margins ](https://en.wikipedia.org/wiki/Convergent_boundary) .

_Images from Google Maps. Note: I edited this answer substantially after the
OP clarified the question... and I learned about[ the difference between
transform faults and fracture zones
](http://www.columbia.edu/~vjd1/MOR_transforms.htm) . _


Question: If I have wind speeds at 80m and 120m above the surface and I am interested in approximating the speed at 100m, is it valid to just average the 80 and 120 speeds? In other words, should I expect wind speed to be a linear function of height or would a nonlinear function be more appropriate?

Explanation: The post wants to know the speed when different winds combines togeter. The anwser gives a explaination based on wind gredient formulas

$\begingroup$

The content below is based on expressions found in [ Wind gradient
](https://en.wikipedia.org/wiki/Wind_gradient) from Wikipedia. See that link
for more background information.

Wind speed as a function of height can be modeled with the following
expression:

$$v_2 = v_1 \left(\frac{h_2}{h_1}\right)^a$$

Where  $v_1$  and  $v_2$  are the wind speeds are heights  $h_1$  and  $h_2$
,  $a$  is a parameter related to the atmospheric stability.

Given your two values for wind speed at different height we want to solve for
$a$  :

$$a = \frac{\ln(v_2/v_1)}{\ln(h_2/h_1)}$$

You can now use this value, along with the original height / wind speed pairs
to evaluate the wind speed at a different height.

The model will likely be best when the evaluated hight is between the points
used to compute  $a$  , basically interpolation will be better than
extrapolation.


Question:
Egypt is planning to create a new delta on the left of the old one by building an artificial river that redirects agricultural waste water into the desert. The water should gradually turn the desert into land that's suitable for agriculture. Here's a video that explains the plan further.

This ambitious plan will cost around 9 billion euro's to complete. What I don't understand about it though is how simply redirecting water to a desert can turn it into arable land. The Egyptian desert consists of sandy soil and this soil type usually lacks the nutrients necessary for crops to grow. So how will this work?

Explanation: The post wants to figure out how the aritificial river can make desert arable land. The anwser reveals that not only it provides water, but also will lead some biological precession

$\begingroup$

Such projects take time to fully achieve their aims. In addition to providing
water, the sand will need to undergo a process of [ soil conditioning
](https://en.wikipedia.org/wiki/Soil_conditioner) . This will include the
progressive addition of [ biomass ](https://en.wikipedia.org/wiki/Biomass) , [
humus ](https://en.wikipedia.org/wiki/Humus) and compost. This will include a
means of water retention, which may include the application of clay or slit
either mechanically or via the outflow of the Nile River.


Question:
According to textbook knowledge, the mass of the earth is about 6×1024kg
. How is this number determined when one cannot just weight the earth using regular scales?

Explanation: The post wants to find the mass of the Earth. By using the law of universal gravitation and law of motion, we can solve that.

$\begingroup$

According to **Newton's Law of Gravity** based on attractive force
(gravitational force) that two masses exert on each other:

$$F=\frac{GmM}{r^2}$$

Where:

  * $F$  is the gravitational force
  * $G = 6.67 \times 10^{-11}\ \mathrm{m}^3\ \mathrm{kg}^{-1}\ \mathrm{s}^{-2}$  is a constant of proportionality
  * $M$  and  $m$  are the two masses exerting the forces
  * $r$  is the distance between the two centers of mass.

From **Newton's second law of motion** :

$$F=ma$$

Where:

  * $F$  is the force applied to an object
  * $m$  is the mass of the object
  * $a$  is its acceleration due to the force.

**Equating both the equations** :

$$F = \frac{GmM}{r^2} = ma$$

$$\frac{GM}{r^2}= a$$  (The  $m$  's canceled out.)

Now solve for  $M$  , the mass of the Earth.

$$M = \frac{ar^2}{G}$$

Where  $a = 9.8\ \mathrm{m}\ \mathrm{s}^{-2}$  ,  $r = 6.4 \times 10^6\
\mathrm{m}$  , and  $G = 6.67 \times 10^{-11}\ \mathrm{m}^3\ \mathrm{kg}^{-1}\
\mathrm{s}^{-2}$  .

$$M = 9.8 \times (6.4 \times 10^6)^2/(6.67 \times 10^{-11})\ \mathrm{kg}$$

* * *

Hence,

##  $M = 6.0 \times 10^{24}\ \mathrm{kg}$


Question:
My question refers to the current process of climate change. CO2 is rising, which leads to the greenhouse effect, which raises temperatures. This leads to more wildfires, which reduces number of trees, increasing CO2 and reducing CO2 capacity absorption. Ice caps start to melt, which reduces sunlight reflection (less snow), trapping more heat on atmosphere. Water rises, taking over land and trees, further enhancing the CO2 absorption capacity. Oceans acidify, lowering their CO2 absorption capacity too. Etc etc.

It seems the process of climate change is a "vicious circle", with a lot of feedback loops reinforcing the trends.

Is this the case? Are there counteracting forces that go against this circle?

Explanation: The post wants to know whether the climate change is purely a negative phenomenon. The anwser gives some pros and cons of climate change, indicating that it's not just a negative thing.

$\begingroup$

There are indeed a lot of positive feedback mechanisms, i.e. a warm climate
leads to a warmer climate. From [ this Wikipedia article
](https://en.wikipedia.org/wiki/Climate_change_feedback) , they are:

  * Carbon cycle feedbacks
  * Cloud feedback
  * Gas release
  * Ice-albedo feedback
  * Water vapor feedback

However, there are also a few negative feedbacks (same source):

  * Blackbody radiation
  * Carbon cycle
  * Lapse rate
  * Impacts on humans

Now the question is: what is the net budget between positive and negative
feedbacks? To assess this, climatologists use some metrics, the main ones
being "transient climate response" (TCR) and "equilibrium climate sensitivity"
(ECS). From [ Knutti et al. (2017) ](https://doi.org/10.1038/ngeo3017) :

> TCR is defined as the global mean surface warming at the time of doubling of
> CO  $_2$  in an idealized 1% yr  $^{−1}$  CO  $_2$  increase experiment, but
> is more generally quantifying warming in response to a changing forcing
> prior to the deep ocean being in equilibrium with the forcing. Based on
> state-of-the-art climate models, and instrumentally recorded warming in
> response to CO  $_2$  and other anthropogenic and natural forcings, the
> Intergovernmental Panel on Climate Change's Fifth Assessment Report (IPCC
> AR5) assessed that the transient climate response is 'likely' (>66%
> probability) to be in the range of 1 °C to 2.5 °C.
>
> By contrast, the equilibrium climate sensitivity (ECS) is defined as the
> warming response to doubling CO  $_2$  in the atmosphere relative to pre-
> industrial climate, after the climate reached its new equilibrium, taking
> into account changes in water vapour, lapse rate, clouds and surface albedo.
> [...] The estimated range of ECS has not changed much despite massive
> research efforts. The IPCC assessed that it is 'likely' to be in the range
> of 1.5 °C to 4.5 °C.

