Is it possible for an active volcano to become less violent over time? Suppose a violent, dangerous composite volcano whose eruptions destroy the nearby countryside exists. Would it be possible for this volcano to change such that future eruptions are less violent, and less dangerous to anyone living nearby?
Alternatively, would it be possible to terraform the volcano to make its eruptions less dangerous?

The ultimate fate of all active volcanos is to become extinct volcanos, and eventually, objects that are barely recognizable (or not recognizable at all) as extinct volcanos.
This process from pre-volcano to active volcano to extinct volcano to barely recognizable extinct volcano takes a long time, even by geological standards of "a long time". The oldest elements of the Hawaiian–Emperor seamount chain predate the Chicxulub crater, and even those oldest elements are easily recognizable as extinct volcanos. Active volcanos are active for only short periods of time (short by geological standards), a few hundred thousand years at most. That of course is a very, very long period of time by human standards.

Alternatively, would it be possible to terraform the volcano to make its eruptions less dangerous?

Consider the follow hypothetical doctor-patient conversation: "Doctor, it hurts when I do this!" (The patient then bonk himself on the head.) The doctor's response is simple: "Don't do that then." The easiest way to do this with regard to active volcanos is to stop having humans live near active volcanos. What is feasible is to vastly improve the ability to predict when volcanos will erupt. The state of the art in this regard is rather lousy (the state of the practice is lousier still),
That is not feasible; there are too many humans who already live near active volcanos. Terraforming also is not feasible. While the amount of energy released by volcanos per year is a lot less (orders of magnitude less) than the amount of energy the Earth receives from the Sun, it is still orders of magnitude higher than the amount of energy consumed by humanity. To make matters worse, that human consumption is widely dispersed while volcanic energy is concentrated in a few places.


Determining a rock age using volcanic ash layer I wanted to know if volcanic ash deposits found in the geologic record are most useful in correlating the age of rock layers if the volcanic ash was distributed over a large area during a short period of time or large area during a long period of time

I was thinking over a long period of time because it helps more with determining a rocks age.

The main interest of tephrochronology (i.e. dating sediments using volcanic ash layer) is specifically its instantaneity (relatively to geological timescale of course). It is precisely used for higher-resolution dating. An example would be the Kawakawa/Oruanui tephra from New Zealand which is a good isochronous marker bed at 26.5 ka, spread over 1500km, but represents probably only a few months of volcanic activity (see e. g. Manville & Wilson 2006)

Important factors vis-à-vis the usefulness of a specific ash layer however are its geographical extent (a volcanic event that will spread an ash layer over a whole basin would be more useful in that it will be used to correlate a large number of sites together) and maybe its volume (you need a minimum amount of material to work with, I'll assume).

Reference: Manville V. & Wilson C. J. N., 2006. The 26.5 ka Oruanui eruption, New Zealand: A review of the roles of volcanism and climate in the post‐eruptive sedimentary response. New Zealand Journal of Geology and Geophysics, 47: 525-547.


Any site for monitoring equatorial oceanic waves? I  use Carl Schreck's site Equatorial Wave Zoo for monitoring atmospheric waves such as Kelvin wave, Equatorial Rossby wave, filtered MJO OLR, Mixed Rossby Wave(MRG),etc.
Here is an example - Kelvin Wave OLR

In addition I use Matt Wheeler's BOM site(unavailable at the moment) for the same information.

Now I am getting interested in the ocean and it's characteristics as part of my effort in learning coupled models i.e. air - sea models.

Is there a site for monitoring equatorial oceanic waves(in real time with lag) such as oceanic Kelvin wave, Rossby wave etc ? That should help me in identifying along the equator at least where upwelling or downwelling is likely to occur.

I would prefer a site for my basin i.e. Indian Ocean.

There are a few sea surface height datasets available on PO.DAAC (the Physical Oceanography Distributed Active Archive Center), but this one seems to be (near-)real-time and global: the SARAL Near-Real-Time Value-added Operational Geophysical Data Record Sea Surface Height Anomaly.
It was launched in 2013 so there is just 3 years of data so far but it should help I think.

Similarly you have a few current datasets such as the OSCAR 1 degree ocean surface currents dataset which was launched in 1992.

These are all raw datasets though, so you'll have to do the data processing and the plotting yourself.

The altimetry data provides a good way of looking at waves. Apart from SARAL, AVISO provides high quality, remotely sensed, gridded and raw altimetry data.


How to download FLDAS data from earth data I am trying to download transpiration and snow cover data from earth data but unable to figure out how to do that

To download the data from earth data, the following needs to be done

Go to https://urs.earthdata.nasa.gov/ and create an account. Note the username and password which will be used to download the data.

In your linux root create a file .netrc and enter the following
machine urs.earthdata.nasa.gov login your_login password your_password

Go to https://disc.gsfc.nasa.gov/datasets and search your dataset of interest and then click it

4.On the right hand side you will see a get data button, click that button.
5.Now a dialog box will open, click get data

Now another dialog box will open, click download links list, a list of the files will get downloaded.

Now in the terminal fire the following command
wget --load-cookies ~/.urs_cookies --save-cookies ~/.urs_cookies --auth-no-challenge=on --keep-session-cookies --content-disposition -i <url.txt>

Meanwhile before step 7 two things need to be done. One, you need to just create this file ~/.urs_cookies and the second is that you have to authorize the data that you are downloading.


Are strong deep earthquakes that result in minimal impacts common? Happened to see notice of fairly strong earthquakes near Fiji and Peru within the past few days, and thought I'd find they had been fairly damaging... only to find the impacts were quite minimal.  Have to think maybe that relates to the depths they occurred at (563 km and 610 km)?

It seems like an overall busy period, with earthquakes in/near Indonesia, Venezuela, and Oregon as well in about the last week.  But I do feel like I've seen similar bursts in the past.

However, I'm naively not sure I've ever seen such deep ones reported where they apparently didn't cause much shaking.  I figure that may well be just because such earthquakes having minimal impacts simply weren't as publicized in the past??

But, not knowing nearly enough about the differences in earthquake types and patterns, I'd be interested to know:

Were the minimal impacts indeed a consequence of the depths of the earthquakes?
Are such strong earthquakes at those depths expected at fairly similar regularity to similar earthquakes at shallower depths?

Depends where you are in the world, New Zealand has a combination of deep the shallow earthquakes, the shallow ones are associated with the slip-strike faults in the south. The deeper ones on the other hand occur where subduction is the dominant tectonic scheme in the north island, particularly in central and eastern areas. A deep earthquake of a given magnitude will cause considerably less damage to human artifacts than a shallow one; there's more, and softer, rock between the rupture zone and the surface to absorb the energy of the 'quake. If they're deep enough then the hot, plastic, rocks between the rupture and the surface will in fact block most of the L and R shear waves that often cause the most surface damage.


Earthquake probabilities A recent report estimated the probability of an earthquake greater than magnitude 7 on the southern section of the San Andreas fault at approximately 60% over the next thirty years.  How is that probability calculated?

The probabilities are generally based on long term recurrence rates. You can read about the details in the Working Group on Earthquake Probabilities page Earthquake Probability Models.

The last major earthquake on Southern SAF was in 1857 which means that the average slip deficit on the fault is around ~8.0 m (the Pacific plate is sliding past at a rate of 55 mm/yr). Using empirical scaling laws you can estimate the magnitude of the earthquake that is required to release the accumulated strain. However, the 8.0 m slip deficit is an upper bound as some of the deformation in the region is accommodated by other fault systems, e.g., Elsinore and San Jacinto faults (sub-parallel to SAF) as well as the Eastern California Shear Zone which was the site of Landers/Hector Mine earthquakes in the 90s.


