How do you stop the Australian wildfires? The Australian fires are the biggest anyone has ever seen and is increasing at a rapid rate causing smog all around the country, approximately 1 billion animals have lost their lives, as of 9 January 2020. The firefighters are rescuing people and animals and doing their best.

But How do you stop such a huge fire? I've heard of a way where in tons of sand are deployed through jets but fire at such a huge scale i wonder how that is gonna help.

As of 7 January 2020, the total area burnt by the fires in the whole of Australia is 8.4 million hectares (21 million acres; 84,000 square kilometres; 32,000 square miles. That is equivalent in area to the nations of Austria or the United Arab Emirates. Greater in area than the Czech Republic, Ireland or Sri Lanka.
The vast majority of that has been on the eastern side of the country, in New South Wales (49,000 sq km) and Victoria (12,000 sq km). In Western Australia, the area has been 15,000 sq km and has forced the closure of the southern road between east and west twice.
With many fires burning in many areas the only course of action is to let the fires burn out by themselves. The efforts of fire fighting crews is to mainly protect areas from being burnt: properties and ecological regions of significance, such as the region containing Wollemi pines and other forested areas.
Dumping sand, as you inquire about, has problems: from where to source the sand, the amount of sand required, how to dump the sand, will the dumped sand have a negative environmental impact afterwards.
The fire fighting methods currently use in Australia are the same as with fire fighting practices for bush fires (wild fires) elsewhere. Fire are left to burn themselves out. The fringes of fires will be doused with water, either by ground crews and water trucks or via aerial bombardment, with water bombers (fixed winged aircraft or helicopters). Beating fire out is also used. Aerial bombardment will also include the used of fire retarding chemicals.
To prevent fires from spreading, firebreaks are also made, where large areas of ground are cleared of flammable material to prevent them from being ignited by ember attacks. This can also include the use of back burning, by fire crews, where forest is deliberately set alight by fire crews. This is only done when considered necessary and when weather conditions are favorable - wind directions and strength, humidity and temperatures.
The trouble with bush fires in Australia is eucalyptus is a significant species of Australian forests.  Eucalyptus requires fire to regenerate. They also shed large quantities of leaves and bark that accumulate on forest floors. A controversial method of minimizing bush fires has been to burn forest deliberately in cooler periods, under more controlled conditions, to reduce the fuel load during the main summer fire season.
By not letting the fires burn themselves out, the unburnt leaf and bark litter on the forest floors will remain and accumulate presenting another fire problem for the future.
Australia will always burn, it's the nature of its forests. Bush fires are an expected, but unwelcome part of summer in Australia. What Australia needs is better forest fire management strategies.

Edit 28 July 2020
Recent information about the Gosper's Mountain mega blaze that burnt more than one million hectares.
Anatomy of a 'mega blaze' - text and short videos of about the fire, from ignition by lightening strike, advance, merger with three other fires and how it was fought.
Gosper's Hill 'mega blaze', a 13 minute long video.


How do I get the velocity of the P-wave or S-wave without using the travel time graph? I'm trying to write a program that would determine the magnitude and epicenter of an earthquake. From http://www.dartmouth.edu/~ears5/handouts/EQ_distance.html, the distance of the epicenter can be calculated with

$d = (ts-tp)/(1/vs)-(1/vp)$

However, I'm not really sure as to what the values Vs and Vp are. From the travel time graph Vs and Vp varies with the distance of the sensor from the earthquake. Given that I have 3 sensors with given latitude and longitude, from which I am able to get the lag time, is there anyway I can estimate the Vs and Vp from these variables?

Otherwise, is there any other way I could get the distance of the sensors from the epicenter?

I think that it is possible with fours stations yes, not with three.

With the assumption of (roughly) homogeneous velocities we get the following forward problem. Assuming that $v_p$ and $v_s$ are constants between the Earthquake and all your receivers yields the following system:
\begin{align}
    d &= \sqrt{\Delta x^2 + \Delta y^2 + \Delta z^2}=\text{3 knowns, 3 unknowns},\quad \text{for} \left\{ \begin{array}{rl} \Delta x &= x_\text{earthquake}-x_\text{receiver} \\ \Delta y & = y_\text{earthquake} - y_\text{receiver} \\ \Delta z & = z_\text{earthquake} - z_\text{receiver}  \end{array} \begin{split} \end{split}\right.,\\
\Delta t & = t_s - t_p = \text{measured},\\
p &= \left(\frac{1}{v_s} - \frac{1}{v_p} \right) = \text{1 unknown}.
\end{align}
Restating the equation of your question then gives the forward problem:
$$ \Delta t = p d.$$
For example, for $v_p=2000$ m/s and $v_s=1200$ m/s, we find $p=1/3000$ and can compute e.g. with $d = 600$ that $\Delta t = 600/3000 =0.2$ s.

Now, we must set=up the inverse problem. You generally need at least as many equations as unknowns. In this case we have 4 unknowns (the velocity and the $(x,y,z)$ earthquake location). That means that we need 4 (unique) equations. I think that the simplest way of obtaining 4 unique equations is to combine 4 measurements:
$$ p\begin{pmatrix} d_1 \\ d_2 \\ d_3 \\ d_4 \end{pmatrix} = \begin{pmatrix} \Delta t_1 \\ \Delta t_2 \\ \Delta t_3 \\ \Delta t_4 \end{pmatrix}, $$
Denoting with $x_{1,2,3,4}$ and $y_{1,2,3,4}$ the latitude and longitude of the receivers, and assuming measurements at the surface, we obtain:
$$ p\begin{pmatrix} \sqrt{(x_\text{earthquake}-x_1)^2 + (y_\text{earthquake}-y_1)^2 + z_\text{earthquake}^2} \\ \sqrt{(x_\text{earthquake}-x_2)^2 + (y_\text{earthquake}-y_2)^2 + z_\text{earthquake}^2} \\ \sqrt{(x_\text{earthquake}-x_3)^2 + (y_\text{earthquake}-y_3)^2 + z_\text{earthquake}^2 } \\ \sqrt{(x_\text{earthquake}-x_4)^2 + (y_\text{earthquake}-y_4)^2 + z_\text{earthquake}^2} \end{pmatrix} = \begin{pmatrix} \Delta t_1 \\ \Delta t_2 \\ \Delta t_3 \\ \Delta t_4 \end{pmatrix}. $$

Finally, we must solve the inverse problem. Unfortunately, the system is non-linear (the unknowns are 'hidden' in the root of the square and can't be separated). You must therefore use some non-linear solver. For example, in MATLAB you can do this

% True earthquake location
x_earthquake=0;
y_earthquake=0;
z_earthquake=1000;

% True station locations
x_1 = 1000; y_1 = 1000;
x_2 =  500; y_2 = -300;
x_3 = -400; y_3 = -100;
x_4 =  -10; y_4 = 800;
X = [x_1;x_2;x_3;x_4];
Y = [y_1;y_2;y_3;y_4];

% True velocity structure
p0 = 1/3000;

% Forward equation (F(1)=x_eartquake, F(2)=y_earthquake, F(3)=z_earthquake, F(4)=p.
t =@(F) F(4) * sqrt( (F(1)-X).^2 + (F(2)-Y).^2 + F(3).^2);

% True recordings
measured_times = t([x_earthquake,y_earthquake,z_earthquake,p0]);

% Misfit function (=0 at optimum)
G =@(F) t(F) - measured_times;

% Invert
%options = optimoptions('fsolve','FiniteDifferenceType','central'); % using these will give better results in MATLAB
%F_inv = fsolve( G, [1000,1000,1000,0.1],options)
F_inv = fsolve( G, [1000,1000,1000,0.1])
x_earthquake_inv = F_inv(1)
y_earthquake_inv = F_inv(2)
z_earthquake_inv = F_inv(3)
p_inv            = F_inv(4)

If you copy this into https://octave-online.net/, for example, it will find an earthquake at (0,0,1000) and p=1/3000, with some small errors due to the non-linear solver.


What type of clouds are these? I would appreciate some help in identifying these clouds:

This was taken on a commercial plane flying between Hong Kong and Japan on the 31st of March around 2pm.

I tried looking up cloud types but was not too sure. This sort of had two different looks - one being a flat kind of sea, and the other being the smaller, noisier clouds popping up from underneath.

Thanks very much.

Mostly stratocumulus with a scattering of emergent cumulus towers which have been lofted by strong internal convection cells.


What sort of instruments do planetary geologists use to predict that there are oceans on the moons Titan and Europa? NASA Planetary Scientists released a video that says they have confirmed that there is an underground ocean in Europa and that Titan has liquid oceans made of hydrocarbons.

I myself am an aspiring planetary geologist and current student, the only instrument I have seen them using is the spacecrafts orbiting these moons.

As casey said, the satellite isn't an instrument in itself, it is a platform hosting many instruments. What you're talking about the discovery of oceans on Titan and Europa, you're referring to the the Cassini-Huygens mission aimed at, among other things :

Determine the composition of the satellites surfaces (such as Titan)
Prove or disprove the existence of oceans or water on those satellites.
Characterize Titan's surface on a regional scale

In order to establish the existence of oceans or water on those satellites, scientists use:

Spectroscopy

More precisely, absorption spectroscopy. AS is an analytical chemistry tool used to determine the presence of a substance in a sample. Thanks to a spectrometer, you'll be able to gather a lot of information about the elements on this Moon, and by further analysis, determine if there's water or no.

N.B : I am not talking about liquid water, but about water. Science now establishes the validity of certain signatures, but liquid water is to be confirmed.

What about Cassini?

Cassini used a lot of spectrometers : the VIMS (Visible and Infrared Mapping Spectrometer) was used on the satellite to gather information on the surface.

  "Our instrument measures the characteristic colors of the many materials that make up planetary surfaces and atmospheres: rocks, ices -- such as water, ammonia, methane -- and organic compounds," says Dr. Bonnie J. Buratti, VIMS Investigation Scientist. "VIMS data makes it possible to know what surfaces and atmospheres are made up of."

Volcanic observation

Indicators such as Geysers can be used to support the theory of water presence in subsoil. If by any radars, such as the ISS from Cassini (Imaging Science Subsystem), you can see geysers, then you can search about the possibility of water. Why? Because as you may know, water is pressurized in subsoil. Then, when water is heated by the heat of rocks, and it will tend to result in a geyser.