Which basically means that the climate will get warmer in the future, until it
will eventually reach some kind of equilibrium.


Question: I've seen models in astronomy that show how the Earth-Moon system must have come together after a collision. However, I have not heard whether there is any actual physical evidence on Earth that points to a prior collision. Is there geological (or other physical) evidence here on Earth that confirms the moon originated from a collision on Earth? If so what is that evidence?

Explanation: The post wants to know if there are any evidence which may proov the moon collided with the earth once. The fact that the collision is only a theory.

$\begingroup$

> Is there geological (or other physical) evidence here on Earth that confirms
> the moon once collided with the Earth?

No, there isn't. This is, however, plenty of evidence that the moon formed due
to a collision of a third body (sometimes referred to as Theia) with the
Earth, and the moon formed from the ring of debris that resulted from the
collision.

This theory is often known as the _[ Giant Impact Hypothesis
](http://en.wikipedia.org/wiki/Giant_impact_hypothesis) _ and searching for
this term may help you find other links and references elsewhere.

> If so what is that evidence?

To summarise the geological evidence, much of it is **indirect** evidence, in
that it compares geology on the Moon with similar geology or features on the
Earth, and draws conclusions to explain the similarities (or discrepancies).

Because of the sheer size of the proposed impact, it would have likely
reconfigured the surfaces of both bodies entirely, and so finding direct
physical evidence on the Earth would be extremely unlikely. (e.g. a massive
hidden crater would simply no longer exist)

##  Geological Evidence

  * Moon rocks collected from the Apollo missions that have almost identical oxygen isotope ratios to similar rocks found on Earth of the same age.
  * A large portion of the Lunar crust is made up of Anorthosite, which is indicative of a large melting event. (with the energy for this supplied from the impact)
  * Zinc. Lunar rocks contain less zinc, but with heavier _isotopes_ of Zn than those found on Earth, which by contrast has lighter isotopes in greater abundance. This is consistent with zinc being depleted from the moon by evaporation, such as during a massive impact event.

  * Density and volatiles. The Moon is 25% less dense than the uncompressed density of Earth. It is severely depleted in volatiles, with practically no water and less than half the potassium abundance that Earth has. The combination of low density and lack of volatiles implies that the Moon was not a simple accretion of early solar system material, but resembles the Earth's mantle in bulk composition. Volatile material would have been vapourised by the impact.

  * The bulk composition of the Moon's crust. (This one does not actually involve the Earth, but I feel it is still important to mention.) The Moon's mantle and crust chemical composition _could_ be explained if the Moon had a large iron core, but its core is actually quite small.

##  Other Physical Evidence

  * The ratio of the Earth and Moon's mass far exceeds any other planet in the solar system, and this begs the question of how did so much material become in orbit of the Earth. (Not evidence as such, but raises the question in the first place)

  * Getting more indirect...there are bands of warm silica-rich dust orbiting nearby stars which is interpreted as planet-sized bodies having collided with each other, so there is precedent for similar events happening elsewhere in the galaxy. (Again, I realise this is one is not strictly answering the question, but Earth Scientists often have to look well outside the Earth to answer their questions!)

Hope this gets you started!

**Update:** I recently came across [ this short article
](http://www.ox.ac.uk/news/science-blog/where-did-moon-come) , which provides
a summary of some of the latest thinking on the Moon's formation, including
the Giant Impact Hypothesis.

_Sources:[ de Pater and Lissauer (2010): Planetary Sciences
](http://books.google.co.uk/books/about/Planetary_Sciences.html?id=RaJdy3_VINQC)
; [ Canup and Asphaug (2001), Nature
](http://www.es.ucsc.edu/~rcoe/eart206/canup_Moon_Nature_01.pdf) _


Question:
I am procedurally generating planets for an open world space sandbox game. I am using a rough simulation of tectonic plates to create mountain ranges and other geological features.

A planet surface consists of several plates, which consists of tiles. The continental plates are randomly generated, with random initial elevation and drift velocities assigned.

Tile elevations are generated by multiplying an envelope ( e^{ -d^2 } ) with a function for different types of plate boundaries and relative drift velocities.

For the sake of realism, is it possible to have a land-ocean divergent plate boundary? I could not find anything online. There also doesn't seem to be any examples on Earth from what I could find. Is it safe to assume that divergent boundaries only occur between land-land and ocean-ocean plate boundaries?

Explanation: The post wants to generate some ocean-continent boundaries. The anwser is no since the boundaries are formed themselves.

$\begingroup$

The oceanic plates are themselves formed from the [ divergent boundary
](https://en.wikipedia.org/wiki/Plate_tectonics) , so probably not. Even if a
new rifting occurred exactly at the boundary, the result would eventually be
that the ocean floor surrounds the divergent boundary.

A very simplified model of an ocean would have a [ divergent boundary
](https://en.wikipedia.org/wiki/Mid-ocean_ridge) in the middle. This oceanic
spreading center might have basaltic volcanic islands along it, and two
mirrored plates on each sides, pushing towards neighboring continental or
oceanic plates.

[ ![From wikipedia, as usual](https://i.sstatic.net/OVQRD.png)
](https://i.sstatic.net/OVQRD.png)

I'm trying to think about any oddity that would fit your description, but
can't really come up with a good example. Locally you might have the
conditions in [ triple junctions
](https://en.wikipedia.org/wiki/Triple_junction) eg. at Afar Triangle.
Somehow, it could also describe parts of New Zealand and eastern
Mediterranean, but it's very complex areas with more parameters controlling
the rifting.