How can I get historical weather data for the United States? I went to the web site of the National Weather Service and clicked around but it was fruitless. How can I get historical weather data in the US?

I want to get daily high and low and precipitation for a particular weather station for time periods such as 1930 to 1940.

This the portal for the NOAA historical records

https://www.ncdc.noaa.gov/cdo-web/search

You can specify what you are looking for (Summaries, or other), the time range, the area.

They have records starting in the 1800`s to today.


Can ionospheric plasma disturbances effect the weather? I've read about how ionospheric discharges like jets can be initiated/triggered by lightning, and how lightning can propagate upwards from clouds.  What I was wondering is if this can happen in reverse.  In other words, can ionospheric plasma disturbances/discharges effect/trigger troposphere to ground lightning?

Apparently it is possible for the ionosphere to influence the conditions in the lower atmosphere.

The following are relatively recent results from a paper in 2009, and a summary and up-to-date state-of-the-art can be found in this paywalled, upcoming chapter in a book.

As it turns out, at least in the arctic regions, the ionosphere can have quite an impact on the temperatures.

Geomagnetic storms provide large numbers of energetic electrons, which follow the magnetic field lines. As the magnetic field lines only intersect with the atmosphere in the polar regions, those electrons will preferentially precipicate there.
What follows is Ozone destruction and generation of $NO_x$-species in the Mesosphere and Stratosphere. $NO_x$ molecules are classical long-lived Ozone killers.

The Ozone depletion leads to direct heating of the Polar surface of up to $\Delta T \approx 4.5 K$ by increased UV irradiation. Furthermore, the $NO_x$ can be transported towards lower latitudes, and geostorm induced chemistry seems to have influenced mid-latitude Ozone concentrations measurable in the past for short times. A nice summary also in the Science case for the EISCAT3D-radar (not paywalled, and one can skip directly to page 7 where those effects are described and further referenced).

Those indirect effects are intriguing in that, while some might argue that the ionosphere cannot have any influence on the troposphere, it is forgotten that the radiative energy balance links the whole atmosphere. And it is this balance that can be altered by geostorm induced ozone destruction in the middle atmosphere. It's not exactly what OP was going for (no rise in lightning strikes in the troposphere), but the ionosphere knows their own version of lightning , called Sprites (also a more recent finding in research), which are sure to appear with those events.

Keep in mind that this is still ongoing research.
While a correlation of geomagnetic storms with changing polar temperatures has been already noted in the past, and seems undisputed, the exact mechanism responsible for the correlation is still under scrutiny. Personally I feel there must be more to it, as $4K$ temperature difference just from increased UV irradiation seems a bit much.


Have the landmasses of North and South America ever been part of the same continent? I know this question have been asked very often, but I could not find a proper answer around StackExchange, so I thought this might be a good place to ask:

Have the landmasses of North and South America ever been part of the same continent?

Apart from today, yes.

Back in the Cambrian (~500 million years ago), there was a continent in the southern hemisphere called Gondwana. It was a combination of several modern day continents: South America, Australia, Antarctica and Africa. A small piece of North America that corresponds mostly to modern day northern Canada existed in the northern hemisphere, called Laurentia. Curiously, some parts of northwestern Europe were part of that as well (Scotland, Norway).

Between the Devonian (~370 million years ago) and the Triassic (~200 million years ago) almost every continent on the Earth was amalgamated into one super continent called Pangaea, including Gondwana and Laurentia. During the Jurassic and Cretaceous (~150 million years ago) the entire thing separated due to the opening of the Atlantic Ocean, sending North America north west and South America south west. Since then the two continents were separated, until very recently (geologically speaking) that a land bridge formed between the two.

A very good website with reconstructions of paleogeographical maps is this one:

http://scotese.com/

Note that these are all interpretations and they are open for debate. But the general idea is correct.


How and why humidity and vapour pressure change as a parcel of air with an unchanging amount of water vapor rises, expands, and cools I know that a parcel of air can rise, cool and create clouds and rain but I'm struggling to understand how:

absolute humidity
relative humidity
actual vapour pressure
saturation vapour pressure

change as a parcel of air with an unchanging amount of water vapour rises, expands and cools.

I think that absolute humidity and relative humidity increase, but is there a difference how absolute and relative humidity behave and how actual vapour pressure and saturation vapour pressure behave?

The imortant point is that absolute humidity is defined as

mass of water per volume of air

So if the parcel of air expands, and still has the same water content, there is less mass of water per volume, hence the absolute humidity decreases.

Maybe most if not all of the question depends on this and is much more clear now?


How do climatologists define the "start" of a rainy season in a unimodal monsoonal climate? For a number of gridcells, all between the 500mm and 800mm rainfall isohyets in sub-Saharan Africa, I've got dekadal rainfall (from FEWS).  They are pretty spread out over the continent and there are a lot of them, so they'll have different seasonality, though they should all be roughly unimodal (particularly given the latitudes that I've got).

I want to define an algorithm to define the average start and end of the rainy season.  So far, I'm thinking of the following:

For each gridcell:

Define the month of average maximum rainfall
Find the month prior to the average maximum rainfall month that minimizes the sum of 3 lags of average monthly rainfall, subject to that month containing at least 20% of annual rainfall.  Define it as the average start.
Find the month after the average maximum rainfall month that does the same thing, only with 3 forward lags of average monthly rainfall.  Define that month as the end of the rainy season.

The 20% figure seems arbitrary, though I'm not sure what else to do.  I'm not really a climate scientist, though maybe I could play one on television.  Is there an accepted convention here?  If not, any suggestions on improving my algorithm?

I find the following method very straightforward and elegant. So maybe I can convince you to consider it.

For the South American (SA) monsoon, Silva et al. (2007) and Carvalho et al. (2012) proposed the Long-scale South American Monsoon Index (LISAM), based on the annual cycle of some fields.

For the 2012 paper, where several precipitation datasets are used to calculate what you want, the methodology is as follows:

Compute the empirical orthogonal function (EOF) of a precipitation gridded data on land over SA. (There are some details as weigth by latitude, you may want to do combined EOF etc etc... but the essence of the idea is the same). Note that EOF is computed on raw precipitation, not anomalies.
The first EOF represents the annual (seasonal) cycle of the monsoon. The First PC (PC1) shows it clearly (Figure 2 in Silva et al. 2007)

At last, having the time series of PC1, you can determine:

Start (onset): the day in which PC1 turns positive.
End (demise): the day in which PC1 turns negative.

Further detailes are given in the cited papers. Hope it helps.


Why are there no deep caves in the continental US? According to http://www.caverbob.com/wdeep.htm and http://www.caverbob.com/usadeep.htm, the only cave over 1000m deep in the US is a lava cave in Hawaii. In the continental US the deepest is 505m.

On the other hand in Europe there are two caves over 2000m deep.

The largest cave systems are in the US (Mammoth Cave in Kentucky. Why aren't they deeper?

It seems that the limestone layers are thinner and have been tilted less than in Europe. If this is the case, why?

It is related with the large-scale geology of the USA and Europe. The USA is rather normal in this regard, but the particular geological history of Europe allowed the formation of exceptionally deep caves there.

During Mesozoic, Europe was a closed ocean: the Tethys Ocean, with shallow carbonated platforms. The Alpine Orogeny formed the continent and the carbonaceous materials emerged creating mountains with large carbonaceous units. The meteorization and erosion of these carbonates created big karstic systems with deep caves.