Determining the paleoelevation of mountain ranges In New Delhi, a lot of people say that the
Aravali range  used to be as tall as the Himalayas. How do scientists know that some mountain range used to be a lot taller millions of years ago, and what is the process by which mountains erode in general?

I'll begin with your second question, as to how mountains erode.
To simplify things, there are two methods:

Physical weathering, where the rocks are broken down by weather. For example, there are cracks in rocks that get filled by water which freezes and expands. This maker the cracks bigger and breaks down the rock. Also wind, earthquakes and any other process that breaks it down.
Chemical weathering, where the rocks are not broken by are altered to softer and weaker types of "rock". For example, take granite, a common rock in mountain ranges. It contains minerals from the feldspar family which are solid and hard. However, give some time and water and they convert to minerals from the clay family (aka "dust").

The first process is more relevant for tall mountains.

Now, how do we know the rate? This is a competition between uplift (the rising of the mountains) and the weathering (the breaking down of mountains). I'll give one example to illustrate it. Let's say that on the mountain top you find limestone with a certain fossil that you know existed 5 million years ago. So you know this used to be at the sea floor, let's say 300 meters below the water, but now it's 2000 meters up in the sky. So it had to rise 2300 meters in 5 million years. You can calculate the average uplift rate from that. Also, let's say that you know that in the past 10 years, a certain amount of rock was eroded from the mountain. You can calculate the rate of weathering. This way you can know the total rate.

There are new modern methods which rely on cosmogenic isotopes to get a much more accurate picture of this process, but that's for another question.


Has the number of natural disasters per year increased significantly in the years since human-caused climate change started? Human-caused global warming has had an increasingly significant effect on the earth since the industrial era began.

Since the beginning of the industrial era, has there been a significant increase in any of the following categories per year?

Hurricanes
Tornadoes
Floods
Forest fires
Extreme blizzards.

Although unrelated to global warming, I am also interested in statistics on earthquakes in the same time period as a 'base case' to account for higher reporting rates.

We have no evidence to suggest that the rate or magnitude of earthquakes has, in general, increased: while earthquakes are having a greater effect on humans, this is entirely - as best we can tell - due to increased population density, and the propensity of people to build cities immediately above major fault planes (because they channel water to the surface along impermeable fault debris). Populations settling above the surface expressions of major faults isn't much of an issue when you're living in tents in a sparse community, but once you get high-density populations living in skyscrapers with poorly-enforced building regulations death and injury rates rise very rapidly. For more detail and onward references see the USGS FAQ on this topic.

For statistics on extreme weather phenomena/natural disasters, particularly relating to precipitation and flooding, the IPCC is an excellent resource. There is of course some degree of difficulty with reporting bias (as with earthquakes), but the IPCC are extremely rigorous in calculating - and providing - bounds on probabilities and certainties. In particular, see Working Group 2: Impacts, Adaptation, and Vulnerability; their summary for policy-makers has to say:

  Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability (very high confidence).

For more details, see the final draft of WG2's contribution to the Fifth Assessment Report (AR5). Particularly worthy of highlight are the FAQ and chapter 18.4.3, which has to say:

  The last several decades have seen changes in the frequency and intensity of extreme weather events including extreme temperature, droughts, heavy rainfall, and tropical and extratropical cyclones with low to very high confidence, depending on the type of extreme event (IPCC, 2012, WGI AR5 Chapter 2). However, the impacts of extreme weather events also depend on the vulnerability and exposure of systems. It is possible that climate change can affect vulnerability and exposure, but typically both are primarily influenced by non-climate confounders, most notably economic development.


What creates so much pollution in Africa? Using AirVisual app I notice there is a huge patch of pollution in Africa. I cannot imagine there is a big economic activity to support that huge pollution across almost the whole continent. What could be the cause of those pollution in Africa.

What you can see in your image is mostly dust from the shara desert https://earth.nullschool.net/#current/particulates/surface/level/overlay=pm10/orthographic=13.33,5.70,766

You will be able to see other types of particles like smoke from combustion like cars-industry-forrest fires,the picture show particles that is 10 microns in size.

On the linked page you can find details of other types of chemical and particulate emissions too like SO2 and CO2 it is a good source for information about the atmosphere.


What produces these distinct shapes in the Rub' al Khali seen from space? update: Searching "Rub' al Khali Empty Quarter" found "Q2: What are sabkhas?" in
https://www.geocaching.com/geocache/GC6BYQ0_rub-al-khali-the-empty-quarter which seems to be related.
From the NASA image gallery; and also Catalog Page for PIA11084 where the caption says:

Original Caption Released with Image: The Rub' al Khali is one of the largest sand deserts in the world, encompassing most of the southern third of the Arabian Peninsula. It includes parts of Oman, United Arab Emirates, and Yemen. The desert covers 650,000 square kilometers, more than the area of France. Largely unexplored until recently, the desert is 1000 km long and 500 km wide. The first documented journeys made by Westerners were those of Bertram Thomas in 1931 and St. John Philby in 1932. With daytime temperatures reaching 55 degrees Celsius, and dunes taller than 330 meters, the desert may be one of the most forbidding places on Earth.
The image was acquired December 2, 2005, covers an area of 54.8 x 61.9 km, and is located near 20.7 degrees north latitude, 53.6 degrees east longitude.
The U.S. science team is located at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate.
Image Credit: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team
Image Addition Date: 2008-08-08

Question: The pattern shown in this image has an amazing amount of approximate repetition of shapes; long straight segments with a periodicity of about 2 kilometers, "stair steps" northward and sharp, pointy extensions southward. Is there any understanding how such a distinct pattern could be formed?

below: "Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1)" cropped and full sized (reduced resolution) images from here.

below: screen shot from Google Maps confirming orientation. Marker is at 20.7N, 53.6E.

This a great example of "self-organization" in a geomorphological system; this one an oscillating system at a fairly large scale. On a much smaller scale, you find similar structures on sand dune surfaces themselves as well as dry dusty dirt roads:

Self organization (and its scaling) is a complex process delicately dependent on just a few variables, in the case of desert dunes, wind carrying capacity, and critically, bed surface. (You can also find examples of oscillating chemical self organization in rocks when, for example, you see "tree-rings" around plagioclase crystals in anorthosites.)

The hidden (and surprising) complexity of self organization in natural systems is reflected in the difficulty in modelling these behaviors. In the case of small sand dunes:

  ...a complex process of ripple coalescence is responsible for this evolution of the ripple field. Small, fast-moving ripples catch up with and are absorbed by larger, slower forms, each such merger resulting in a growth of the mean wavelength and a decline in the relative dispersion of the wavelengths.

This mechanism may figure in modelling the formation of large dune fields:

  Because dunes reorient only at their crest terminations with a change in wind regime, the rate of formation of a new pattern of small dunes is typically faster than the rate of reorientation of the existing pattern, resulting in the superposition of simple patterns to give rise to complex patterns. Complex patterns are distinct from spatial changes in a simple pattern, and from the type of superposition that characterizes compound/complex dunes.

There are literally tons of examples of self organization in earth systems, at all kinds of scales, but that NASA image is a stunner!


Does anyone know of a DBMS with global geospatial search? We need to store coverage areas and search by location over arbitrarily large areas. It has to be able to handle polar searches and coverage areas that span multiple hemispheres. No GIS can do this. Informix has the Geospatial Datablade, but it costs like a hundred grand. Does anyone use something more affordable--preferably free?

The best open-source and free geospatial database is in my opinion PostGIS. It is easy to use and has a huge support group (also for example at  https://gis.stackexchange.com/)

It also connect to all sorts of different open-source programs and web-interfaces which should make data editing, viewing and sharing easy.

It can handle the date-line and the poles:

 4.2.3.2.
 What about the date-line and the poles?
 All the calculations have no conception of date-line or poles, the coordinates are spherical (longitude/latitude) so a shape that crosses the dateline is, from a calculation point of view, no different from any other shape.

(http://postgis.net/docs/manual-1.5/ch04.html#id358535)

Link: http://postgis.net/


How does quartz form in calcite veins? This question might be a bit silly, but I have to ask since I am very curious and I did not find any good answer. I visited two places, where quartz crystals (var. smoky quartz) occurs - and both sites are pretty similar and could host the same formation process.

My question is - could you explain how exactly these crystals of quartz could form in calcite veins in limestone ? Hydrothermal fluids ? What does it mean, could it be easily described ? Where did the SiO2 come from ? Was there any radiation, too ?

Your question is

  How does quartz form in calcite veins?

But maybe a more appropriate question will be

  How does quartz not form in calcite veins?

Quartz is one of the most common vein material in Earth's crust, and for a reason. SiO2 is not a particularly soluble compound at the surface, but once you get deeper it becomes extremely soluble (source):

The solubility of SiO2 is almost 0 at at 1 bar and 25 °C, but increases strongly with pressure and temperature. For example, at 200 °C and 2000 bar (2 kbar, 0.2 GPa, about 5–6 km depth), conditions that are reasonable for an unmetamorphosed but deeply buried limestone, you can fit about 500 mg of SiO2 in 1 kg of H2O. At atmospheric pressure that's one litre of water, but it's much less voluminous at depth. That is a lot of SiO2! Once the fluid goes up towards the surface, it depressurises and cools, causing precipitation of quartz.

There is no shortage of sources for SiO2. It is the most common chemical component in the crust. Even in seemingly pure limestone there are more than enough impurities of silicates to provide a source for SiO2.

The fact that quartz and calcite occur together in a vein is no problem at all. You might expect them to react, but they actually do not. Here's a diagram to show it (source):

Calcite and quartz can peacefully coexist, unless it's extremely hot and then they react to form wollastonite (a Ca-silicate). This is the kind of stuff you find in high grade metamorphosed marbles, for example.

As an aside, another question that may be asked is why only calcite and quartz? If hot dense water are so good in dissolving stuff? Where's the rest of the periodic table? Well, most of the elements are readily soluble in acidic fluids. In this case, the abundant calcite buffers the pH of the water to rather basic conditions, which are not that great in dissolving stuff.


How many mole of oxygen gas is there in the atmosphere? At standard conditions, one liter of air, 0.21 L of oxygen gas, contains 0.0094 moles of $\rm O_2$. What is the total quantity of oxygen gas molecules in the atmosphere?