Earth is not controlled by any simple equation, its formed by 4.56 billion
years of interaction between rocks, magma, life, atmosphere and even
astronomic influence, so I'm not sure that divergent boundary never occurred
between existing continental and oceanic plates, but at least it's very
uncommon and couldn't last long.

A way to understand plate tectonics better, and maybe even a good inspiration
for world building games, is to have a look at the (free) program [ Gplates
](https://www.gplates.org/) that is used by students and scientists to
simulate tectonic history. Don't worry about your ignorance, interest is more
important than knowledge. The game might be a good inspiration for someone to
learn more about something really [ awesome
](https://www.google.com.au/imgres?imgurl=https%3A%2F%2Fwww.nasa.gov%2Fsites%2Fdefault%2Ffiles%2F1-bluemarble_west.jpg&imgrefurl=https%3A%2F%2Fwww.nasa.gov%2Fcontent%2Fgoddard%2Fearth-
from-space-15-amazing-things-
in-15-years&docid=t97PKMHHmGUgVM&tbnid=I6mUIX2BemSxbM%3A&vet=1&w=2048&h=2048&noj=1&client=firefox-b&bih=1082&biw=1831&q=earth%20from%20space&ved=0ahUKEwi4sYL4u87RAhWKHZQKHUr8B_QQMwgzKAMwAw&iact=mrc&uact=8)
.


Question: I remember Journey to the Center of Earth and wonder: What is the deepest in the Earth surface or below sea level we have traveled either by foot, sub, drill or camera?

Explanation: The post wants to know the deepest surface that human have traveled to in the earth. The anwsers is the Mponeng Gold Mine in South Africa

$\begingroup$

Probably a bit over 4 km, in this South African mine: [
https://en.wikipedia.org/wiki/Mponeng_Gold_Mine
](https://en.wikipedia.org/wiki/Mponeng_Gold_Mine) But as the link mentions,
the mine operators go to considerable lengths to reduce the mine temperature
to endurable levels from the 66°C/151°F of the surrounding rock.

Note: This answer is for the original question, where the OP asked for the
deepest depth below the surface. It's since been changed to ask for depth
below sea level.

[ ![enter image description here](https://i.sstatic.net/VwkFT.jpg)
](https://i.sstatic.net/VwkFT.jpg)

[ http://cracked.tumblr.com/post/162183647834/the-mponeng-gold-mine-in-south-
africa-is-the ](http://cracked.tumblr.com/post/162183647834/the-mponeng-gold-
mine-in-south-africa-is-the)


Question:
In the movie Into the Storm (2014) near the end, storm chaser Pete sees the eye of a massive tornado.

In 1928 (real life), Will Keller was in his barn when a huge tornado passed through. He reported seeing an opening in the center of the tornado "about 55 feet (17 meters) across and extended some 2,500 feet (762 meters) up." Source

Is there any "official" documentation or evidence that tornadoes, especially stronger ones, have eyes like a hurricane does? Or is it just an urban legend?

Explanation: The post wants to know whether the tornado has an eye in it. By coriolis function we can compute that.

$\begingroup$

Yes, if one takes the common meaning of the term [ "eye of the storm"
](http://glossary.ametsoc.org/wiki/Eye) to be the area of relatively low wind
speed near the center of the vortex, most tornadoes can be said to have eyes.
Cyclostrophic balance describes a steady-state, inviscid flow with neglected
Coriolis force:

$$ \dfrac{v^2}{r} = -\dfrac{1}{\rho}\dfrac{\partial p}{\partial n} $$

where centripetal force balances radial pressure gradient. Here, $v$ is
tangential wind speed, $r$ distance from vortex center, $\rho$ is air density,
$p$ is atmospheric pressure and $n$ is the radial direction pointing inward.
From here, tangential wind speed is simply:

$$ v = \sqrt{-\dfrac{r}{\rho}\dfrac{\partial p}{\partial n}} $$

suggesting that $v\to0$ when $r\to0$. While the flow in tornadoes is highly
non-stationary and subject to friction, this idealized model shows why there
must exist an "eye" inside a vortex or an area of closed circulation. This
"eye" may or may not be easily recognized by a hypothetical human observer
inside a tornado.


Question:
I have heard from many people that sinks do not empty in a particular pattern depending on what hemisphere you are in, but I have also heard from people who adamant that a sink of water would empty clockwise in one hemisphere and anti-clockwise in another.

While I acknowledge the above idea is probably a myth, is there any simple experiment that one can do do determine what hemisphere they are in, utilizing the Coriolis effect, or otherwise?

Explanation: Besides using Coriolis effect, the author wonder if there is anthoer simple ways to find the hemisphere you are in. The solution is Foucault pendulum.

$\begingroup$

You can use the [ Foucault pendulum
](http://en.wikipedia.org/wiki/Foucault_pendulum) to determine the hemisphere:
Its plane of movement rotates:

  * anti-clockwise in the southern hemisphere;
  * clockwise in the northern hemisphere.

The rotation of the plane can be explained by the Coriolis force.


Question:
Are there any estimates of the amount of Uranium there is in Earth's Crust?

From what I know, it's supposed that there are large amounts of Uranium in Earth's Core, the decay of which is responsible for the core maintaining its high temperature. Mining the core is hardly imaginable, while the resources in Crust are more accessible available for mining: so how much is in the Crust, and at what levels of concentration? (which affects the viability of accessing it).

Explanation: The post wants to know how much Ur  in the Earth. The anwser gave some estimations in different perspectives.

$\begingroup$

According to wikipedia, there are around 5.5 million tonnes of [ uranium in
ore deposits ](https://en.wikipedia.org/wiki/Uranium#Resources_and_reserves)
that are commercially viable at current prices, and perhaps 35 million tonnes
that are potentially viable if prices increase.

Also according to wikipedia, [ the Earth's crust
](https://en.wikipedia.org/wiki/Uranium#Biotic_and_abiotic) (to 25 km depth)
contains an estimated 10^14 tonnes (100 trillion tonnes), while the oceans may
contain 10^10 tonnes (10 billion tonnes). This presumably includes the ore
figures stated above.