Conversely, the USA has a lot of Paleozoic units where the carbonaceous materials are too compact to form deep caves. You may have shallow Mesozoic caves near the surface but Mesozoic units are thinner than some folded zones of Europe.


How to derive the total potential energy of the atmosphere? In one of Lorenz’s paper (I mean the meteorologists Edward Lorenz), he stated that the total potential energy of the whole atmosphere $P + I$ (means the sum of potential energy and internal energy) is
\begin{equation}
P + I = c_p p_{00}^{-x} \int p^x \Theta dM
\end{equation}
where $p$ is pressure, $p_{00}$ is a standard pressure, and $x$ is the ratio $(c_p - c_v)/c_p$, $c_v$ and $c_p$ are the specific heats of air,$\Theta$ is the temperature and M is the mass.
Based on my knowledge, the internal energy should be a simple form that
\begin{equation}
I = \int c_v T dM
\end{equation}
and the potential energy is actually the gravitational potential energy, which is
\begin{equation}
P = \int g h dM
\end{equation}
where $g$ is the gravitational acceleration and $h$ is the height. So my question is how did Lorenz derived his form of the total potential energy of the atmosphere? The original paper is Energy and numerical weather predictionLorenz 1960, and this equation was the first equation in the paper.

First, let's acknowledge this fact: $$c_p=R+c_v \tag{1}$$,
where $R$ is the specific gas constant.
This means that $x=R/c_p$.
Rearranging the equation, we can see that $$P+I=\int{c_p\left(\frac{P}{P_{00}}\right)^{R/c_p}\Theta dM}\tag{2}$$
Notice that $\left(\frac{P}{P_{00}}\right)^{R/c_p}$ is the Exner function. By extension,$$P+I=\int{c_p T dM}\tag{3}$$ If we rearrange (1) we can get
$$P+I=\int{c_v T+RT dM}\tag{4}$$.
You observed correctly that $I=\int{c_v T}dM$. This leaves $$P=\int{RT dM}\tag{5}$$.
Using the ideal gas law,$$P=\int{P\alpha dM}$$, where $\alpha$ is specific volume. This makes (4) look like $$P+I=\int{c_v T+P\alpha dM}\tag{4}$$ which is just a restatement of the first law of thermodynamics: $\delta q = c_v dT + p d\alpha$


Why is it colder in the mountains than at sea level? I have heard numerous reasons why that is. I just want to know which one is right.

The traditional answer basically comes down to the physics concept of adiabatic cooling, a description of which is:

There is less pressure as you go up in the atmosphere (basically due to less air weighing down)
Air takes up more volume at lower pressures
Since there's nothing else to supply the energy needed to expand, the air employs the energy that was in the temperature (which is the amount of air molecule movement).  The result is that air higher up is cooler by about 10 C per km (for dry air... moisture releases heat, reducing the cooling to about 6 C per km)

But, in summary, air is cooler higher up in the atmosphere.
And that is certainly the dynamics explaining why air temperature decreases as you go up throughout the troposphere (the only layer of the atmosphere we generally encounter in our lives... other things do come into play in other layers... such as ozone collecting and rereleasing energy from the sun in the stratosphere, actually causing warming with height there).
However, it seems that this typical explanation actually falls short.  Because if you pay attention to the wording of that "because air higher in the atmosphere is cooler" phrase, it doesn't actually answer the question.
Because all locations you're considering are surface locations, whether they're at sea level or in the mountains.
And the surface layer basically does all the absorbing of the troposphere's sunlight (the solar radiation absorbed by the troposphere is negligible).
And after all, mountainous places receive the same amount solar energy as lower elevation sites (in fact, technically a tiny bit more, that tiny tiny bit I mentioned that the troposphere takes out for sea-level locations).
Where am I going with this?
If the ground gets the same energy.  And if it's made of the same material and has the same color, it would therefore gain the same amount of temperature.  And then the air's heating comes up from the ground below.
So then, once again, why are mountains cooler?!?
Because of their EXPOSURE.
Air predominantly flows horizontally rather than vertically (basically consider this an effect of gravity and the resultant air density distribution).
And it's that horizontal wind that changes everything.  Because most sites on mountains are generally quite proximate (at least regionally) to places where winds will blow in air that was higher above the lower elevation ground surface, and thus colder.  The air on mountains, with limited protection (and because winds are higher aloft) is mixed strongly with the neighboring adiabatically cooler air that's sitting above the sea-level sites.
Just for more convincing this is vital, I invite you to look at these climatographs comparing two cities in the US.

What are these towns?
Red: Wendover, Utah (elevation 4,291', latitude 40.73° N)
Blue: Latrobe, Pennsylvania (elevation 997', 40.32° N)
Over a km difference in height.  So you'd expect Latrobe to be about 18°F warmer.
But it isn't.  Instead it's cooler (especially during summer afternoons, which average up around 10 degrees apart).
Why?  Because a lot of the intermountain west is actually quite a wide flat elevated basin\plateau.  So in the central part of this area, there isn't as much exposure to the colder air coming in from places with lower topography.
Rest assured, it was very hard to find examples like this.  You need a very wide flat elevated area, which is fairly uncommon.  Plus just the right place to compare to for emphasis.
There are other common factors that do enhance the cold predilection on mountains, including the lower moisture content at elevation, the often lighter land color of raised terrain, and the increased cloud coverage due to air rising orographically.
But the fundamental reason it is colder on mountains is adiabatic cooling and the exposure of raised locations to that colder air.


Is surface temperature relevant for the creation of high- and low pressures areas? Trying to truly understand high- and low-pressure systems, I find explanations contradicting.
The question is: Where is the mistake in my line of thoughts?
Looking from a surface point of view:

More sun = more heat.
More heat = higher ground pressure (just like heating a bottle)
Higher ground pressure = Hot air wants to expend to all directions
Expending air (also) the vertical axis = rising air, cloud formation.

That is coherent with the notions

that high-pressure areas come from the equator while low-pressure areas come from the poles
that there is a circulation of air rising around the equator and sinking at the poles.

But it conflicts with the notion

that air sinks into high-pressure areas and
hence high-pressure areas have fewer clouds than low-pressure areas.

If I flip everything, that conflict is solved while suddenly global circulation and "H from Equator" doesn't fit into the picture.
Additionally, it seems that sometimes air sinks, creating the higher pressure underneath, and other times it starts with the high pressure and air rises. So I might assume that both high and low pressure can origin from both high and low temperatures, and at both, air can rise or fall. Or in other words: whatever, it's free-for-all.
Where is my mistake?

Your logic is wrong. Do not think of the atmosphere as a closed system. Your analogy of heating a bottle is not appropriate.
In uncomplicated terms, the Sun heats the ground, the ground radiates thermal energy heating the air just above the ground and the heated air rises. In rising, the air heated has less density than the surrounding air. By rising it exerts less pressure on the ground than the surrounding and thus has a lower pressure relative to the air surrounding it - it is a low pressure system.
The rising air reaches the tropopause, around 10 to 15 km above sea level, where it is no longer buoyant. Forced by the continually rising air it travels away from the heated zone. As it moves at such elevations  the air cools, increases in density and falls to the ground. As it falls the pressure it exerts on the ground is higher than the air surrounding it and it forms a high pressure system.
The system is best explained with Hadley Cells.
Air tends to rise around the equator and at latitudes 60° north and south, where most of the low pressure systems are located. Air tends to fall around latitude 30° north and south and the poles, where most of the high pressure systems are located.