It is easier to calculate:

The pressure of air on the ocean level is $\approx 100 \rm {kPa}$.
Thus, its weight is $\approx$ 100000N over $1 \rm m^2$.
Considering that the overwhelming majority of the atmosphere is below some km over the surface of the Earth, we can ignore the decrease of the gravity with height.

Now calculate the mass of the air above a square meter (ten tons). And multiple it with the surface of the Earth (in square meters). Then, divide it with the mean molar mass of air and multiple with the partial pressure of $\ce{O2}$ in it ($\approx$ 29%).


What are some good resources to learn about geophysics? What are some good textbooks and online resources for learning about geophysics? That is, physics, as it relates to the earth's geology, shape, and internal structure.

There are two universally loved geophysics texts : Turcotte and Shubert  and Stein and Wyessesion. Both are upper division/lower graduate level texts. Fowler is great if you do not have a strong mathematical background (multivariate calculus). Beyond that, it might be difficult to get a a good understanding without an Introductory physics course, and a few semesters of calculus. A good way to learn about geophysics, is in fact, just learning about physics and mathematics in general. Hopefully this stack exchange will also become a good resource.


Is it reasonable to approximate the swinging building (base of the building) during an earthquake to a simple harmonic motion? I am thinking of making a model of swinging building during earthquake and study the methods to minimize the swinging.

So does it even makes sense to approximate the swinging buildings (the base of the building) to simple harmonic motion? if not then what kind of motion does the buildings do.

By the definition of simple harmonic motion (restoring force proportional to displacement), no. You need to take into account damping.


Where does the biggest land-based ice cap reside? I'm thinking biggest in volume, regarding which area of the planet will contribute more to a raising in sea level - were the ice in those regions to melt.

I can basically think of to candidates, namely Greenland and Antarctica. So maybe some comparison between the contributions of the two would be great.

Antarctica is the ice sheet (cap) that will contribute most IF it would melt completely. The 2013 IPCC report (Ch. 4, the Cryosphere) provides an estimate of 58.3 m of sea level equivalent (sle). Greenland would if completely wasted away provide 7.36 m sle. Remaining glaciers provide an additional 0.41 m sle. The likelihood of Antarctica completely wasting away seems unlikely with our current understanding although the so-called West Antarctic Ice sheet (closes to the Antarctic Penninsula is sitting with its base deep below the current sea level) is far more likely to be lost than the East Antarctic Ice Sheet. Hence the contribution from Antarctica is likely less than the maximum number. Greenland on the other hand is thought to have a "point of no return" beyond which it will irreversibly be lost given the current or warmer climate. Since Greenland is mostly land-based, much of the mass loss will be by surface melting while West-Antarctica can lose much of its mass by ice berg calving which is likely a much faster loss mechanism. Estimates on the scenarios are emerging but there are still uncertainties and there may also be feedbacks that we either do not fully understand or have not yet seen that can change these scenarios (particularly for West-Antarctica). This Science article
published online May 12 2014 is a good example of emerging research on the stability issues of West Antarctica.


CO2 density, is it really a greenhouse gas? Since there are only 400 ppm in the atmosphere how can a molecule, which doesn't generate heat but absorbs it, possibly have such a great heating effect on the other 999,600 molecules?

During the Jurassic/Mesozoic era the density of CO2 was as high as 7000 ppm and the earth was lush with vegetation and animal life. Aren't we actually in an exceptionally low CO2 density, historically?

Yes, carbon dioxide truly is a greenhouse gas.

With regard to the immediate greenhouse effect, 99.96% of the dry atmosphere might as well be vacuum. Nitrogen (N2), oxygen (O2), and argon are extremely weak greenhouse gases at the temperatures present in the atmosphere. The only greenhouse gases in our atmosphere are trace gases. (The most powerful greenhouse gas is water vapor, and that, too is a trace gas in our atmosphere.)

The 99.96% of the atmosphere that doesn't directly participate in the greenhouse effect is however present. At sea level, a molecule of carbon dioxide will travel freely for about a tenth of nanosecond before colliding with another gas molecule. This mean time between collisions increases to a few tens of nanoseconds at the top of the troposphere. A greenhouse gas molecule such as CO2 that has just absorbed a thermal infrared photon will very quickly transfer that absorbed energy to the non-greenhouse gases that comprise the vast majority of the atmosphere.


Which are the points on Earth that receive the least solar radiation on average? The points which, instantaneously, receive the least radiation during daylight hours must be (I assume) those that lie tangent to the Sun rays. These points rotate, so there's a whole circle of them surrounding each geographical pole. These points are not colder than the poles because (again, I assume) the radiation they receive is higher on average, although (instantaneously, during the day) the lowest anywhere on Earth.

So, the points on Earth that receive the least radiation on average must be within those circles. Which are they; are they the poles?

I should perhaps add that I am interested in the version of the problem that idealizes away from complications having to do with cloud cover or other atmospheric effects.

On an average those would be the poles. As you correctly pointed out, due to the tilt of the Earth's axis, there are large areas that receive very little and sometimes no sunlight at all and those change throughout the year. But on an average, poles are the ones that get the least amount of solar radiation.

For example, see this paper and in particular have a look at figures 2.6 and 2.7. The last of the two figures, which I will link here shows the graph of annual mean insolation as a function of latitude.

Here it's obvious that poles do receive the least amount of radiation on an average. But this is the idealized case, i.e. insolation at the top of atmosphere.

On the other hand, if you do take prevailing atmospheric conditions into account then it's a competition between the area between Spitzbergen and Iceland in the North and areas just off the coast of Antarctica in the South. Taken from Wikipedia article on Solar Irradiance the top picture shows top of the atmosphere insolation and the bottom one - ground-level insolation.


Why would geostrophic theory be invoked in the tropics with regards to the MJO? In this paper reviewing the Madden-Julian Oscillation, frequently geostrophic theory (or approximately geostrophic) is often invoked. However, for geostrophic theory to be invoked, $$Ro=\frac{U}{f_0L}<<1$$. However, along the equator, $f_0=0$ and near the equator, $f_0\approx0$, which makes $Ro>>1$. But since this is a published paper, shouldn't the editor or reviewers caught this problem?

Some examples:

Page 815, second to last paragraph in the first column: "These results are consistent with an oscillation that is -- at 850 hPa at least-- in approximate geostrophic balance."
Page 816, first paragraph in the second column: "For geostrophic balance, the in-phase relationship is consistent with maximum pressure variations at the equator, while the more nearly out of phase relationship suggests maximum pressure variations off the equator."

Recent literature points to an attempt to understand the theoretical dynamics behind the MJO as seen in these two publications - Dynamics moisture mode vs. moisture mode in MJO dynamics and A general theoretical framework for understanding essential dynamics of Madden–Julian oscillation

As concluded in the original paper by Madden and Julien oscillation the key is to observe that original MJ observations were either in a in phase relationship of the surface pressure and the zonal wind(850 hPa) or out of phase relationship. No such relationship was observed with the meridional wind.

So the "geostrophic balance" in the MJO(in it's non dimensional form) is simply this equation

$$  \beta * y*u = -(\frac{\partial \Phi}{\partial y}) $$

So $\Phi$ here represents the first baroclinic mode (the meaning of baroclinicity in the tropics is quite different to it's meaning in the mid latitudes). In the tropics what it means is that vertical structures of winds and currents that reverse directions (cf. MJO review). So the MJO has a baroclinic mode of vibration because the winds reverse direction from 850 hPa to 200 hPa.

So in the assumption (proven by observations) that the  MJO can be characterized as a low frequency wave with an anisotropic horizontal length scale where the zonal length scale is much larger than the meridional scale by a magnitude of 10.So the meridional acceleration in the v-momentum equations can be neglected. With this approximation the zonal wind is in approximate geostrophic balance with the meridional pressure gradient  and all high frequency inertio gravity waves, mixed Rossby-gravity waves as well as short Rossby waves are all filtered out. So that only leaves the moist Kelvin and long Rossby waves as well as the MJO.

Alternatively if one were to consider the Rossby radius of deformation in the case of the MJO the horizontal wavelength of the disturbance if equal to the Rossby radius of deformation then both the velocity and mass fields undergo mutual adjustment as illustrated in these slides - Waves in Atmosphere  on page 52.

Similar conclusions were arrived at to derive balanced dynamics in the tropics by assuming linear theory and a shallow water model - Balanced dynamics in the tropics

UPDATE
Looking at other peer reviewed references that attempt to provide an explanation of the MJO it appears that an assumption of linear theory wrt to the MJO is not universally accepted and that non linear advection does have a role to play in the slowly propagating planetary scale disturbance(cf.Simple model of the 40-50 oscillation. A later paper by Madden(cf.Seasonal variations of the 40-50 oscillations in the tropics reports that  there can be substantial meridional wind perturbations bringing into question whether the assumption of geoostrophic balance is universally valid or not in the case of the MJO.

The original paper by MJ in the light of the observations obtained then perhaps made a conclusion that the structure and dynamics of the signal could resemble a Kelvin wave in which case the geostrophic balance of the Equatorial Kelvin wave (no large scale meridional wind perturbations and the equator acts as a wave guide ) could be used to explain the geostrophic balance in the case of the MJO. Later observations have proved this assumption incomplete.


Can concretions form in non-sedimentary rocks? Most modern works (e.g. Nichols 2009; Allaby & Allaby 1999) seem to restrict the definition of ‘concretion’ specifically to formations in sedimentary rocks. I have recently come across a taxonomy of concretions  in Crimes (1966), which includes concretions ‘formed as a result of metamorphism’. He gives little detail about formation processes but writes that ‘[m]etamorphic concretions have also received much attention and have been reviewed briefly by Barth and Ramberg.’ (I don’t have access to the cited references.) It seems plausible to me that the concretionary processes that operate in sedimentary rock would also be possible at very low grades of metamorphism, provided that some pore space remains. However, Crimes’ concretions are apparently formed as a result of metamorphism.

Do such metamorphic concretions have a place in currently accepted taxonomies of sedimentary features, or have they been reclassified as a different phenomenon altogether?