The previous link states that "The decay of uranium, thorium, and potassium-40
in the Earth's mantle is thought to be the main source of heat", however no
estimate is given for quantities. Allègre, Lewin and Dupré (1988) state that
"The concentration of U in the primordial mantle (bulk Earth) has been
determined to be ~21±1 ppb". Back of the envelope calculations would then give
us that the [ mantle ](https://en.wikipedia.org/wiki/Mantle_%28geology%29) is
84% of the [ Earth ](https://en.wikipedia.org/wiki/Earth) by volume (probably
slightly less than that by mass?), and the mass of the earth is 6*10^21
tonnes, which would give us, very approximately, 0.84 * (6*10^21) *
(2.1*10^-8) ~= 10^14 tonnes, or roughly the same as is in the Earth's crust.

  * Allègre, C.J., Lewin, E. & Dupré, B., 1988. [ A coherent crust-mantle model for the uranium-thorium-lead isotopic system. ](https://www.sciencedirect.com/science/article/pii/0009254188900940) Chemical Geology, 70(3), pp.211–234.


Question:
Someone shared a video with me in which clouds were forming a ring around the Sun. I took this screen shot of that video:

What is the reason behind this?

Explanation: The post wants to know what the ring around the sun is. It's the 22 degree ring

$\begingroup$

This optical phenomenon is called a [ 22° halo
](https://en.wikipedia.org/wiki/22%C2%B0_halo) which is a subset of other [
halos ](https://en.wikipedia.org/wiki/Halo_\(optical_phenomenon\)) . This
arises from sunlight refracting through hexagonal ice crystals, which can be
found in high level cirrus clouds. Light that would otherwise not make it to
your eye enters an ice crystal and then exits at an angle of approximately 22
degrees. This produces the arc of light you see in the video. You see
(inverted) rainbow coloring of the halo because the light is not uniformly
refracted but varies from 21.7 degrees for red photons to 22.5 degrees for
violet photons.

![enter image description here](https://i.sstatic.net/4gojQ.png)
Image by donalbein, Wikemedia Commons, CC-By-SA-2.5 [
https://commons.wikimedia.org/wiki/File:Path_of_rays_in_a_hexagonal_prism.png
](https://commons.wikimedia.org/wiki/File:Path_of_rays_in_a_hexagonal_prism.png)

There are many more optical phenomenon you can see, but all are based around
sunlight and the optical properties of ice crystals and water droplets. Here
is a phenomenal picture taken in winter showing many examples:

![enter image description here](https://i.sstatic.net/kv1zw.png)


Question:
It is said about ozone:

a layer in the earth's stratosphere at an altitude of about 10 km (6.2 miles) containing a high concentration of ozone

Over the Earth’s surface, the ozone layer’s average thickness is about 300 Dobson Units or a layer that is 3 millimetres thick. (nasa)

So, how does the Ozone gas stays in that altitude without dissolving?

Explanation: The post wonder how the ozone stay in the place it should be. The reason behind it is a series of chemical reaction.

$\begingroup$

For the ozone layer to dissolve, it would need something to bind to. [
Chloroflurocarbons ](https://en.wikipedia.org/wiki/Chlorofluorocarbon) are one
very notable thing that it can bind to, but that doesn't exactly answer the
question.

While the ozone layer would be 3 mm thick if it was compressed down to the
pressure of earth's surface, since it is further up in the atmosphere, it can
take up a larger volume (like 20 km thick). The ozone layer is maintained by
the [ Chapman cycle ](http://glossary.ametsoc.org/wiki/Chapman_mechanism) .

That is, the oxygen molecule (  $\ce{O_2}$  ) is dissociated by UV radiation
(photolysis) to oxygen atoms (  $\ce{2O}$  ). Then the photolyzed oxygen atoms
(  $\ce{2O}$  ) can bond with two oxygen molecules (  $\ce{2O_2}$  ) to make
an two ozone molecules (  $\ce{2O_3}$  ), provided another molecule can take
away some excess energy.

Since it is a cycle, there has to be a way for the ozone to return to oxygen.
One ozone molecule (  $\ce{O_3}$  ) can photolyze to make an oxygen atom (
$\ce{O}$  ) and oxygen molecule (  $\ce{O_2}$  ). The oxygen atom (  $\ce{O}$
) can react with another ozone molecule (  $\ce{O_3}$  ) to make two oxygen
molecules (  $\ce{2O_2}$  ).

To just answer your question directly, the ozone layer is replenished by the
sun's UV energy reacting with the oxygen in the atmosphere.


Question:
This question asked: What is the evidence it is feasible to reverse ocean acidification by adding large quantities of a base (bicarb soda)?

The result was:

Danny Harvey of the University of Toronto has already looked into this. His solution is to deposit 4 Gt/a of limestone into the oceans. To put this in perspective, global coal production in 2013 was nearly double this at 7.823 Gt and it is more than global iron ore production in 2014 of 3.22 Gt.

The respondent then posed the question: Are there sufficient quantities of easily obtainable limestone to do this?

My question is: Are there sufficient quantities of limestone to dump in the ocean to reverse acidification?

Explanation: The post intends to find out limestone's quantity to stop the ocean from being acid. The anwser gave a way to estimate, but it's really hard.

$\begingroup$

Getting figures on the amount of limestone available is difficult.

Apparently " [ limestone ](https://en.wikipedia.org/wiki/Limestone) makes up
at least 10% of the total volume of all sedimentary rocks".

One way to answer your question is by inference. [ Cement
](https://en.wikipedia.org/wiki/Cement) is manufactured from limestone.
Current [ global production of cement
](http://www.statista.com/statistics/373845/global-cement-production-
forecast/) is in excess of 3.27 Gt/a and by 2030 it is forecast to be
approximately 4.83 Gt/a. Thus if all limestone currently earmarked for cement
production were diverted to neutralizing the acidity of the world's oceans it
could be done at short notice. How sustainable it would be over the 200 years
required, is another matter. It also raises the question of what happens to
all the industries and projects that current rely on a cheap and ready supply
of cement?

With much tongue in cheek, the Florida peninsular in the US is predominantly
limestone, likewise a significant portion of eastern England, with the "white
cliffs of Dover, the [ karst ](https://en.wikipedia.org/wiki/Karst) regions of
the eastern Adriatic, southern Europe, southern and eastern Asia and the
Nullabor Plain in southern Australia. Would these countries be will to
sacrifice their lands to neutralize acidity in the worlds oceans?