Do volcanos really create fertile soil? I'm developing an ecology for a portion of my world that's geologically active and I remembered hearing that volcanic ash in soil makes it more fertile to agriculture.

After extensive googling I've found surprisingly little information about the subject, and nothing convincing. Is this just a science "fact" that gets spread around without actually being completely true? If it is true, or partly true, what is the chemical process at work, and under what conditions is it the most effective?

As always: It Depends.

Assuming enough water and sunshine, crop growth rate boils down to the concept of limiting nutrients. These may be: nitrogen (via ammonia or nitrates), phosphorous (via phosphates), potassium, and sometimes others.  In typical continental settings (i.e. subduction related vulcanism), lavas may be enriched in potassium and phosphorous but not nitrogen. So there will be some effect from rock composition.

Perhaps more important, there will be a refresh effect. If we have regular ash falls, this will 'top up' the soil as compared to normal soils, replacing nutrients. This is a similar effect to flood plains.

So, to answer, it tends to be true that volcanic eruptions increase fertility, but the most effective kind are from explosive ash-fall vucanism you get in regions like Italy or the US Cascades. The kind of flood-plain basalts you get in Iceland probably don't help.


How does El Niño affect thunderstorm/tornado/CAPE activity in the central United States? Or in other words, does it increase the amount of heat+moisture advection from the Gulf of Mexico into the Central United States? And is this increase associated with increased CAPE/tornado/thunderstorm activity?

According to the NOAA document A Comparison of Rawinsonde Data from the Southeastern United States during El Niño, La Niña, and Neutral Winters (Sankovich et al.) report that

  Schaefer and Tatom (1998) considered
  the number of tornadoes per year in the United
  States and sea surface temperatures (SST) in
  various portions of the Pacific Ocean to try to
  discern an impact of ENSO on the occurrence of
  tornadoes. Their statistical inquiries showed no
  major influence, but their data does show a signal
  that more tornadoes tend to occur in the mideastern
  states during the La Niña phase.

However, another study cited by Agee
and Zurn-Birkhimer (1998) state

  that tornado
  occurrences do not favor one ENSO phase but
  rather exhibit a shift in geographic location. For
  example, their results suggest that more
  tornadoes occur in the lower Midwest, Ohio Valley,
  Tennessee Valley, and mid-Atlantic region during
  the La Niña phase than in any other phase.

This is further concluded by Bove (1998) concluded that

  increased tornado activity over
  the Ohio and Tennessee River Valleys during the
  cold La Niña phase.

Sankovich et al. conclude that

  Like all climatological signals, they [ENSO]
  represent long-term trends. They do not provide a
  tool for making seasonal or other extended
  forecasts; rather, they show that other things being
  equal, the atmospheric stratification over the
  Southeastern United States is typically less
  compatible for the occurrence of severe
  thunderstorms (3/4” or larger hail, thunderstorm
  gusts of 58 mph or faster, tornados) during a La
  Niña winter than during Neutral or El Niño winters.

In the paper A seasonal-scale climatological analysis correlating spring tornadic activity with antecedent
fall–winter drought in the southeastern United States (Shepherd et al. 2009), state that to

  remove the uncertainty
  associated with the ENSO debate, we consider a new approach
  by focusing on antecedent drought conditions for a region.

Whereby, they conclude that the relationship between drought and incidences of tornadic and severe thunderstorm behaviour to possess a

  statistically significant tendency for
  fall–winter drought conditions to be correlated with below-normal tornado days the following
  spring in north Georgia (i.e. 93% of the years) and other regions of the Southeast. Non-drought
  years had nearly twice as many tornado days in the study area as drought years and were also
  five to six times more likely to have multiple tornado days

References:

Schaefer, J. T., and F. B. Tatom, 1998: The
relationship between El Niño, La Niña, and
United States tornadoes, Preprints, 19th Conf.
on Severe Local Storms, Minneapolis, MN,
Amer. Meteor. Soc., 416-419.
Agee, E., and S. Zurn-Birkhimer, 1998: Variations
in USA tornado occurrences during El Niño
and La Niña, Preprints, 19th Conf. on Severe
Local Storms, Minneapolis, MN, Amer.
Meteor. Soc., 287-290.
Bove, M.C., 1998: Impacts of ENSO on United
States tornadic activity, Preprints, 19th Conf.
on Severe Local Storms, Minneapolis, MN,
Amer. Meteor. Soc., 313-316.


Looking for a complete table of minerals for a database I am building an open source spatial geochemical/geological observational database for mineral exploration.  I need to fill the mineral lookup table.

Where can I find a complete list of all minerals and mineral-groups, chemical formula, Strunz and/or Dana number, or any other data.

I want the Strunz and/or Dana numbers so I can query the data and recode the data based upon mineral groups.

After looking at the source from nrmima.nrm.se/IMA_Master_List_2016-03.pdf, I discovered the:
IMA Database of Mineral Properties
Created and maintained by the RRUFF Project in partnership with the IMA
http://rruff.info/ima/

I downloaded all the minerals in a csv file.  This worked very well for two reasons.  First they had various chemical formula formats that were favorable for entry into a database.  Second they had a mineral group column.  The mineral group column was very important since often in the field the exact mineral is not noted but a group such as chlorite or garnet.  I will worry about Strunz or Dana numbers/codes at a later date.


Human hurricane forecasting? I'm referring to an article I've read on Ars Technica about the differences in various weather models.
One thing that stood out for me was a mention of

  Forecast models typically show their skill with three-, four-, and five-day forecasts. For simplicity's sake, we will focus on 120-hour forecasts. At this lead time, the average error of the European model with respect to Irma has been about 175km in its position forecast. The next best forecast is from the hurricane center, which is slightly more than 300km. An automated model, then, has so far beaten human forecasters at the National Hurricane Center (looking at all of this model data) by a wide margin.

A similar thing about models beating human forecasters was also mention in another article linked in this article and nowhere is there any other explanation for it, so:

Do humans seriously forecast extreme weather?
I get it, that meoteorologists get trained in physics, hydrodynamics etc, but I can't believe that humans try to compete with computers in crunching the numbers on differential equations. Or how else does 'human forecasting' work?

The forecasts issued by the National Hurricane Center are made by people. Those people use every single tool they have at hand. This includes forecasts made by automated tools such as the European Centre for Medium-Range Weather Forecasts (the "European model"). It also includes forecasts made by several other models. The human-generated forecasts account for the skills of those various models, for the ages of the forecasts made by those models, and for data that are not used by those models.

When multiple models more or less agree, the human forecasters use techniques and their judgement to split the difference. There is no splitting the difference when models disagree massively, as currently is the case with Jose. The current forecast (advisory 20A at 0800 AST on 10 September 2017), depicted graphically below, appears to be exactly that made by the ECMRWF.


Difference between sulfuric clouds and sulfuric haze (on Venus)? In a conversation in comments below this answer to the question in Space Exploration Could we see the surface of Venus after the explosion of a H-bomb in its atmosphere? we're speculating about what terms like sulfuric clouds and sulfuric haze mean, and in particular if they are both made of aerosols based on water.

What might be the difference between sulfuric clouds and sulfuric haze, and do both still require water in the form of an aerosol?

According to Wikipedia, haze is an atmospheric phenomenon in which dry particulates obscure the clarity of the sky.

Whereas haze is thought of as a phenomenon of dry air, mist formation is a phenomenon of humid air.