Allaby, A. & Allaby, A., 1999. A dictionary of earth sciences, 2nd ed. Oxford University Press.
Barth, T. F. W., 1952. Theoretical Petrology. New York.
Crimes, T. P., 1966. The relative age of some concretions in Cambrian sediments of St. Tudwal's Peninsula, North Wales. Geological Journal, 5(1), 33-42.
Nichols, G., 2009. Sedimentology and Stratigraphy, 2nd ed. Wiley-Blackwell.
Ramberg, H., 1952. The origin of metamorphic and metasomatic rocks; a treatise on crystallisation and replacement in the earth’s crust. Chicago.

Not really. Concretions are features of sedimentary rocks almost by definition. Here's my go at a definition; it essentially presupposes a sedimentary rock:

  Concretions are spatially discrete zones of above-average (compared to the rest of the rock) cementation by authigenic minerals. Common cements include calcite, quartz, siderite, and pyrite.

Concretions form during the process of diagenesis. However, diagenesis and metamorphism are really on a continuum of 'burial effects' — increasing temperature and pressure, compaction, changing porewater chemistry, and so on. One could almost call diagenesis a metamorphic facies. Concretions can start to form immediately after burial, and grow during burial as long as geo- and electro-chemical conditions allow, or until the pores are completely occluded. According to some workers (e.g. Bjoerkum & Walderhaug), total occlusion usually happens somewhere between about 150 °C and 200 °C. By this time, clay minerals have started altering and we are into low-grade metamorphism (zeolite facies).

Conversely, I think most so-called soft-rock geologists just lump all alteration into 'diagenesis' (I know I did until I started working on geothermal reservoirs). So, if you're prepared to explain yourself because it sounds weird, you could claim that concretions form in metamorphic rocks.

Footnotes

I've never seen one, but I don't see why you couldn't have concretions in volcaniclastic rocks (which I count as sufficiently like sedimentary rocks to warrant no more than a footnote).
If you were desperate to call something else a concretion — a mass of phenocrysts in a granite, say — you could always broaden your definition of 'concretion'. We geologists love arguing about the definitions of words!


What factors influence cloud height and type? For example, why do altocumulus clouds fail to form at 6400m in high latitude locations, but will form at this height at a lower latitude? Is it simply an issue of temperature?

It depends on the height of the troposphere. On the equator troposphere has at least 12 km height, but in the polar latitudes it might be even less than 8 km. That difference in troposphere altitude allows clouds to form differently depending on the latitude. The second reason is that higher parts of the troposphere closer to the equator are warmer and contain more moisture than in the polar region.


Does the regional localisation of global temperature trends suggest contamination by urban development? Much of the raw data behind the various datasets reporting the average temperature of the world are now available. So it is possible to look for strange patterns in the daily or monthly temperature data. The reported "average temperature of the world" is, ultimately, derived from these datasets by various corrections, normalisations and regional aggregations.
Recently I came across a claim that the temperature anomalies are remarkably concentrated geographically. The hypothesis suggested, tentatively, by the author (Clive Best, blog on topic here) suggested that this might reflect contamination of the raw data by rapid urbanisation. NB his argument is very different from the idea that US data is corrupted by the urban heat island effect which is addressed here: https://skeptics.stackexchange.com/questions/10341/has-the-urban-heat-island-effect-on-the-us-temperature-record-been-underestimate)
He reports (my emphasis):

The anomalies from ~4000 stations all over the globe are then combined to give one  global anomaly, yielding  the familiar graph we know and love which shows ~0.6 deg.C rise since 1850. Looking in more detail however we discover that some parts of the world are not warming at all and some  are  even cooling.

His argument develops:

It immediately becomes obvious  that  the bulk of  observed warming is concentrated in the Northern Hemisphere : Eastern Europe, Russia, central Asia, India, China, Japan, Middle East, North Africa. These are all areas of rapid population increase, development and industrialisation. There is essentially no warming at all in the Southern Hemisphere. Bolivia, Peru, Paraguay and Argentina all appear to be cooling. Even Australia and Zealand are static or  cooling. The US is evenly divided and the UK shows essentially no signal at all.
...Could much of the observed temperature rise over  the last 6 decades  be simply due to increasing urbanisation and development since ~1960 ?

Climate models don't produce even warming across the world so there could be explanations perfectly consistent with the observations of major regional differences. So my question is: does the large variation in regional temperature anomalies have a straightforward explanation in mainstream warming theory or is it a sign that some regional records are contaminated by urban development?
NB Best used the data from HADCRUT3 released by the UK Met Office, but it is also obvious in the NOAA GSOD dataset which is conveniently available to 2010 in Google's BigQuery as a sample dataset (you can also download it if you have the capacity process it).

The blog article suggests that the increase in "waste heat" from growing urbanisation is biasing the temperature observations.  The problem with this argument is that the heat generated directly from burning fossil fuels is far too small to make a difference, compared to the change in the Earths energy balance due to increasing greenhouse gasses.  See:

  Integrating anthropogenic heat flux with global climate models Authors

  Mark G. Flanner

  Atmospheric Science, Volume 36, Issue 2, January 2009.
  DOI:10.1029/2008GL036465

  Abstract 1 Nearly all energy used for human purposes is dissipated
  as heat within Earth's land–atmosphere system. Thermal energy released
  from non-renewable sources is therefore a climate forcing term.
  Averaged globally, this forcing is only +0.028 W m−2, but over the
  continental United States and western Europe, it is +0.39 and +0.68 W
  m−2, respectively. Here, present and future global inventories of
  anthropogenic heat flux (AHF) are developed, and parameterizations
  derived for seasonal and diurnal flux cycles. Equilibrium climate
  experiments show statistically-significant continental-scale surface
  warming (0.4–0.9°C) produced by one 2100 AHF scenario, but not by
  current or 2040 estimates. However, significant increases in
  annual-mean temperature and planetary boundary layer (PBL) height
  occur over gridcells where present-day AHF exceeds 3.0 W m−2. PBL
  expansion leads to a slight, but significant increase in atmospheric
  residence time of aerosols emitted from large-AHF regions. Hence, AHF
  may influence regional climate projections and contemporary
  chemistry-climate studies.

The blog article says:

  My guess is that 80% of this energy ends up as heat (2nd law
  thermodynamics). Assuming that  energy consumption is concentrated
  mostly in urban areas then the net “anthropogenic”  heating in those
  areas works out at around 5 watts/m2. This  then leads to  an average
  1.4 degreeC. rise in temperature for urban areas.  Anyone who has lived in the city knows from experience that the surrounding
  countryside is indeed  some 1-2 degrees colder.

Indeed the climatologists know that as well, they are not stupid, and hence "homogenise" the observations to remove these sorts of biases.  Note that if the bias were due to increasing urbanisation, this would be easily detectable by comparison with the observations from rural stations in the same region.  This is not rocket science (see my answer here for links on homogenization)!

Note also that increasing urbanisation would not necessarily cause an increase in local forcing, because as you increase the fossil fuel use, you also increase the area over which it is dissapated, which the author of the blog post also appears to have missed.


Sediment boundarys produced by largest meteors? Vredefort, Sudbury and Chicxulub Chicxulub crater is the third largest crater on earth, which is said to have produced the KT boundary.   What is the sediment layer produced by the 1st and 2nd largest craters?

Also, there many craters before and after Chicxulub was created, but the KT boundary is the only one commonly mentioned…why?

Vredefort

No extraterrestrial iridium anomaly: http://adsabs.harvard.edu/full/1989LPSC...19..733F

I'm not sure I fully agree with them, because there is quite a lot of Ir there and their threshold of 200 pg/g seems somewhat arbitrary to me. They claim the Ir was derived from local rocks, but that's a hell lot of iridium!

Sudbury

There is an an iridium anomaly. Read all about it here (paywalled) and here (open access).

  but the KT boundary is the only one commonly mentioned…why?

Because it's the first one found, a nice story of serendipity, because it's the last really big one, and the one that killed the dinosaurs. Everything you need to make a good story - even though it's not unique.

Iridium (and other PGE) anomalies and pretty much common feature of impacts. This paper shows attempts to link mass extinction events with impacts. This is a controversial issue, because you can also link massive volcanic events with mass extinctions, not only impact. But that's not for now - they do show iridium anomalies for all impacts. That's another paper showing iridium anomalies for impacts, much older this time.


Please could you help me finding global warming video with ice melting in glass time messurment scene? Few months ago I watched a video posted on reddit about climate change and in it there was scene showing glass of water with ice cubes.

They were measuring how long it takes for ice to melt and then after ice is melted how rapidly water warms. My best guest was Guy McPherson or Bill Nye video, not sure about Cowspiricy, but I have now seen I do not know how many and I could not find that scene.

I am looking video (scene) that shows how glass full of water stay for 40min+ at the same 1C temperature while ice is there, but as soon as ice melts, temperature of water increases to 25C (room temperature) just in 5 minutes. There was a glass of water with timer and thermometer, displaying melting ice and rise of temperature.

Basically it makes a point that without ice water warms more rapid.

Is there anyone there that has maybe seen the video please can you send me the link or tell me where it was?

This is probably not the specific video you are looking for, but here is one:

The video Latent heat of fusion shows the effect that you mean in a real world situation where the water is not mixed well.
The video Determination of Melting Point of Ice shows it under more controlled conditions.

The video Heat in changes of state explains it with a graph.

The video Energy required to melt ice and warm the water gives you an actual calculation example and shows the energy involved in melting ice vs. heating water.