Question:
I was just reading this about how Hurricane Ethel could have merged with Hurricane Dora in 1964.

Has such a merge ever happened before in history? If so, what was the result? Would storms become twice as powerful? Or would they disrupt and dissipate each other?

They don't have to be hurricanes or typhoons per se, just large storms. I would think the low pressure regions of two storms would tend to attract each other if they were nearby, but it's apparently rare or unheard of because a quick google search showed nothing.

Explanation: The post intends to figure out some merged hurricanes. The anwser is yes, and the phonomenon can be explained by  Fujiwhara effect.

$\begingroup$

Yes two hurricanes/tropical cyclones/typhoons can merge with each other and
the effect is known as Fujiwhara effect- [ Fujiwhara effect
](https://en.wikipedia.org/wiki/Fujiwhara_effect) .

The National Weather Service defines the Fujiwhara effect as "Binary
Interaction where tropical cyclones within a certain distance(300-375 nautical
miles depending on the size of the cyclones) of each other begin to rotate
about a common midpoint". What really happens is that centers of both systems
begin to orbit in a counter clockwise direction about a midpoint that is
determined by the relative mass and cyclone intensity. Eventually the smaller
cyclone may merge into the larger cyclone. There are several examples of the
Fujiwhara effect and one example would be Hurricane Connie [ Hurricane Connie
](https://en.wikipedia.org/wiki/Hurricane_Connie) and Diane [ Hurricane Diane
](https://en.wikipedia.org/wiki/Hurricane_Diane) way back in 1955. Shimokawa
et al [ Fujiwhara Effect Types ](http://dil-
opac.bosai.go.jp/publication/nied_natural_disaster/pdf/45/45-02E.pdf) talk
about the various kinds of interactions that can take place among various
typhoons(Please note that the Fujiwhara effect is not restricted to two
systems). The various kinds of interactions are

  * Complete Merger
  * Partial Merger
  * Complete Straining Out
  * Partial Straining Out
  * Elastic Straining Out

Complete straining and complete merger interactions lead to destruction of one
of the vortices. Partial merger and partial straining lead to partial
destruction of one vortex and elastic straining is an interaction in which
both vortices survive with their initial circulation. Partial merger and
partial straining out have received less attention in the literature on binary
tropical cyclone interactions as the interactions are extremely complex.
Prieto et al claim that during a partial merger repeated mass exchanges occur
between vortices. As these are nonlinear effects a quantification is only
possible by a direct numerical integration and precise initial condition [
Binary TC Vortex Like Interactions
](http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493%282003%29131%3C2656%3AACOBTC%3E2.0.CO%3B2)

The period of orbit maybe as small as one day or there are others such as
Cyclone Kathy and Cyclone Marie [ Kathy/Marie Fujiwhara
](https://en.wikipedia.org/wiki/1964_Pacific_typhoon_season#Typhoon_Marie_.28Undang.29)
orbited for a period of 5 days prior to merging into each other as pointed out
by [ Lander et al
](http://twister.ou.edu/QJ/CD3-1990-1995/1993/v119n514/s5.pdf?origin=publication_detail)
. If the period of orbit is longer then there is a greater probability of a
merger.

Region wise binary cyclones are more common in the Western North Pacific than
the North Atlantic as pointed by Dong et al [ Relative Motion Of Binary
Tropical Cyclones
](http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493%281983%29111%3C0945%3AOTRMOB%3E2.0.CO%3B2)
Regarding the predictability of the track of binary cyclones Dong et al. state
that prediction of steering forces of a single tropical cyclone are replete
with numerical forecasting uncertainties and the problem is accentuated by the
presence of another tropical cyclone in close proximity. Those who followed
the progress of Hurricane Sandy in late October 2012 (an instance of the
Fujiwhara effect but in this case a tropical cyclone merged with an extra
tropical storm) will remember the ECMWF model correctly predicted the landfall
location.


Question:
I've had an inconclusive chat about the Sveconorwegian belt of the Fennoscandian shield, the western parts in particular, and reasons as to why there would not be any deposits of hydrocarbons there. Perhaps someone here can provide a definite answer?

Is it simply that the belt is of primarily Gothian origin, or might there be more to it?

Explanation:
This post wonders why there's no  hydrocarbons in the Sveconorwegian belt. There are mainly 3 reasons: 1. These old rocks never had a good source rock: too old, too little life.
2. They are so old that any source would have been mature hundreds of millions of years ago.
3. In any case, metamorphosis has destroyed any potential reservoirs, and a combination of tectonism, orogenesis, and exhumation has destroyed any traps.

$\begingroup$

There's a little more to it than its substantial age, but not a lot.

First, a bit of background. We need a few conditions for accumulations of oil
or gas:

  * A [ kerogen ](https://en.wikipedia.org/wiki/Kerogen) -rich source rock that has been 'cooked' to thermal maturity. Insufficient temperature, or insufficient time, and it's undercooked; if the temperature is too high, or the rock is very old, then the volatiles have all been discharged or cracked. I don't know of any source rocks older than about Ordovician; the best ones are Jurassic in age.
  * A migration pathway from the source to a trapping configuration, and time for this to have happened.
  * A [ structural ](https://en.wikipedia.org/wiki/Structural_trap) or stratigraphic trap, such as an anticline, and the geological conditions that have preserved it since filling.
  * A reservoir rock — a porous and permeable rock. Usually this is a sedimentary rock, though hydrocarbons have been discovered in fractured granites (in Vietnam for example; this is rare).

Only 'conventional' hydrocarbon deposits need all these conditions (sometimes
called a [ 'petroleum system' ](http://subsurfwiki.org/wiki/Petroleum_system)
, such as you might find in the North Sea). 'Shale gas' is basically just a
mature source rock — we can extract hydrocarbons from it by fracking, for
example.

As you might guess from the list of conditions, there are lots of things that
can destroy a hydrocarbon accumulation, or preclude its formation. For
example, deep burial destroys porosity by diagenesis and compaction. Time
increases the likelihood of trap leakage or thermal destruction of volatiles.