However, haze particles may act as condensation nuclei for the subsequent formation of mist droplets; such forms of haze are known as "wet haze".

In meteorological literature, the word haze is generally used to denote visibility-reducing aerosols of the wet type.

According to Wikipedia,  a cloud is an aerosol with a visible mass of liquid droplets, frozen crystals or particals suspended in the atmosphere, and the droplets don't have to be made of water.

This article states that standard cloud models for Venus consider that all clouds and hazes are composed of liquid droplets of sulfuric acid mixed with water, with sulfuric acid making accounting for 75% to 96% by weight of the cloud composition.
However, other minor constituents may make up the cloud particles.

Clearly regarding the mixture of water and sulfuric acid there is no distinction between clouds and hazes on Venus.

In another article it can be seen on images that the altitude from 30-48 km of the atmosphere of Venus is named "the thin haze region ", whereas
the altitude from 48-50 km is called "lower cloud region ".

The figure on page 14 of the dissertation of Yeon Joo Lee also shows that below the cloud layers there is this "thin haze region" which has almost a thousandfold less extinction coëfficiënt than the clouds above it.

Apparently this huge difference in absorbing the light explains the difference and appearance between sulfuric clouds and sulfuric haze on Venus.

Figure 4 of Bullock and Grinspoon 2001 (paywalled) The Recent Evolution of Climate on Venus (Icarus 150, 1, Mar. 2001, pp 19-37) shows the abundances of H$_2$SO$_4$ and H$_2$O in the cloud region of the atmosphere of Venus. A non-paywalled version can be found with an internet search, but since it does not use https I won't include the link explicitly here.

Explicitely it shows H$_2$SO$_4$ as a gas in the thin haze region and only in the cloud region there is a mixture of water and sulfuric acid liquid.


How do simulate a cloudy day for a science fair project? I'm trying to help my 5th grade daughter with her science fair project.  She wants to test that water heats up more on a sunny day than a cloudy day.

To help her, I'm trying to figure out how to block a lot visible light but allow IR and UV through.

I know I could use smoke or dust, but can't figure out how to contain it for an experiment.

My suggestion would be to use shade cloth. You can get it in different densities and different colours. I know this is not the same as clouds, but get her to think about what the differences are and how that might affect the results. If I were a judge I would be very impressed to see a section like that on the poster.


Colder winters along the North American Atlantic northeast Why are winters along the North American Atlantic northeast coast colder than winters along the North American Pacific Northwest coast?

As air moves over the surface of the earth, the properties of the surface (such as temperature) can alter the air. Since at those particular latitudes, the predominant wind direction is from the west and partially the north, it is important to note what is north and west of both locations.

West of the northeast Atlantic is land, and northwest is Canada. This means that the air is drier, and prone to have characteristics of mimicking continental- polar (cP) air. Continental polar air tends to be dry (since it is over land) and cold (since it is coming from an area that receives less sunlight).

West of the northwest pacific is the Pacific Ocean, and northwest is still the Pacific Ocean. This means that the air is more moist and prone to have characteristics mimicking the air over the ocean, called Maritime polar (mP). Since the water tends to warm slower than the land and also slower than the land, the air over the water is usually warmer during the winter and cooler over the summer than the air over the land.

To summarize:

At those latitudes, the wind usually blows from the north and west.
During the winter, air over the ocean is warmer than air over land.
North and West of the Northwest coast is the Pacific ocean, while land is west of the Atlantic coast.
Therefore, the winter in the northwest coast is usually warmer than the winter along the northeast coast.


Has PAZ satellite been launched? There are several mentions in the net about Spanish radar satellite called "PAZ", which should work in tandem with German TerraSAR-X and should have been launched in 2015. But most results I can find talk about plans, and no real announcement that it has been successfully launched. Anyone knows the current status of it?

The launch was scheduled for 2017 using a DNPER launch vehicle. It seems that the contract contract between Kosmotras and Hisdesat was canceled and that resulted in an uncertain launch date. The satellite has been finished since 2013 and it has been waiting for over 3 years to be launched.

Source

The dispute between Russia and Ukraine resulted in EU imposing sanctions on Moscow. As a result, Russian President Vladimir Putin supposedly signed a secret decree halting launches of the Dnepr rocket (based on the Soviet-era R-36M missile that was made in Ukraine). It also affected the launch of several satellites of the Iridium constellation. With the Russian government withholding approval for the launch, everything felt apart and now the parent company (hisdeSAT) has to find another launch platform. The two main candidates are the Soyuz rocket from Roscosmos and the Falcon rocket from Space-X, but the cost of either alternative is much higher.


What causes smog to clear up on some days? I notice that in my town sometimes the pollution/haze/smog sticks around on some days but then on others the sky can be completely clear. I don't necessarily see any difference between windy and non windy days so it seems like it is something else at work.

My guess would be a combination of boundary layer, cloud cover, and emissions (though that isn't terribly specific).

Smog is a combination of nitrogen oxides (which cause smog's brown color), ozone, and particulate matter. These in turn form from emissions of hydrocarbons and nitrogen oxides from vehicles and industrial combustion. If those emissions are lower on some days, the smog's going to be better. But that's a boring answer if you don't have a way of tracking your town's emissions.

Ozone and nitrogen oxides break down and reform in sunlight, driving a number of chemical reactions; this is why smog is usually more of an issue in the daytime. Cloud cover can reduce these sunlight-driven reactions, which can help with smog. Rain washes out most of the particulates, so even light rain overnight can help with smog.

Since the emissions are coming from the ground, they're usually trapped within the boundary layer (https://en.wikipedia.org/wiki/Planetary_boundary_layer). Air within the boundary layer is fairly well-mixed. If the boundary layer is low, then the concentrations are high and the smog is bad. Boundary layers increase in size throughout the day as the ground heats up and convection mixes higher into the atmosphere, so a number of places (including my hometown of Los Angeles) have their worst smog in the morning. How long the smog sticks around depends on how quickly the boundary layer can grow. This depends on a ton of interesting factors -- there are entire textbooks on the subject (The Atmospheric Boundary Layer, An Introduction to Boundary Layer Meteorology) -- but start with the Wikipedia article and let us know if you have more questions about it!


Do desalination plants affect the ground water table? I saw an article recently in the newspaper stating that a desalination plant near the seashore was causing the ground water to become saline. Usually I would expect that areas where saline water already sometimes intrudes would see this, but in this scenario the residents around the plant claimed that before the establishment of the plant the ground water had tasted "as tender coconut".

If they are right then what may be cause for that saline water intrusion...

With desalination plants the import thing to consider is from where such plants are taking water and where they are disposing of their waste.

The waste from desalination plants is either salt of very salty water. If the plant you are referring to is just dumping its waste on the ground nearby or into a nearby river or water channel it is possible that the salt in the discharge water from the plant will enter the ground water, making it salty.

Generally this should not happen and in most situation it does not. With desalination plants near the seashore, the waste water is discharged back into the sea, but far from the intake for the plant.


How do fold mountains form? I know mountains form due to the collision of tectonic plates. But how does such a slow collision have such great impact as to create huge mountains.
Is there any intuitive explanation for this?

Some mountains form from the collision of tectonic plates; volcanoes are different, and there are some other mechanisms too.

IANAgeologist, and perhaps one will be along to give a more technical explanation, but intuitively, it's about viscosity:

If you push an object very slowly into a bowl of water, nothing much happens - the water flows around the object.

If you push the object equally slowly into a tray of treacle, it's harder work, and you can form a "hill" in front of the object. The treacle flows too slowly to get out of the way.