Trying to calculate the percentage of carbon emissions that is offset by 45,000 acres? In my problem there is an average net ecosystem production  rate    of:
0.25 kg$^o$C/m2/yr
Am asked:
a. What percentage  of  140,000 tons/yr would be offset from reforesting    45,000 acres?
b. With that what area of reforestation/afforestation would need to be offset by 10% by the same emissions?
How I solved (not sure if correct):
Since 45,000 is in acres I converted to square meters:
45,000 Acres = 182,108,539.008 m2
Then I could say for every 0.25 there would be one m2 from the above number. Therefore I multiplied.
0.25*182,108,539.008 = 45,527,134.752 kg $^o$C
So now I need to convert kg to ton:
45,527,134.752 kg = 50,185.08 tons
So now I just need to divide into 140,000 to find how much it knocks off of the total carbon emitted:
140,000 / 50,185.08 = 2.8%
Is this right?
I'd imagine for 10% I'd just have to adjust the acreage to get it just right. With land area being 25,414,258,332.672 m2

What kind of forest, 45,000 acres of What?
If it's boreal forest it's a slow carbon sink
If its tropical, it'll be faster
Forests are largely carbon neutral in terms of carbon emissions. In temperate forests they emit carbon when they go inert for the season.Trees are roughly 50% carbon (dry weight) by mass. Increases in standing timber are directly correlated with increases in bound carbon. But once those trees die they decompose to anerobic co2 again. To grow a pound of wood, a tree uses 1.47 pounds of carbon dioxide and gives off 1.07 pounds of oxygen. An acre of trees might grow 4,000 pounds of wood in a year, using 5,880 pounds of carbon dioxide and giving off 4,280 pounds of oxygen in the process. However deciduous temperate forests put out co2. Tree's with Long life spans and ideal growing conditions are however better carbon sinks, as they accumulate cellulose they sequester carbon. Once they're mature they slow down and become carbon neutral. Trees that largely never stop growing are good carbon sinks
Redwoods
Eucalyptus
douglas fir
Lawson cypress

Tall Tropical specimens
Koompassia
Menara
Bertholletia (Brazil Nut)


Seismology: What causes an earthquake? I am doing a research on earthquakes but I cannot fully understand what causes earthquakes.

  "Caused by a sudden releases of energy by the Earth's crust, causing
  seismic waves which causes the ground to shake."

  "Energy release associated with rapid movement on active faults is the
  cause of most earthquakes."

  "Earthquakes are usually caused when rock underground suddenly breaks
  along a fault. This sudden release of energy causes the seismic waves
  that make the ground shake."

I have been puzzled by these statements and I really can't seem to understand.

Do these mean that energy is released by the earth's crust then produces seismic waves, then the seismic waves make the ground shake?

Does it also mean that earthquakes often most on faults because the release of energy and seismic waves occurs near or on a fault?

A lot of stress is accumulated along the fault by the time it breaks. For example, on the San Andreas, the Pacific plate is moving north with respect to the North American plate, but the fault resists this motion. As a result, a ton of elastic energy builds up around the fault as the stress increases.

Eventually, the stress becomes so much that the fault slips. The fault slipping releases the stored elastic energy, some of which is radiated as seismic waves. (Most of the energy becomes heat.)

Seismic waves "carry" stress and motion. When a wave first arrives, the rock at that location experiences a change in stress. This causes the rock at that spot to move slightly. But then, the rock at that spot has moved with respect to nearby rock. This differential motion of the rock creates stress, which then affects nearby rock similarly. So, the wave's stresses propagates motion in the rock, and motion in the rock propagates stress.

A few types of waves exist (P waves, S waves, and surface waves, in descending order of wave propagation speed). But the general idea behind them all is the same.

The ground therefore shakes when the waves arrive.


What is the purpose of this large structure on a Japanese hillside? A Google satellite view shows an unusual structure on a hillside in Miyazaki Prefecture, Japan (Latitude 31.782600°N, Longitude 131.233881°E):

It measures about 170m by 120m and is structured in steps of around 10m, with some parts also showing a grid structure on a scale of around 2m. It appears to be made out of concrete. What could its purpose be?

The structure looks similar to this photograph of a "Japanese land retention system" mentioned in passing towards the bottom of this webpage. From the linked page:

  Land retention systems in Japan, for example, are often designed as heavy waffle grids which are molded to the topography and cover it to a uniform structural depth.

This seems to correspond roughly to the structure in the question, although the "waffle-grid structure" is only evident in some parts.
I've been unable to find any more details online about these land retention structures and their design.

As farrenthorpe writes in a comment, the three-dimensional view is probably more informative:

At first glance the siting seems strange, since there is no major settlement or road below the structure. However, it does overlook a river which, around 3 km downstream, feeds into the Tenjin water storage dam. In the current Google satellite imagery the riverbed is mostly dry and ongoing engineering work is visible in it, presumably also related to debris/sedimentation control. Similar hillside structures can also be seen on some of the slopes around the dam itself.


Rivers - How to calculate maximum velocity from average velocity in a cross section I'm studying flow velocities in a small river. I have a lot of data of average flow velocities in cross sections throughout the entire catchment. And I know the river develop a stream velocity profile per cross section (see below).

Now I'm also interested in the maximum flow velocity in a certain cross section. So I'm looking for a way to calculate the maximum velocity from the average velocity. Is there some kind of formula or method to do this?

Any help is appreciated.

The velocity profile that is shown is parabolic. It is characteristic of laminar flow in tubes or channels. A development of the equation is shown in this link. An exposition about flow in open channels is at this link.

The maximum velocity over the entire profile is at the surface (see Equation 4.7 and statement 17 in the first reference). The maximum velocity at an other range of positions as measured vertically from the bottom of the stream is always found at the top of the range (i.e. at the point closest to the surface of the stream).

Let's propose a velocity profile from the bottom to top of the channel as this.

$$ v(z) = Cz^2 $$

This satisfies the boundary condition that $v(0) = 0$ at the bottom of the channel. The parameter $C$ is found by taking one additional measurement anywhere in the stream. With this function, we find the average velocity over a range of positions $zo$ to $zt$ is an integral over the parabolic profile ...

$$ <v> = \frac{\int_{zo}^{zt} v(z) dz}{zt - zo} = \frac{C}{(zt - zo)} \int_{zo}^{zt} z^2 dz = \frac{C\left(zt^3 - zo^3\right)}{3\left(zt - zo\right)} $$

An alternative way then to find $C$ when given an average velocity over a range of distances is to use this expression.

A distinction is required to understand the average velocity expressed above. An alternative view of average velocity is from measuring at a set of points over a range $zo$ to $zt$ and then averaging the measured values. This measured average has the expression

$$ <v>_m = \frac{1}{N} \sum {v_j} $$

With $H$ as the height of the channel, in the limit that $zt - zo << H$, we can assume $<v> \approx <v>_m$. Otherwise, the better approach is to use a one-point method with $v$ at $z$ and the source equation for the profile.

Finally, consider when you have multiple measurements at different positions where the positions are not differentiated but the velocities are differentiated. One approach here is to average the velocities and use any one value to the profile for $C$. The better method however will use non-linear regression method to fit data. By example, consider this hypothetical "measured" data set generated using $v = 4 z^2$ as its base.

z=1, v = 3, 4, 5, 6

z=2, v = 14, 15, 16, 17

z=3, v = 32, 33, 35, 45

A plot of the data and a non-linearized regression fit to $v = Cz^2$ is shown below. The result is $C = 4.0 \pm 0.2$.


Why are cobalt deposits so unevenly distributed? The Democratic Republic of the Congo's Katanga Province contains almost 40% of the world's reserves of cobalt [1].
Why are deposits concentrated so strongly in such a small portion of the earth?
I would have thought that this is due to similar reasons to: Why do gold deposits form only in certain areas of the earth?
But the distribution of cobalt seems far more uneven than that of gold.
Do the geological processes causing this uneven distribution significantly differ from those that cause the uneven distribution of gold?
[1]: British geological survey, 2009

Part 1

The Democratic Republic of the Congo's Katanga Province contains
almost 40% of the world's reserves of cobalt [1].
Why are deposits concentrated so strongly in such a small portion of
the earth?

Cobalt isn't as unevenly distributed as it seems. It is correct that most of the world's cobalt reserves are in DRC, but most of the cobalt resources are not in DRC. And this is a very important point.
Resources are known and estimated quantities of economically extractable materials. Reserves are resources for which detailed plans for extraction have been made. This means that to turn a resource into a reserve you need to have plans for the extraction and refining plants, have environmental approval, have the workforce figured out, have authorisation from the owners of the land, and more. As you can guess, making a resource into a reserve is a time consuming and expensive process.
The reason why most of the Earth's reserves are in DRC is simply because it's easier and cheaper. The infrastructure already exists, and the expertise exists. At current technology levels and cobalt prices, the investment in defining a reserve from a resource elsewhere is simply not economical.
This does not mean that DRC has most of the cobalt. It only means that getting cobalt out of the ground and making it into a product is cheaper in DRC than it is in other countries.
Part 2

Do the geological processes causing this uneven distribution significantly differ from those that cause the uneven distribution of gold?

The reasons are the same, in principle. Every element behaves differently and will separate from other elements and concentrate because of various geological processes. Cobalt tends to follow copper and nickel, and in most cases it is mined as a by-product of those elements. That's why cobalt is extracted from the African Copperbelt, and why it's extracted from nickel deposits in Australia. Not much point going into the chemistry and thermodynamics of why this happens, but the point is that elements which are concentrated by rare geological processes will be found in fewer places, and elements that are concentrated by common geological processes will be found in more places.


Landsat 5 DN's to Top of Atmosphere reflectance The landsat 7 etm+ users handbook does the the Dn to top atmosphere correction (pages 79-80, link below). Is this applicable to landsat 5 tm as well?

Landsat 7 User Handbook

Short answer: yes.

The Radiance to Reflectance correction described in the Handbook is just physics based. Therefore, it is valid for any Earth observing satellite. However, the mean solar exo-atmospheric irradiances (table 5-2) are specific to the wavelength bands of the ETM+ sensor. Therefore, if you want to correct other bands you would need new coefficients valid for the corresponding wavelength range.

But in the case of Landsat 5 and 7, the bands are almost identical (with very small differences on bands 4 and 7). Therefore, unless you need very high accuracy I would say that it is safe to use ETM+ coefficient (Landsat 7) to correct TM imagery (Landsat 4 and 5).

The bands for TM and ETM+ are as follows:

(taken from USGS Landsat Mission)

Note that it is not only the boundaries of the bands what matter, but also the spectral response within the band. Therefore, even for bands that seem identical, the coefficients of exo-atmospheric irradiances can present small differences.
Ideally, you should use the coefficients calculated specifically for the TM sensor, but using the ETM+ values would provide a very good approximation. At least good enough for most applications. But if you want to use the TM specific values, they are as follows:

(Taken from USGS Q&A "Where can I find the solar exoatmospheric spectral irradiances (ESUN) for the Landsat 1-5 MSS, Landsat 4-5 TM, and Landsat 7 ETM+ sensors?")


What "g" would be needed to keep helium on Earth? 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?

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 and the thermal escape mechanism is known as the Jeans escape.

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.


Meaning of ECMWF model levels I am trying to understand the actual meaning of the ECMWF L137 model levels. In the link I provided, both pressures (in hPa) and heights (in m)  are given for every level. But which actually defines the model level, and which is merely included as an indicative value?