So, why no hydrocarbons in the (mostly Precambrian) Swedish-Norwegian shield?
Maybe these are the top 3 reasons:

  * These old rocks never had a good source rock: too old, too little life. A bad start.
  * They are so old that any source would have been mature hundreds of millions of years ago.
  * In any case, metamorphosis has destroyed any potential reservoirs, and a combination of tectonism, orogenesis, and exhumation has destroyed any traps.


Question:
I know that helium is a very light and rare gas on Earth because Earths gravity is not strong enough to keep it. Instead, helium and hydrogen are rising through the atmosphere and escape into outer space.

My question is: How massive would Earth have to be so that it could keep helium in the atmosphere? 2, 5, or 10 times the actual mass? Could we, for example, compare it to Neptune or Saturn?

Explanation: The post wants to calculate the 'g' which guarantee helium on earth. It's actually a possiblity distribution.

$\begingroup$

[ Atmospheric escape ](https://en.wikipedia.org/wiki/Atmospheric_escape) is
the loss of planetary atmospheric gases to outer space. You'd never be able to
contain ALL of any gas forever by gravity. Ultimately you end up in the
rarefied atmosphere where there is some probability that a molecule of gas
will reach escape velocity. The probability distribution is given by the [
Maxwell-Boltzmann distribution
](https://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution) and
the thermal escape mechanism is known as the [ Jeans escape
](https://en.wikipedia.org/wiki/Atmospheric_escape#Thermal_escape_mechanisms)
.

On earth the problem is compounded by the fact that helium is lighter that the
other gases. So (1) helium migrates to the upper atmosphere because of its
density and (2) helium atoms on average have the same kinetic energy as oxygen
and nitrogen molecules which means that helium atoms are on average traveling
much faster than oxygen or nitrogen molecules.

All of this is made more complicated by needing a temperature profile of the
atmosphere as a function of the height.

It doesn't help to assume temperature of background radiation because even at
that temperature you can calculate the probability of a helium atom having 80%
of light speed. This sort of like being able to calculate the probability of
throwing $n$ heads in a row regardless of how big $n$ is.


Question:
The magnetic North (or South) Pole moves due to activities in the inner of Earth. And earthquakes can tilt the Earth's orientation a tiny bit. But can and does the axis' orientation relative to the surface change too? A major impact could surely do it, but could the Earth's inner activities?

I don't mean precession, nor tectonics or continental drift, but the movement of the rotation axis. Could for example the geographic North Pole move to Greenland? (Not Greenland moving to the NP)

Explanation: The post wonders whether the earth's geographic north point move. It's a phenomenon called polar motion.

$\begingroup$

**Yes, it's called[ _polar motion_
](https://en.wikipedia.org/wiki/Polar_motion) . **

The rotational pole moves continuously, as you can see from the right-hand
side of this figure (below) by the [ Earth Orientation Centre
](http://hpiers.obspm.fr/eop-pc/) (EOC) and the [ International Earth Rotation
and Reference Systems Service
](http://www.iers.org/IERS/EN/Organization/ProductCentres/EarthOrientationCentre/eoc.html)
(IERS). The figure shows about 650 days of time; ` mjd ` is _modified Julian
day_ and time goes along the locus in the polar motion diagram.

[ ![The daily polar motion](https://i.sstatic.net/lP4kg.png)
](https://i.sstatic.net/lP4kg.png)

The left-hand side of the figure shows Length of Day, and I think that's
chiefly why they are making these refined polar measurements — to keep track
of leap seconds, etc.

It's been established, by Fong et al. (19996), among others, that [
earthquakes could change the earth's rotational axis
](http://www.nasa.gov/topics/earth/features/japanquake/earth20110314.html) ,
by redistributing mass on a large scale. Their table 1 shows [the sort of
effect that individual earthquakes theoretically have — _edited after comment
from David Hammen_ ] on the length of day; $\Psi$ is the excitation vector:

[ ![Table 1 from Fong et al 1996](https://i.sstatic.net/BsKx1.png)
](https://i.sstatic.net/BsKx1.png)

The EOC's website lists [ other geophysical excitations
](http://hpiers.obspm.fr/eop-pc/index.php?index=excitation&lang=en) , among
them:

  * Atmospheric angular momentum
  * Oceanic angular momentum
  * Hydrological excitation function
  * Axial angular momentum of the core

###  References

Chao, Benjamin Fong, Richard S. Gross, Yan-Ben Han (1996). Seismic excitation
of the polar motion, 1977–1993. _Pure and Applied Geophysics_ , September
1996, Volume 146, Issue 3, pp 407-419. [ DOI 10.1007/BF00874727
](http://dx.doi.org/10.1007/BF00874727)

**Update:** [ This article ](http://www.nasa.gov/feature/nasa-study-solves-
two-mysteries-about-wobbling-earth) on some new research in this field is
worth a read.


Question: I was checking the weather forecast just now, and it is showing that it "feels like -999 °C". I never heard or saw -999 °C before. I searched other weather channels, and they were showing that it feels like 2 °C. What is the meaning of -999 °C, irrespective of the weather?

Explanation: The post wonders what the meaning of -999 celsius degree is. It doesn' t mean anything since the absolute zero is higher than it.

$\begingroup$

The value -999 is likely the "fill value" used in the dataset when data is
missing or is not being properly filtered or handled when displayed. In the
specific case on the website you cite, it is likely a problem with the
algorithm for wind chill (the "feels like" temperature this time of year).

It isn't a physical value and only means the value is missing. Furthermore,
-999°C is not a possible value because [ absolute zero
](https://en.wikipedia.org/wiki/Absolute_zero) is –273.15°C, and it's not
possible to be colder than this (at least [ not in any meaningful way
](http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146) ,
and certainly not because of wind chill). The [ coldest recorded temperature
](https://en.wikipedia.org/wiki/Lowest_temperature_recorded_on_Earth) on earth
is around –90°C.


Question:
between 1950 to 1980 there were negative temperature anomalies in the years of those decades.

decade        difference (°C)        difference (°F)
1950–1959        −0.02        -0.0360
1960–1969        −0.014        −0.0252
1970–1979        −0.001        −0.0018
This seems odd considering massive oil consumption started in 1880 IIRC, and by 1980 over 350 billions of oil barrels were already consumed (much more probably, since the data before 1950 isn't considered because there weren't reliable records).