At very high pressures, rock behaves a bit like the treacle[1] - only much more so. It takes a huge force to push one lump of rock into another, but because the rock flows very slowly, by pushing very slowly you can still create a mountain.

[1] I realise it's not exactly like treacle. If you're an expert here, feel free to post another answer getting it right :-)


What is the difference between an inertial instability and a symmetric instability? Inertial and symmetric instabilities are fluid instabilities present in atmosphere and ocean.

What are the difference between an inertial instability and a symmetric instability?

Inertial instability is similar to the centrifugal instability in that we are looking at the stability of parcels to horizontal perturbations.  In the inertial case, however, the initial state is geostrophic balance rather than cyclostrophic balance.

Symmetric instability is the case where a parcel is inertially stable to horizontal perturbations and statically stable to vertical perturbations but unstable to perturbations between the horizontal and vertical.  An additional constraint is that the flow is symmetric and only varies in two dimensions.  This instability was also first examined as centrifugal instability, but with added baroclinity.  Conditional symmetric instability (CSI) is related to symmetric instability with the difference being that $\overline{\theta}^*_e$ rather than $\overline{\theta}$ surfaces are used to asses the instability.

For more information on these mesoscale instabilities, you can find information on them in Markowski and Richardson (2010) pages 49 (inertial instability) and 53 (symmetric instability).  Holton (4th ed, 2004) treats these instabilities on pages 204 (inertial oscillations) and 279 (symmetric instability).

Markwoski, P., and Y. Richardson, 2010: Mesoscale meterology in midlatitudes. Wiley, 430 pp.
Holton, J. R., 2004: An introduction to dynamic meteorology. Elsevier, 4th ed., 535 pp.


Movement of the Prime/Greenwich Meridian The line on the ground at the Greenwich Observatory is itself on a tectonic plate that is moving, so is the prime meridian still 0 degrees, 0 minutes, 0 seconds?

If not - what are the implications (if any)? For example, if my GPS says I am 100 km west of Greenwich Observatory, am I 100 km west of 0 0 0 or 100 km west of the line on the ground? Who (and what) decides this kind of thing?

Curious punter, not a scientist....

Remember it is just an imaginary line on the Earth's surface used for georeferencing purposes, so movement of the reference line has no implications so long as we can still reference to it (hence the International Reference Meridian).

In fact, the position of the Greenwich  meridian has changed throughout history, mainly due to the Airy Transit Observatory (the original location of the Greenwich meridian) being built next door to the previous one (to maintain the service to shipping). Such changes had no significant practical effect. Historically, the average error in the determination of longitude was much larger than the change in position. The adoption of WGS84 ("World Geodetic System 84") as the positioning system has moved the geodetic prime meridian 102.478 metres east of its last astronomic position (measured at Greenwich). The position of the current geodetic prime meridian is not identified at all by any kind of sign or marking (as the older astronomic position was) in Greenwich, but can be located using a GPS receiver.
Due to the movement of Earth's tectonic plates, the line of 0° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres; that is, towards the Airy Transit Observatory  (or the Airy Transit Observatory  has moved toward the east, depending on your point of view) since 1984 (or the 1960s). With the introduction of satellite technology, it became possible to create a more accurate and detailed global map. With these advances there also arose the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning - the International Reference Meridian.

See:

Malys, S., Seago, J.H., Pavlis, N.K. et al.  (2015) "Why the Greenwich meridian moved" Journal of Geodesy. 89 (12) pp 1263–1272

Dolan, G (2013). "WGS84 and the Greenwich Meridian". The Greenwich Meridian.


Calculating Love numbers (tidal deformation) for non-homogeneous planets I have a question about planetology. I'm not sure if it belongs here, but the astronomy stack exchange seemed a bit odd, too.
I would like to enhance my understanding of Love numbers. Love numbers quantify the deviation from equilibrium tides on a planet. For a homogeneous body, you can write, for example
$$ k_2 = \frac{3}{2}\Big(1 + \frac{19}{2}\frac{\mu}{\rho gR}\Big)^{-1}$$
where $\mu$ is the rigidity, $\rho$ is the density, $R$ is the radius of the planet and $g$ is the gravity constant. I have been taught that for non-homogeneous bodies, e.g. planets with a core etc, there is no simple analytical expression and you need numerical techniques. I have been reading a paper about exactly that, but I don't see what equations are used to obtain $k_2$. In this article, the interior of the planet is described by Maxwell models and a pseudo-period Andrade model. These rheological models are used in conjunction with so called structural models for the interior. It's not clear to me what these are, either. I think they are equations for pressure and temperature. The article then says: "Based on these, the tidal Love number $k_2$ is calculated and compared against measurement."
Does anyone know what equation needs to be solved to obtain the $k_2$ Love number? Or what steps need to be taken to do such a calculation? Or if you know a relevant reference, anything is greatly appreciated, because I don't know where to start.
EDIT: I found a calculation for planets in hydrostatic equilibrium (see my own answer below). If someone knows the equations for a terrestrial planet with viscoelastic behaviour, I would still be interested in that.

I actually just found an article that explains such a calculation in detail, using a matrix propagator method: Padovan et al. 2018
This is a calculation for gas giants or bodies in hydrostatic equilibrium. The calculation of the Love numbers for terrestrial planets that have viscoelastic behaviour requires the rheology (which includes models for the rigidity). I did not find the equations that need to be solved in that case, but a similar matrix propagator method can be used apparently.
I will not repeat the matrix propagation method here, as it is quite well elaborated in the paper I just mentioned, except for the equation you have to start from, which is explained in another article, but looks a bit too complicated for me at this point.


Global areas below sea level dataset? Anyone know of an existing dataset of areas below sea level? Vector extents would be ideal. Doesn't need to be super detailed for my purposes, but it would be good if it included all areas (e.g. over-estimate is better than under-estimate).

There is a List of places on land with elevations below sea level on Wikipedia, which looks like it's fairly complete, but finding the extents of the below-sea-level area for each of those locations is not likely to be simple.

You could try building one using a global Digital Elevation Model. There are several freely available like TanDEM-X, SRTM, or ASTER GDEM. You'd have to look for all the pixels containing a negative value. Adjacent negative pixels would give you connected regions below sea level, and you could easily estimate their area just by multiplying the number of pixels of a region by the area of one pixel.
I'm not saying it would be easy. The main challenge would probably be that some regions would be split across several tiles.

An update on this. I spent a couple hours trying this as an exercise. I downloaded three tiles from the ASTER global DEM centered around Badwater Basin (Death Valley, USA), one of the regions in the Wikipedia list. I wrote a small (~20 lines) Matlab script that does the following:

Binarization of the images, so that every pixel above sea level is set to 0 (black) and every pixel below sea level is set to 1 (white).
Concatenation of the three arrays in order to have only one image for the next step (I manually added a fourth, all black array for the sake of dimension consistency).
With a binary image, Matlab has a useful function called regionprops that finds all connected regions and returns their properties, such as their area (as a number of pixel).

Here is the result:

I'm pretty happy with it, as it looks very similar to the yellow area labelled "below sea level" on this USGS map.