Since it discusses the pressure values setup first, then says:

Also provided are the Geopotential and Geometric heights, the
temperature and density of the level based on the 1976 version of the
International Civil Aviation Organization (ICAO) Standard Atmosphere...

I would take that to indicate that the vertical coordinates are defined by pressure, which is how I've always believed it to be.  And then the other values are just estimates from the standard atmosphere approximation.
In addition https://rda.ucar.edu/datasets/ds115.4/docs/levels.hybrid.html looks to give more details on the vertical setup...

The vertical coordinate parameters define the edges of the atmospheric
layers in terms of the surface pressure (in Pascals). The parameters
can be considered as being true at the mid-point (in pressure) of a
layer for each point. The formula to use to determine edges of the
layers is $P_{k+1/2} = A_{k+1/2} + B_{k+1/2}P_s$, where $P_s$ is the surface pressure and $A$ and $B$ are defined in the table.

So others who work on modeling may be able to offer further input on the setup, but from my casual understanding, it looks to definitely be based on pressure :-)


Easy homework questions about seasons and daytime The following homework assignment is supposed to be easy, but there seems to be information missing from the questions asked (or at the very least knowledge from OP).

At which width circle does the sun shine perpendicularly?
How long is a day south of the south polar circle?
Which season is starting in the southern hemisphere?

The following picture was included.

The figure depicts the Northern hemisphere summer solstice, given that, there is no information missing:

At which width circle does the sun shines perpendicularly?

The one labeled "13 1/2 uur" that corresponds to the Tropic of Cancer

How long is a day south of the southpole circle?

If we understand "day" as the length of daytime, the answer is zero: The sun never rises that day south of the southern polar circle.

Which season starts at the southern hemisphere?

Winter


What place on Earth is closest to the Sun? What is the location on Earth that is closest to the Sun?

I've seen this question asked many times, and answered in varied and contradictory ways:

The most common answer is “the summit of Chimborazo volcano in Ecuador”. This volcano is the point on Earth's surface that is furthest from the center of Earth, and that is then equated to being the closest to the Sun.
This is very commonly spoken of around the Chimborazo volcano area and among the people involved with tourism there (here is an example of this answer)
Others argue that it is Cayambe volcano in Ecuador, it being the highest point along the equatorial line (answer example).
Others say Mount Everest in Nepal/China because it is the highest point on Earth (example).
And others argue it is Sairecabur volcano in Chile/Bolivia, because it is the highest point at the latitude which is closest to the Sun on January 5th, when the perihelion happens (i.e. the point in Earth's orbit that is closest to the sun) (example).
A fifth answer, with the same logic as the previous, is Licancabur volcano in Chile/Bolivia, which quite as close to the latitude of the perihelion but is significantly higher than Sairecabur.

What is the correct answer and why?

What place on Earth is closest to the Sun?

This is an interesting question, but it lacks a key factor that is crucial to the answer: TIME.

The point on Earth closest to the Sun varies through time, so the question can be asked about any moment in time, or over periods of time. Let's analyze the factors involved.

At any given moment in time, the point on Earth's surface that is closest to the Sun is what is called the “subsolar point”. This point corresponds to the point on the surface that intersects the imaginary line that connects the center of the Earth to the center of the Sun. In other words, the subsolar point correspond to the point on Earth surface where the sunlight hits the Earth perpendicular to the ground, therefore, a vertical object would project no shadow.

(image from Wikipedia: subsolar point)

The longitude of the subsolar point corresponds to that of the meridian experiencing solar noon. Over Greenwich (longitude 0°) that happens at the actual noon, and as the Earth rotates 15° every hour, that will happen one our later (at 13:00 h UTC) at longitude 15° W, two hours later (at 14:00 h UTC) at longitude 30° W, and so on. In general terms, you can use the following formula for the subsolar point longitude ($\text{SSP}_{\text{long}}$).

$\text{SSP}_{\text{long}} = \left(\text{UTC} -12\right)*15°$

This is a simplified formula, but accurate enough for our purpose. Let's take as an example the following date

July 20, 1969, at 20:17 UTC

In that moment, the longitude of the subsolar point was 124° 15' West:

$(20+(17/60)-12)*15°=124.25°=124°15'$

Finding the latitude of the subsolar point is a bit more complicated, we need to know the declination of the Sun. Declination is the equivalent of latitude for celestial coordinates. For that, use a formula, a table, or a online calculator like the NOAA Solar Position Calculator.

Just enter the date, and even that the location doesn't matter here, we need to select “Enter Lat/Long -->” to be allowed to enter the offset to UTC as 0, otherwise the time won't be interpreted as UTC time.

From there we can find that the solar declination for our example date is 20.58° (20° 34') which corresponds to the latitude of the subsolar point: 20° 34' North.

Therefore, on July 20, 1969, at 20:17 UTC, the subsolar point was at 20° 34' N, 124° 15' W, which is somewhere between Mexico and Hawaii. That was the point on Earth closest to the Sun at that moment.

Now, what would happen if there were a very tall mountain close to the subsolar point? Would that mountain be closer to the Sun?

The answer is: probably. It depends on how far and how much higher it is relative to the subsolar point.

We can do a quick calculation based on the following diagram (in this approximation we assume that Earth is spherical, that the sun is infinitely far away and other simplifications)

From there we have

$r-r'=\Delta H$

$D = r ~ \theta$ ($\theta$ in radians)

$\frac{r'}{r}=\cos(\theta)$

After some algebra you can write that the extra height $\Delta H$ needed to be as close to the Sun as the subsolar point is

$\Delta H = r \left(1-\cos\left(\frac{D}{r}\right)\right)$

Where $D$ is the distance and $r$ is Earth's radius (in this case makes sense to use the equatorial radius of 6378.1 km)

If we plot this equation we get the following

(the vertical axis is logarithmic)

We can see that around 10 km away from the subsolar point, ~10 meters are enough to be closer than it to the Sun. ~30 meters at 20 km, ~800 meters at 100 km, ~3,000 m at 200 km, and if you go further than 340 km, not even Mount Everest will get you closer to the Sun.

So, the closest point to the Sun will be whatever geographical feature that maximizes the value $\text{Altitude}-\Delta H$, where $\text{Altitude}$ is the altitude of the geographical feature. Let's call that point “proxisolar” point. I just made up that name, but it will be handy for the following discussion.

Now that we understand the basis to establish what is the closest point to the Sun at a given moment, we can tackle the question that probably most people meant when asking this question:

What is the point on Earth that gets closest to the Sun over a year?

The most important fact to keep in mind, is that the variations of the distance between the Earth and the Sun over the year dwarf any topographical feature and even the diameter of the Earth itself. Earth’s distance from the Sun (center-to-center) varies from 147,098,074 km at perihelion (closest) to 152,097,701 km at aphelion (most distant). Therefore, the difference is 5 million kilometers!.

The perihelion happens around January 4th, when the solar declination is about -23°, therefore, the latitude of the subsolar point is around 23° South. That rules out Chimborazo, Cayambe and Everest, because they are too far to be the “proxisolar” point. In contrast, Sairecabur (5,971 m at 22.72° S) and Licancabur (5,916m at 22.83° S) are reasonable contestants.

The problem is that the perihelion happens on different days of the year and at different times of the day every year, so the point that gets closest to the Sun on a given year is just the one that happen to be the “proxisolar point” at the time of the Perihelion.

People who argues that Sairecabur or Licancabur are the points that get closer to the Sun, are implicitly assuming that the distance Earth-Sun doesn't vary much during the day of the perihelion. Therefore, the extra elevation of these mountains allows them to get closer to the Sun during that day. Unfortunately, that assumption is completely wrong. Let's see why:

An approximation of the distance Earth-Sun can be obtained from the following formula

$d = \frac{a(1-e^2)}{1+e \cos\left(\text{days}\frac{360}{365.25}\right)}$

Where $a$ is the semi-major axis of Earth's orbit, $e$ is the eccentricity, and $\text{days}$ is the number of days elapsed since the perihelion. To see the simplifications behind this equation look here (note that the conversion factor 360/365.25 is erroneously inverted in that link, thanks @PM2Ring for spotting that).

If you solve the above equation for the perihelion and for one day before/after it, you will get that the difference is 358 km, and for half a day you get 89 km. Therefore, if the subsolar point happens to be on the opposite side of the Earth than, let's say, Licancabur volcano, this volcano would need to be 89 km higher that the subsolar point to get closer than it to the Sun that year. 89 kilometers!

Therefore, we can discard the idea that a given mountain could be the point that gets closer to the Sun on EVERY year.

If we plot the above equation with distances relative to the perihelion we get the following (using $a$ and $e$ from here)

Here we can see, that if the perihelion happens a bit more than 3 hours before or after the solar noon at Licancabur, the ~6,000 m of elevation advantage would not be enough to get closer to the Sun than the subsolar point at the perihelion, even if such point is at sea level.

Note that three hours corresponds to 45° in Longitude, which at that approximate latitude corresponds to approximately 4,600 km.

Therefore, it can be argued that Licancabur is the point on Earth that has more chances to be the closest to the Sun in an arbitrary year. But in a given year, it might or might not be the closest depending on where the subsolar point is at the moment of the perihelion.

Finally, it is important to note that the distance Earth-Sun at the perihelion varies widely from year to year. If you look at this table of perihelions between years 2001 and 2100, you will see that perihelions often vary by several thousand of kilometers.

Therefore, for example between years 2001 and 2100, the closest perihelion by far is the perihelion of next year (2020), and it will happen when the subsolar point is in the middle of the Indian ocean, about 12,700 km away from Licancabur and Sairecabur volcanoes. Therefore, the point that will be closest to the Sun this century will be one in the middle of the Indian ocean about 320 km south of Rodrigues Island.

Said this, the question of which point on Earth will get closest to the Sun depends on the period of time on which it is considered. For each year, each century and any other arbitrary period of time, the answer will be different.


Relative UVB reflectance of white cloth vs aluminum Which reflects more UVB from the sun, a flat laying white cloth sheet, or a flat laying aluminum sheet? Ie. Which material would reflect a wavelength of 280 to 315 nm best?

I understand that polished aluminum surface reflects more that white. However , the white surface re-emits more energy  so the net affect is that the white retains less energy.