Why was there a negative temperature anomaly between 1950 to 1980?

Explanation: The post observes that there's a negative temperature falling in the year of 1950-1980. It's a result of global dimming

$\begingroup$

This phenomenon is known as [ global dimming
](https://en.wikipedia.org/wiki/Global_dimming) .

It was due to the particles and aerosols mostly released by combustion of
fossil fuels such as diesel. Those particles block the radiation from the sun,
so they have a cooling effect. For some decades this effect counterbalanced
the warming effect of greenhouse gases, although it is no longer the case at a
global scale (see for instance [ Wild et al. 2007
](https://doi.org/10.1029/2006GL028031) ). Particles emission has been reduced
thanks to better engines and new regulations, which stopped their masking
effect on global warming. Which is a good thing since those particles have a
serious impact on health.


Question:
We know that plates can subduct, causing one plate to be pushed into the core by another. As the plates move, the subduction continues, pushing one plate under the other.

If this process continues, logic says, the entire plate could be subducted under the other.

Has this ever happened? How would we know?

Explanation: The post tries to find out if there's an entire plate subducted before. The anwser is yes, such as Intermontane Plate

$\begingroup$

Excellent question! Indeed, entire plates have subducted before.

One example is the Intermontane Plate ( [ Wikipedia
](https://en.wikipedia.org/wiki/Intermontane_Plate) ). This plate sat west of
North America around 195 million years ago. The plate contained a chain of
volcanic islands on its western edge (known as the [ Intermontane Islands
](https://en.wikipedia.org/wiki/Intermontane_Islands) ). As the plate
subducted under the North American plate, that island chain stood too tall and
merged with the North American plate.

An illustration of the plate:
![enter image description here](https://i.sstatic.net/m3oXq.png)
Source: [ Black Tusk, Wikimedia Commons
](https://en.wikipedia.org/wiki/File:Intermontane_arc.png)

The entire Intermontane Plate was subducted under the North American plate.
However, we are able to see the remnants of the plate in the volcanic islands
it left behind.

Once this Intermontane Plate was subducted, the Insular Plate became the new
subduction zone.

This entire process was actually repeated with the Insular Plate, subducting
the entire plate, leaving behind yet another chain of volcanic islands that
fused to the western coast of the North American Plate.

The Burke Museum has an excellent article that describes the subduction of
both the [ Intermontane Plate
](http://www.burkemuseum.org/static/geo_history_wa/The%20Omineca%20Episode.htm)
and the [ Insular Plate
](http://www.burkemuseum.org/static/geo_history_wa/Coast%20Range%20Episode.htm)
. Well worth the read.


Question: I'm using ADXL345 accelerometer with Raspberry Pi to build a seismograph. I've successfully hooked it up and can plot the accelerometer data in three axis. Is there any way to express these data in the form of the magnitude of an earthquake, of course, at the point of sensing? I know that it might be imprecise, but any representation would be helpful (e.g. Richter scale), and how to accomplish that.

Explanation: Using an accelrtometer as a seismograph is not possible, it can only be used to measure the local sesmic intensity. Instead, if we have two, it's possible as it can triangulate the location of the earthquake.

$\begingroup$

The magnitude of an earthquake is related to the total energy released,
therefore to estimate it from a seismogram you need to know the distance to
the source. In the case of the [ Richter scale
](https://en.wikipedia.org/wiki/Richter_magnitude_scale) for example, the
relationship between magnitude and seismogram amplitude is defined for a
standard distance.

If you have only one seismograph, you can not triangulate the location of the
source ( [ hypocenter ](https://en.wikipedia.org/wiki/Hypocenter) ).
Therefore, you can not estimate the magnitude of a seismic event (Richter or [
moment magnitude ](https://en.wikipedia.org/wiki/Moment_magnitude_scale) ).

**But you can estimate the[ local seismic intensity
](https://en.wikipedia.org/wiki/Seismic_intensity_scales) of the event ** at
the particular location of your instrument. With the accelerometer data you
can easily measure the [ peak ground acceleration
](https://en.wikipedia.org/wiki/Peak_ground_acceleration) , that can be used
to estimate the intensity in any of the [ existing scales
](https://en.wikipedia.org/wiki/Seismic_intensity_scales) . For example, the
peak ground accelerations associated to each intensity level in the commonly
used [ Mercalli intensity scale
](https://en.wikipedia.org/wiki/Mercalli_intensity_scale) are:

[ ![table of intensities](https://i.sstatic.net/F6aGo.png)
](https://i.sstatic.net/F6aGo.png)

Those _g_ values would be easy to calculate with the accelerometer data and
proper calibration constants.

_Table taken from the Wikipedia page for[ peak ground acceleration
](https://en.wikipedia.org/wiki/Peak_ground_acceleration) _

You might want to have a look at [ this question
](https://earthscience.stackexchange.com/questions/4681/characterizing-
earthquakes-using-accelerometer-data) . There are some nice answers and
references that you might find useful.


Question: What is the scientific reason for the majestic sights of the northern and southern lights, otherwise known as Auroras near the magnetic poles, and why do the northern lights differ from the southern lights?

Explanation: Cause of auroras: Auroras are colorful displays in the upper atmosphere caused by collisions between charged particles from the solar wind and atmospheric gases like oxygen and nitrogen. The colors (green, red, blue) depend on the atmospheric gas, altitude, and energy levels involved in the collisions. Auroras are more intense during peak solar activity when the solar wind is stronger.