Now, how does it translate in terms of area? According to Matlab, the white region has an area of 1 236 989 pixels. Each pixel being approximately 30x30 meters, it yields a below sea level area of 1 113 km$^2$. The only figure I could find online is 1 425 km$^2$ from Encyclopedia Britannica, with no source. But for me that's close enough to have confidence in my estimate!
Some shortcomings:

I knew exactly which tiles contained this particular below sea level region. If you were to go through all tiles (almost 23 000!), you'd have to figure a way to detect when a region is split across multiple tiles in order to reconstruct the region by concatenation. Doesn't seem that hard, but not trivial.
I did not take into account the fact that there is a one pixel overlap between adjacent tiles (it should not change change the result much).
I did not take into account the fact that pixel size actually vary (the resolution is one arc second, which is ~30 meters at the equator but changes with latitude).


Where can I get mineral testing done? Where can I send a sample of material to be tested to see exactly what it is?

Do you have an address where items can be sent for testing and evaluation?

I have a softball size rock that was given to me by my father some 50 years ago and the story that goes with it is awsome if true and would make this a great item to be passed down from generation to generation as a keepsake. (But only if I can verify the rock.)

I would like to try and validate the material and the story just for my own satisfaction.

I don't have a lot of money so if you know of someone or somewhere I could send a sample for testing and evaluation I would greatly appreciate it.

It depends on what type of test. If you just want simple identification this site offers a tentative one and any museum with a mineralogy section will have someone on staff who can do identification and they are usually happy to do it. Ditto for an college with a geology class/program. you might want to call/email the professor in question first to make sure they are willing. All these are usually free, although for the museum you should at least pay the admittance fee out of courtesy. You can also look for a local rockhound club.


Tasmanian earthquake patterns While researching across the Australian Earthquake Engineering Society archive about Seismology, I came across an earthquake map of the Australian island state of Tasmania (below):

I am aware that Australia experiences intraplate earthquakes, so I am not surprised the earthquakes occurring. However, what I am wondering is, is there a geological explanation for the hook-like pattern of earthquakes occurring off the north-east coast of Tasmania?

I can't prove it, but I strongly suspect that most, if not all, of the earthquakes in that north-south lineation along the eastern Bass Strait are caused by slumping of sediments in a series of listric faults. That whole continental edge is a very thick heap of deltaic and continental sediments of about Kimmeridgian (late Jurassic)to late Eocene age. There is a brief summary in: Submarine canyons of the continental margin, East Bass Strait (Australia).

As to 'the hook', it is surely no coincidence that this exactly coincides with 'Flinders Canyon', which you can see on Google Earth. What you can't make out on G.E. is the enormous size of the Canyon and its sedimentary catchment area. The sediments here are up to about 3 kilometers thick, and a massive submarine amphitheater funnels sediment into the canyon in periodic surges. I would bet a month's beer money that the 'hook earthquakes' were caused by occasional turbidite collapses of sediment down this steep canyon. The magnitudes, location, slope and approximate frequency are all spot on.
If you include the smaller canyons along the same continental edge then it is estimated that some 80,000 to 90,000 cubic kilometers of sediment have collapsed from this continental edge since the Late Eocene, i.e. over the last 40 million years.


What is this "hole" that blows sand into the air? This video clip, "Strange Hole in the desert Blows Sand into the air in Saudi Arabia" (alternative link), shows a hole in the desert that erupts like a geyser, except with sand instead of water. It is apparently located in the Aljof area in the north of Saudi Arabia.

The commenters on this video (and its duplicates) have come up with many conjectures, including:

natural gas from a freshly-drilled mine
a broken pipeline underneath.
a pressure build up of certain gases, like methane.
a fake video.
seismic or volcanic activity.

Can anyone explain what this phenomena is?

Blowholes are formed through small openings in the ground through which air will blow out or suck in.They have been likened to natural fans or vacuums.Blowholes are dependent on weather: air temperature and pressure interact to create the stunning effects when air travels from higher to lower density. When outside air warms up, it may eventually become the same density as air underground, causing air flow at the blowhole to stop.This outside air continues to warm up, causing air to begin blowing out of the hole.


is this a rose quartz pink quartz looking stone 19 pounds found in a vehicle in new york state

Yes, it is rose quartz. There are a number of mines in eastern New York near Vermont where rose quartz been found. Massive rose quartz can occur in granitic pegmatites.  I could not google to find images of any massive rose quartz from the northeastern United States but it is very similar in appearance to the rose quartz from Black Hill of South Dakota.  The rose quartz deposits from the Black Hills are the largest on earth and the material is sold world-wide.  Locally here in Ohio, I know of the landscaping company that has bulk rose quartz for sale.

You're specimen might be local to New York but might also be from elsewhere.

See Mindat.org


What causes these mound-like ground formations? Whilst riding on Mam Tor in Castleton, England I came across this scene (not my photo) and I would like to know what causes the formations which I have ringed in red. They look like piles of earth have been deposited a long time ago, but clearly that can't be the case, so what causes them?

Another image of these mounds

They're landslide deposits; Mam Tor gets its name, which translates as "mother hill", from the regular landslides that come off the higher slopes and form hillocks further down into the valley.


Are there four-times daily sea surface temperature (SST) data product for Tropical Cyclone research? Now I am working on a Tropical Cyclone project, and want to figure out the SST status when a TC passes by. I've been searching for SST products, yet there only show up to daily SST. My question is does there exist four-times daily SST, and if no, why is that?

The issue with satellite SST is that in most cases it is captured via infrared radiometers. Those sensors can't see through clouds. Daily products are used to try to remove the cloud effects. Alternatively, passive microwave radiation can be used (e.g., TMI, AMSR2, AMSR-E). It is not affected by cloud cover, but has lower horizontal resolution (~25km, vs 1-5km from IR).  You can try to get the individual passes of each satellite and deal with gaps: For instance


Clarification of overbank flows Apologies for what may be a very basic question, but I am struggling to understand the definition of overbank flow.

Overbank flow is defined along the lines of this: Water flow over the top of the bankfull channel onto the floodplain.

Does this flow include the flow that is in the channel? or is it only the flow that is over the channel?

I am working through a document that seems to say one thing with the words, and a different thing with the mathematics.

If Qf = overbank flow; Qb = bankfull flow:

Is the total flow = Qf, where Qf >= Qb
so if Qf = Qb all flow is in the channel

Or, is total flow = Qf + Qb, flow in channel plus flow out of channel?

Overbank flows means flows that are greater than the channel can handle. Thus the water flow goes over the (river/stream) banks and onto the flood plain.

Bankfull flow would be the maximum water flow that the channel could contain.

Note that flow in the channel isn't necessarily the bankfull flow. Of course if there is overbank flow then there must be bankfull flow.

total flow = Flow in channel + Overbank flow


What are the farthest Time Zone locations? What place in the world has the farthest time zone? I heard it was Kiribati but then I found that Apia (Samoa) is further. So, is there anyplace that is the farthest in time than these two? What about Solomon Islands?

The original wording of your question revealed that you are missing something about what time zones are, and how they are defined. So let me start from the begin. If you are impatient, you can jump forward for the answer in bold font after the figure, and come back here for the explanation.

Time zones are defined by by man just to facilitate time keeping. In principle the idea is that time anywhere should be such that noon happens when the Sun reaches its maximum elevation over the horizon.

Time keeping wasn't easy in the past. Therefore, time was kept by astronomical observatories. And due to the extent of the British Empire, in history was the observatory at Greenwich the one that stayed as the global reference point for time and longitude (because at first the main utility of precise time was to compute precise longitudes).

As Earth's rotates 360° in 24 hours, that's 15° per hours. So if you know the longitude of a place, you just divide that longitude by 15 and you know how many hours you have to add/subtract from Greenwich reference time to get your local time. For example: If you are in Eastern Island at longitude 109°W, it means your local time will be the time at Greenwich (know as Universal Time or UT) minus 7 hours 16 minutes ($109/15=7.26\, h = 7h\, 16m$), if you use that time, the Sun will effectively be at its highest point at 12:00 h.