Difference in surface temperature between the Northwest Atlantic and Northeast Atlantic Why is the surface waters in the Northwest Atlantic Ocean colder than the surface waters of the Northeast Atlantic Ocean?

The main reason is that the Gulf Stream transports warm surface water from the tropics, driven by the thermohaline circulation. The tropical trade winds push surface water towards Western Atlantic and build up stress. Further north the dominant wind direction is towards the east. The Coriolois force also helps to move the warm water masses towards the east.

The Labrador Current, on the other hand, brings cold water down West Atlantic Ocean. The thermohaline turnover is further south in West Atlantic Ocean.


doubt about the 'white spaces' in the maps from satellite I have a doubt about the 'white spaces' in the maps from satellite. For example, for the atlantic areas I have plotted AOD and appear many areas where sistematically do not have any data (white spaces). So I want to know why that happens or which factors are behind that.

thank you for your answer

Expect every daily retrieval of AOD to have many missing pixels, mostly where clouds appear.  Data is removed when the quality assurance flags do not meet some minimum criteria.  This can happen when the retrieval is contaminated (e.g. from clouds or sun glint on the surface).  If you are interested, you can obtain raw level-1 and level-2 data with all data, including arrays for quality assurance flags.

There will also be systematic gaps in the data where the satellite had no coverage.  Polar orbiting satellites will get complete coverage near the poles, but widening gaps will appear the closer you get to the equator.


How hot the earth will be if we have 20% CO2 in the atmosphere instead of ~0.04? and how cold would it be if doesn't have any CO2? I know Earth once had this amount in the past, but if we got the former atmosphere with around 20% of CO2, how much the temperature will rise? assuming the atmosphere is still dominated by 78% nitrogen and 2% of other gasses.
Would it go runaway greenhouse effect?
And what about if Earth has the same atmosphere that we have today but with 0% CO2, how cold it will be? assuming other greenhouse gasses aren't changed and still affecting the planet.
And I know that neither the temperature nor the atmosphere will be friendly, at least not to us.
I don't want to know what will happen to life or if the Earth will still livable because it's obvious. (Edited)

The concentration of CO2 in the Earth's atmosphere has remained less than 1% in the past 470 million years when land plants first appeared--long before mammals, and long before humans graced the earth.
If we are talking about having a biosphere comparable to the one we know, we need to be talking about CO2 and global average temperatures that are comparable to levels during the relatively stable Holocene epoch of the past 11,000+ years.  Those are the conditions that support life, with the current overshoot beyond 350 ppm being brief (in terms of geological time).
As for 0% CO2 concentration in the Earth's atmosphere, that would remove warming that essential for life on the planet. The goal is to have CO2 levels that are stable and also at a level which provides the warming for which life in the biosphere is accustomed to--which is less than 350 and probably even less than 325 ppm.  An accepted scientific paper is Target Atmospheric CO2: Where should humanity aim? [https://arxiv.org/pdf/0804.1126.pdf].
This discussion should also switch from the concentration of CO2 in the atmosphere to global emissions from a few human activities (roughly 90% being combustion of fossil fuels). The question to ask is how much do those human emissions need to change for the current annual increases in CO2 concentrations to stop rising?  They need to return to emission levels that were essentially zero as they were prior to the Industrial Revolution.  That is when natural global emissions and absorption (aka sinks) cancelled one another out to keep levels stable.  However, cutting global human CO2 emissions by half (or let's say 60% to be safe) will stop the rise in CO2 levels for a few years.  If cuts do not go further within the decade toward 100%, CO2 levels in the atmosphere will start to rise again.  Reference: IPCC AR4 FAQ 10.3 [https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/faq-10-3.html].


What is the core of the lightning? The post Lightning mentions about "the core of a lightning bolt":

  The core of a lightning bolt is a few centimeters in diameter.

I couldn't find any article discussing about this. The reference link has been changed, so I couldn't read the original post. So what is it?

I found the paper you initially referred to here:

http://onlinelibrary.wiley.com/doi/10.1029/JB073i006p01889/full

based on the hyperlink by the xkcd webpage you referred

http://www.agu.org/pubs/crossref/1968/JB073i006p01889.shtml

The full reference for this paper is: Oetzel, G. N. (1968), Computation of the diameter of a lightning return stroke, J. Geophys. Res., 73(6), 1889–1896, doi:10.1029/JB073i006p01889.

In the abstract:

  The diameter of a first return stroke is found to lie in the range 1–4 cm. Subsequent return strokes are smaller, with diameters in the range 0.2–0.5 cm. In many cases, strokes to towers have shown evidence of the smaller strokes occurring alone. These are explained as strokes initiated by an upward leader.

This paper is quite old - but is relevant nevertheless.

A more recent paper review the physics of lightning in this review paper, titled 'The Physics of lightning' by Dwyer and Uman (2014). They present the various steps implied when a lightning form, illustrated by this illustration (Figure 1):

Basically, there is several stages when a lightning strike. There is the stepped leader component, that somewhat lead the way for the exchanges that will follow. In the figure, we notice the positive and negative signs (+ / -) and how they change position depending on the discharge.

The stepped leader can be followed by return strikes and so on. In this paper they mention diameters for the stepped leader, which may appear to be much larger than the return stroke, such as in the range of one to ten meter in diameter:

  The luminous diameter of the stepped leader has been measured photographically to be between 1 and 10 m.

But - this is by using photography - yet the core of the lightning at the center (the core) seem to be a few centimeter in diameter:

  It is thought, however, that most of the stepped-leader current flows down a narrow conducting core a few centimeters in diameter at the center of the observed leader.

So in the end both papers tend toward similar centimeter-scale measurements for the lightning core.

The core: aforementioned literature refer to the component of the actual lightning as a 'current carrying core' surrounded by a corona sheath. The sheath is what appear to be 1 to 10 m in diameter, where the actual 'electricity channel' is centimeter wide;

so this core as the centimeter wide channel is the electron pathway.


What type of fossil is this found at northern Ohio? I need some help identifying this fossil.

I found the rock 30 years ago along the Cuyahoga River in northern Ohio.
Northern Ohio is glaciated so it is possible that the specimen was transported to Ohio.
The rock is approx 1.5 x 2.5 inches (3.8 cm x 6.3 cm).
The rock is calcareous reacting to HCL.
The rock feels the correct weight for its size.

Of course 5 minutes after I post the question, I find the answer myself:
Halysites (Chain Coral) fossil extinct coral from the Silurian period.


Why divide total insolation by $4$? From Nature:

We divide by $4$ since the solar energy is spread over the surface of the planetary sphere. The Earth intercepts a circular area of incoming sunlight, and this area is spread over a sphere with the same radius as the circle (area of circle / area of sphere of same radius = $0.25$).

It would seem by dividing by 4 it results in the sun's energy being evenly distributed over the entire globe at the same time as if the earth was flat, and weakening the actual energy required to create a climate. I don't see any logical reason to do this as we know this doesn't physically happen.
What is the explanation? Why do you need to average out the sun's energy over the whole planet at the same time when that's not what actually happens?

Why does climate science divide total insolation by 4?

For two reasons, each of which halves the effects of insolation:

At any point in time, only half of the Earth is illuminated by the Sun. Another name for this factor is "daytime" versus "nighttime".
At any point in time, the surface area of the Earth's hemisphere that is illuminated by the Sun is about $2\pi r^2$, where $r$ is the radius of the Earth. But the cross section of that hemisphere to solar radiation is the area of a circle of radius $r$, which is $\pi r^2$, half of the area of that hemisphere.

? Why do you need to average out the sun's energy over the whole planet at the same time when that's not what actually happens?

The Earth rotates fairly quickly, so that is what happens. If the Earth was tidally locked with the Sun, there would be one side of the Earth that would be very hot and one side of the Earth that would be very cold. That is not what happens.
If the Earth rotated once per month (which is what our Moon does do), the sunlit side would eventually come close to being in thermal equilibrium with the incoming sunlight while the dark side would cool to much colder temperatures than are experienced anywhere on the face of the Earth. That also is not what happens. Even the sunlit Sahara does not get as hot as do equatorial regions of the Moon, and even South Pole does not get as cold as do portions of the Moon.
The periods of daytime heating by sunlight and periods of nighttime cooling by the lack of sunlight are both far too brief to bring those portions of the Earth into thermal equilibrium with the sunlight at daytime or with the 2.7K cosmic microwave background radiation at nighttime.


Strange coating in old lime mine shaft Near Prague,Czech Republic is old lime mine. Most of it is closed but one short transport tunnel under one hill is publicly accessible. Tunnel has profile around 2,5x2,5m and it's slightly bended and it's about 100m long. I estimate that hill above it is about 50m high from floor of tunnel. Aproximate location: 50°02'28.0"N 14°22'32.1"E
In middle of tunnel this immediately caught my attention, as it was reflecting light from my headlamp a lot. Its like some very wet yellow coating on rock side of tunnel.
Area on photo is 2m wide and 1-1,5m high. Tunnel was completely dark, and only source of light was headlamp and flash in my camera so the photo is quite dark
According to geological map of area the base rock is Lime and Hornblende from lower devon,emsian
What is this coating? I found this site post-trip, so I tried to answer all question in quide as best I can and I can go into that tunnel again and provide all needed information

Given your description of the base rock and the age of the mine I would suggest that what you are seeing is what in a cave we would refer to as flow stone (A deposit of Calcium Carbonate in one or more of many crystal structures).

Simplified Process:

Ground water often has varying degrees of dissolved carbon dioxide in it.  This carbon dioxide within the water creates a mild acid, carbonic acid.  The acid in the ground water will corrode out Calcium Carbonate in the base rock.  Most ground water will be saturated with Calcium Carbonate, the lime, depending on the concentration of dissolved carbon dioxide in the water.  When the water seeps out of the walls of the mine it has an opportunity to off gas some of the dissolved carbon dioxide.  When this occurs the water can no longer hold as much dissolved Calcium Carbonate and the excess is deposited on the walls in various forms.  The mechanism of dissolving the rock and re-deposition is driven by various equilibrium reactions( involving temperature, water mixing, partial pressures of the gases...).  A karst hydrologist/geologist would be able to give you much more detailed information on the processes. It is this process that forms caves and causes the formations with the caves to form.