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Straight from [ wikipedia
](https://en.wikipedia.org/wiki/Aurora#Auroral_mechanism) :

> Auroras are associated with the solar wind, a flow of ions continuously
> flowing outward from the Sun. The Earth's magnetic field traps these
> particles, many of which travel toward the poles where they are accelerated
> toward Earth. Collisions between these ions and atmospheric atoms and
> molecules cause energy releases in the form of auroras appearing in large
> circles around the poles. Auroras are more frequent and brighter during the
> intense phase of the solar cycle when coronal mass ejections increase the
> intensity of the solar wind.
>
> Auroras result from emissions of photons in the Earth's upper atmosphere,
> above 80 km (50 mi), from ionized nitrogen molecules regaining an electron,
> and oxygen atoms and nitrogen molecules returning from an excited state to
> ground state. They are ionized or excited by the collision of solar wind and
> magnetospheric particles being funneled down and accelerated along the
> Earth's magnetic field lines; excitation energy is lost by the emission of a
> photon, or by collision with another atom or molecule:
>
>   * **oxygen emissions** : green or brownish-red, depending on the amount of
> energy absorbed.
>
>   * **nitrogen emissions** : blue or red; blue if the atom regains an
> electron after it has been ionized, red if returning to ground state from an
> excited state.
>
>

>
> Oxygen is unusual in terms of its return to ground state: it can take three
> quarters of a second to emit green light and up to two minutes to emit red.
> Collisions with other atoms or molecules absorb the excitation energy and
> prevent emission. Because the very top of the atmosphere has a higher
> percentage of oxygen and is sparsely distributed such collisions are rare
> enough to allow time for oxygen to emit red. Collisions become more frequent
> progressing down into the atmosphere, so that red emissions do not have time
> to happen, and eventually even green light emissions are prevented.
>
> This is why there is a color differential with altitude; at high altitude
> oxygen red dominates, then oxygen green and nitrogen blue/red, then finally
> nitrogen blue/red when collisions prevent oxygen from emitting anything.
> Green is the most common of all auroras. Behind it is pink, a mixture of
> light green and red, followed by pure red, yellow (a mixture of red and
> green), and finally, pure blue.


Question:
When thinking about the formation of the current continents from a super-continent, it's clear that this is a gradual process, but there must be certain areas on earth which were once joined and were the "last" to break apart from Pangea.

Do we know when and where these rifting / break-up events took place?

Explanation: There's no way to predict when and where a continent can rift, it's not on the human's time scale.

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[ Rifting ](https://en.wikipedia.org/wiki/Rift) is an ongoing process. At the
moment, e.g. East Africa is [ rifting
](https://en.wikipedia.org/wiki/East_African_Rift) apart from the Nubian
plate, we also see rifting of continental plates elsewhere, eg [ Rio Grande
](https://en.wikipedia.org/wiki/Rio_Grande_rift) . New continents are being
formed, but it doesn't happen on a human time scale. Modellers of [ future
geography ](http://www.livescience.com/18387-future-earth-supercontinent-
amasia.html) have a [ difficult task
](http://www.sciencearchive.org.au/nova/newscientist/104ns_011.htm) to decide
what rifts that will stop and what rift that eventually will result in a new
continent.

See yellow marked faults and rifts: ![enter image description
here](https://i.sstatic.net/Y22u1.gif)

[ Madagascar ](https://en.wikipedia.org/wiki/Madagascar_Plate) is sometimes
refereed to as the _eight continent_ , not only of biogeographic reasons. It
was rifting away from Africa and later India during mesozoic time, [ starting
160 Ma
](http://www.researchgate.net/publication/248564376_The_Great_Rift_Valley_of_Madagascar_An_extension_of_the_AfricaSomali_diffusive_plate_boundary)
.

The last rifting that resulted in the present continents I can think about
(probably the community can come up with a later event!), would be the North
Atlantic breakup, that is still ongoing, but the last part of separation of
present continents was the [ opening
](http://www.sciencedirect.com/science/article/pii/S0040195108000024#) of the
[ Fram Strait ](https://en.wikipedia.org/wiki/Fram_Strait) [ 16-10 Ma
](http://epic.awi.de/16218/) . This separated Greenland from the European
plate and had a [ large impact
](http://www.sciencedaily.com/releases/2011/01/110127141659.htm) on the Arctic
Ocean conditions.


Question: What causes the different colours of the salts pits in the Saloum Delta in Senegal?

Explanation: The reason behind the colorful salt pits is variable algal concentrations. As the salinity of the pond increases, Micro-organisms change their hues.

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Writing about the San Francisco Bay salt ponds, [ Dr R. J. Rusay notes
](http://chemconnections.org/general/chem120/solutions-mixes.html) :

> Due to variable algal concentrations, vivid colors, from pale green to
> bright red are created. The color indicates the salinity of the ponds.
> Micro-organisms change their hues as the salinity of the pond increases. In
> low to mid-salinity ponds, green algae are predominant. In middle to high
> salinity ponds, an algae called Dunaliella salina shifts the color to red.
> Millions of tiny brine shrimp create an orange cast in mid-salinity ponds.
> Other bacteria such as Stichococcus also contribute tints.

Rusay says nothing about the Saloum Delta, but the explanation could be
similar there.


Question:
Methane clathrates are

solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice.

They mostly are on the bottom of the world's oceans; however, the overly brief wikipedia section for continental deposits suggests that it also occurs in Siberia, northern Canada and Alaska.

How much methane clathrates are believed to be buried in these continental deposits?

Explanation: The question just asks about the mass of methane clathrates are buried in continental deposit.

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According to [ Role of methane clathrates in past and future climates
](http://link.springer.com/article/10.1007%2FBF00144504) ,

> Methane occurrences and the organic carbon content of sediments are the
> bases used to estimate the amount of carbon currently stored as clathrates.
> The estimate of about 11,000 Gt of carbon for ocean sediments, and about 400
> Gt for sediments under permafrost regions...

So 400 billion tons.


Question: Supervolcanos have occurred in the recent geological past, but not within the past 74,000 years. Is it possible to find the magma chambers with no previous history of supervolcanic activity that are likely to produce supervolcanos?

Explanation: To find the magma chambers which produces super volcanals, use the method of seismic tomography.

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I am a seismologist, not a volcanologist, but we can use a method called
seismic tomography to understand the size of magma chambers.

This method is similar to a medical CT scan - the magma mush will have a
slower seismic wavespeed than the surrounding 'normal' rock. Therefore, we can
use this to determine the size of magma chambers from a tomographic image.

Even if a volcano does not appear to be active at the surface, it could still
overlie a magmatic anomaly.

This approach was recently used to estimate the seismic of the Yellowstone
magmatic body. See this paper: [
http://onlinelibrary.wiley.com/doi/10.1002/2014GL059588/abstract
](http://onlinelibrary.wiley.com/doi/10.1002/2014GL059588/abstract)

Seismic tomography can theoretically be applied to any region, but we just
need to have enough seismic sources nearby (e.g. earthquakes) to be able to do
this accurately.