The following web site gives a pretty straight forward explanation of what I believe you are seeing. https://en.wikipedia.org/wiki/Karst


Why do greenhouse gases affect ocean temperature so much? As I understand it, greenhouse gases absorb infrared radiation from the sun.  Much of that radiation would otherwise continue and be absorbed on the planet surface in the ocean or on land.

If CO2 increases are causing the oceans to warm, does that happen mainly by convection?  It's counter intuitive to me to think about air convection having that much effect on ocean temperatures.  Why wouldn't greenhouse gases' soaking up of infrared instead cool the oceans, that would otherwise be warmed by the radiation?

No, greenhouse gases do not absorb infrared radiation from the sun... the Earth is really the source of infrared.  The amount of infrared energy from the sun that reaches Earth is insignificant.  Visible light from the sun heats the Earth, NOT infrared light.  The visible light passes through the atmosphere and is absorbed by the surface of the Earth.  Then, the Earth re-radiates that energy back into the atmosphere in the form of heat (aka infrared radiation).  The infrared radiates back into space, but if there are infrared absorbers in the atmosphere, the energy is not lost to space.  That has a net warming effect.  In fact the Earth needs greenhouse gases, or we would all be frozen.  It is a delicate balance between GHG concentrations and climate effects on the hydrologic cycle.

It is important to note that ocean water expands in response to temperature increase... so even just a modest increase in ocean temperature can lead to significant sea level changes.  Human civilization is thus directly affected by oceanic temperature increase due to sea-level rise.

As a side note, the absorption of carbon into the ocean is really of more environmental concern than the actual heating of the ocean, in terms of acidic effects on the marine ecosystems.  The ocean absorbs roughly half the carbon emitted into the atmosphere and there are areas already seeing the negative effects of acidification.


Spatial allocation of road emissions and unit conversion I have a grid shapefile that has a resolution of 1km x 1km.

This grid is then used to spatially allocate road emissions.

My goal is to get the emissions in the unit: μg m-2 s-1(microgram per square meter per second) since this is the emission unit for aerosols in the WRF/Chem

I manage to get the emissions for a certain grid cell by multiplying the total emissions in (micrograms/s) to the ratio of the road length present inside the grid cell to the total road length

Now I have the emission for that grid cell in μg s-1 (micrograms/second).

My question may be silly or simple, but I am thinking if I still need to divide my emissions by 1000 (since 1km = 1000m) to get the final emission unit I need

(Final unit needed:  μg m-2 s-1(microgram per square meter per second))

Am I right? Hoping someone could help me with this. Thanks!

You have got your emissions over a square kilometer. To get to square meters you divide by 1000000 since $1km^2=10^6m^2$.


seismic wave detection standing at the epicenter If a person is to stand at the epicenter of an earthquake. Is it true that he would not detect P-waves nor S-waves echoing off the mantle core boundary?

Since if P-wave that enter perpendicularly into the core is not refracted or reflected and S-waves are absorbed by the liquid outer core?

A person would not detect much else than the surface waves (Love and Rayleigh waves). The later body waves phases are of very low amplitude and are only detected by a sensitive seismometer. Also, at a short offset, it is often hard to pick later phases, even if they occur. There is a critical angle for seismic refraction, but not for reflection. 0-offset reflection originates just beneath the source.

The core-mantle boundary represent a large change in elastic properties, so some energy will bounce back to the surface. We call these phases PcP, ScP and ScS phases (Pressure-core-Pressure etc). The PcP is the reflected P-wave and ScS is the reflected S-wave. The ScP wave is an interesting phenomena, as it's formed as a part of the energy in the S-wave is converted into a P wave and reflected. There are also higher orders of seismic wave phases that can be detected (or modelled in synthetic seismograms) near the epicenter, the PcP2 and ScS2 e.g. (not shown in figure). The PKiKP is a reflection from the inner core.

The travel time diagram gives a rather good idea of when to expect seismic phases at different distances from the earthquake. The distance is in degrees, so 180$^\circ$ is on the opposite side of the planet. Note how the core shields all direct shear waves, that is why we know that the outer core is fluid.


If a specific mass of air is polluted, will the pollution stick with it? Let us say a car pollutes 1m3 of air. Is that pollution attached to that specific air mass or will it fall to the ground or take its own path through the air, independent of the air it first polluted?

And how far can it travel, typically?

Air is a fluid, as such it tends to mix and concentrations, of anything, tend to disperse into the wider volume. Erik's cup of tea metaphor is pretty accurate, currents of air will pull apart your exemplar 1m3 and disperse the pollutants therein, the heavier particulate, which is mainly soot, water droplets from combustion, and unburned fuel, will full to the ground fairly quickly. Finer particulate and reactive gases like nitrogen and sulfur oxides will react with water to form acids and fall back to earth with the next rain storm. The exception to this general rule of mixing occurs when there is an inversion layer that puts a "lid" on vertical circulation in a particular area causing a build up of pollutants which often results in smog formation.


stretch phenomena after NMO application on seismic data after the NMO application we have stretch for far offset , i read a lot of articles but i can't have a complete idea how this phenomena is really created ! any one have a rich explanation ?

My understanding is the following (NB: it could be wrong!).

The assumption is that on a common shot gather, your travel time follows a hyperbolic curve:
$$
f(x)=t^2=t_0^2 + \frac{x^2}{v^2},
$$
where $t_0$ is the zero-offset travel time, $x$ the offset and $v$ the speed of the medium above the interface. The hyperbolic travel-times, for example, create gathers like the one shown in https://wiki.seg.org/wiki/NMO_stretching#/media/File:Ch03_fig1-9.png (which is figure '(a)' below).
Furthermore, the recorded wavelet has the same shape along the time ($t$) direction, regardless of which offset ($x$) you recorded it. Hence, on the figure below, check that there is no discernible change in the wavelet shape in figure '(a)' below as you move along the hyperbolic curve.
Then, NMO correction essentially comes down to stretching the original data. In the figure '(a)' below, I put a red line close to the zero offset point, and a purple line at far offset. In figure '(b)', which is an NMO-corrected picture, I highlight that same lines, with identical colors. You see that the zero offset  (red line) hardly requires a change, but the far offset (purple line) needs to be stretched considerably to create a flat curve in the diagram! A neat feature is that at at ~90% of the offsets, you still see 2 events cross in both figures '(a)' and '(b)', to indeed confirm that we just stretch the traces with no real further magic behind it...

So, the wavelet shape is constant in the vertical direction everywhere in figure '(a)', but is seriously stretched out in figure '(b)'.

From the page of the linked figure: "Note that stretching is confined mainly to large offsets and shallow times. For example, a waveform with a 30-Hz dominant frequency at 2000-m offset and t0 = 0.25 s shifts to nearly 10 Hz after NMO correction. Because of the stretched waveform at large offsets, stacking the NMO-corrected CMP gather will severely damage the shallow events. This problem can be circumvented by muting the stretched zones in the gather."

tl;dr: because the red and purple line in figure '(a)' must become the red and purple line in figure '(b)', the wavelets of particularly the purple line are stretched.


Ocean level rising, or the floor getting higher? I randomly had a question pop into my head: "Why are oceans salty, but rivers are not?"

This article seems to provide a plausible explanation: http://oceanservice.noaa.gov/facts/riversnotsalty.html

It basically says:

Rain keeps putting fresh water into the rivers, which is why they aren't "salty".
Rivers, as they flow, pick up sediment from the earth and push that into the oceans.  It's more or less this sediment that makes the oceans taste "salty".

That leads me to my question:  Is the ocean really "rising", or is the floor just getting "thicker/higher".  The article states that millions of tons of sediment is being deposited on the ocean floor every year.  Most people say/believe that melting ice is causing the ocean level to rise.  I'm not trying to counter that, but couldn't all that sediment be a significant factor as well?  i.e. Maybe the ocean level isn't "rising" as much as we believe, but, rather, the water is getting displaced by all that sediment?

In a geologically static world you might have a point. Erosion would wear down the land surface, depositing the sediment in the ocean - where 'Archimedes principle' would apply, and seas level would rise by displacement. There are two main reasons why this doesn't account for the rate of sea level rise that we are seeing: plate tectonics and observed rates.

The Earth is a tremendously dynamic environment in which plate tectonics drive an approximate balance between crustal destruction by erosion and subduction, as against freshly generated land at 'constructive margins'. This 'wobbly balance' has been going  for the past 3+ billion years, and is likely to continue for several billion years more. The balance of rise and fall in sea level averages close to zero but has positive and negative anomalies over time. In all cases, fluctuations are slow - in the order of millions of years (excluding the much larger short-term fluctuations caused by ice ages). So for all practical purposes the base-line is 'zero change', upon which is superimposed the effects of sea level rise as we exit the last ice age, i.e. the last 12000 years and continuing. On top of that is the recent effect of human-induced climate change. Let us consider these  rates compared to the hypothetical rate of erosion and sea level rise in a static world.

During the 20th century, sea level rise was estimated from tide-gauges, which raises the problem of 'are we measuring rising sea or sinking land?'  For the last decade or so we have been able to measure absolute mean sea levels by satellite. That is, independent of land levels. Now we can re-evaluate old data, and come up with 'real' average rates of rising sea level. They turn out to be about 2.5 +/- 1.5 mm/year for most of the last century. Now they have accelerated to more than twice that amount. See, for example:

https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level

This is the global average, and yes, there are all manner of positive and negative anomalies. Now consider sediment displacement. Measuring the global sediment flux from rivers to oceans is extraordinarily difficult, for reasons which would take several pages to explain. But of the diverse range of estimates the consensus seems to be between 20 and 60 Gtonnes per year for the world's major rivers. Leaning towards the upper bound, and adding in a factor for the minor rivers, and assuming an average sediment density of about 2.5, we arrive at about 40 billion cubic metres of sediment entering the oceans each year. There are about 360 million square kilometres of ocean, so on average, the annual increase in ocean sediment is about 1.1 mm. In reality, this is an upper bound with the a more realistic estimate being about 0.6 mm. So, even without taking oceanic subduction into account, the observed sea level is at least an order of magnitude more than can be accounted for by sediment displacement. Or to answer your question, no, the ocean floor isn't rising at any rate that we can measure. Conversely, sea level is rising at a rate that is consistent with thermal expansion of the ocean surface, combined with nett global ice melt.