But as you can see, if you move in the east-west direction, every point would have a different time if we just use the above rule, which is very inconvenient, as the time in the east side of a city would be different than the time on the west side.
For this reason they invented Time Zones: to keep the same time within reasonable large areas. In principle they created time zones every 15°, so that no point would be more than 30 minutes off their solar time. In other words, the moment the Sun reaches its maximum elevation in the sky won't be more than 30 minutes off from clock's noon.

However, this time zone lines crossed political borders, so they were adapted and bent so that whole countries or at lest states remained within a single time zone. That way the current designation of time zones was established and is neatly summarized in this figure from Wikipedia commons.

You can see that the most extreme time zones are +14 hours at Line Islands (Kiribati), and -12 hours in and around Baker Islands (US).

Therefore, the maximum possible difference between times on Earth is 26 hours. That means that at 11:00 PM of a Monday in Baker Island, it is 1:00 AM of a Wednesday in Line Islands.

As both places are fairly close to the equator, I doubt they use daylight saving. Therefore, that would be the maximum. In the case of Apia in Samoa, it also uses +14 hours but only in summer.

If you are mostly concern to the time difference with the UK, the largest would be +14 hours to Line Islands.

Finally, take into account that time zones are politically defined, so at any time a political decisión can change the actual answer to this question.


How to start studying dinosaurs and pre-historic mammals/sea creatures I'm kind new to this hole thing of dinosaurs that I'm really interested in, are there any good books/websites/webpages to study the biology of pre-historic creatures? Dinosaurs, mammals, fishes, anything that is not alive anymore. Also, any good books about the history of how these species evolved and the history behind them would be appreciated.

Here's what it takes to really study this: you need to go through the whole bachelor program for geoscientists, that includes fundamental geodynamics like plate tectonics, magmatism, volcanism, volcanic and metamorphic rocks and generally the cycles that make up earth's internal dynamics.
Then there is the huge field of external factors, like sediment geology (that's really complicated stuff), weathering and transport and how soils come to being, diagenesis and the structures sediments can form and their classifications. Role of the ocean (that's where it starts, before all) and the atmosphere, of course.
When through that, usually 4 semesters or so, you can start to specialize. For paleontolgy you need knowledge of earth history, of course, it's subdivision, and the conditions at certain times as far as they are known. Once that's done, then comes real paleontology: Animals (invertebrates and vertebrates), plants, and their development, biological evolution (that's frequently underrated, I find), taphonomy, ... For a sturdy base count another 2-4 semesters.
You may see that even a bunch of websites, maybe all of them together, cannot replace actual study. I am not aware of any site that even gives a reasonable overview of the field. Geoscience, and thus paleontology, touch many fields of natural science.
That said, when asked "How to learn about animal paleontology ?" I allways mention Micheal Benton, Vertebrate Paleontology. It needs a basic understanding of geoscience, evolution and skeleton anatomy. Functional morphology, phylogeny and an overview over sediment geology and earth history also won't harm, but you could give it a try. Some things are explained in between.
I'd recommend to at least get a good introduction and overview into geology, in form of one or two books on the matter. Maybe a colleague knows other good books on the subject. Actually, I'd like to update my bookshelf myself :-)
Also, Museums of Natural History have publications that could interest you, but best would probably to sacrifice some time, visit them and then browse their book shops. Depending on your time and where you are.


What is this ring of cloud? We were in Legoland in Windsor (SE England) recently, and towards the end of the day, we noticed what looked like a ring of cloud in the sky. It was quite large (hard to say as being up in the sky we didn't have a reference point to measure it by). It gently changed shape as it rose, presumably due to the wind, until it dissipated.

Anyone able to explain what it was? Thanks

It might be a smoke ring, but it's not known for sure.

There have been several sightings of similar objects over the past few years.

A ring was seen in Leamington Spa in 2014. Theories about its origin included a group of insects, as well as an ordinary smokestack (perhaps the ring stayed in one piece for an inordinately long time). Eventually, it was discovered that it was the result of a fireworks test.

A similar object was seen in Kazakhstan in 2015. Video of it received quite a lot of publicity, and many people labeled it a UFO. In that instance, the object was ~100 meters in diameter and ~200 to 1000 meters in altitude. The smokestack hypothesis was once again brought forth. Another article mentions that some people attributed the ring to thermal microbursts (see also here); however, microbursts move down, and your ring rose up.

An irregular ring was observed in Birkenshaw earlier this year (2017). This case is notable because the ring in this case was distinctly non-planar; it was warped, just like yours. It may have been from an explosion from a military reenactment.

This article notes that another ring was seen in Windsor, right near where you were:

There are many more examples besides these.

Some notable features about these rings:

They are "hazy" or smeared in parts. In your case, you can see a dark section on the left; the Leamington Spa ring has a smear on the upper left and the Kazakhstan ring has a smear on the right.
The rings are pretty circular. This may be an indication that they are artificial.
Most are flat, but some (like yours) exhibit some warping, which could indeed be cause by wind.
The rings are darker than the clouds around them, as we can see in all cases but that Kazakhstan ring.
They were sighted near places where explosions were taking place on the ground (a similar ring in California was seen at Disneyland).

I'm willing to rule out microbursts in your case because the ring rose. Insects are still a possibility, and perhaps the smears are due to individual bugs flying outside the pattern, but I have yet to see any behavioral study to explain it. This leaves the theory that these rings are from smokestacks or certain detonations, which is well-supported. Not all of the rings were seen near heavy industrial areas, mind you, but that doesn't rule out that hypothesis in those cases. Smaller smoke machines or explosions could still be the culprits, possibly at a ride in Legoland itself.

The most similar type of cloud I can think of is a lenticular cloud, a common explanation for UFO sightings:

Image courtesy of Wikipedia user Omnisource5 under the Creative Commons Attribution-Share Alike 4.0 International license.

However, lenticular clouds aren't ring-shaped, but disk-shaped, and they usually aren't quite that flat. Therefore, I'd rule a lenticular cloud out here.


The Medieval Climatic Anomaly Trying to understand this question, posted to me by a lecturer at University:

  "The Medieval Climatic Anomaly was distinctive in terms of the
  prevailing climates and forcing mechanisms operating at the regional scale."
  Critically evaluate this comment using case-studies to support your answer

Is this statement saying that the MCA was caused only by the prevailing climate and mechanisms operating on a regional scale?
If so what is an example of a regional scale forcing mechanism?

Distinctive in this case means, does not match well with the first but is explained by the later. This is not the greatest sentence format, but distinct means stands out from. Generally when you refer to something as distinct with respect to X, then it means X is the odd part about it or it stands out from X. In this cased because they are talking about a predictive model and the thing being predicted, then it means it is odd because it does not fit the predictions of the prevailing climate model. But it is also distinct because it is better explained by local factors, for a given value of "local", which is not normally the case for an event so large. It also might be a reference to more recent research showing the anomaly is likely not global. Combining these two slightly different usages is not great sentence composition but is grammatically correct.
In this particular case the regional forcing mechanism is believed to be a large volcano erupting.


Does a greenhouse need to be built on agricultural land? Is it possible to build a climate-controlled greenhouse just about anywhere assuming that it contains soil-beds, a light-controlled , temperature-controlled and moisture-controlled environment and other resources needed for plants to grow?

Is there any advantage in installing such a greenhouse on agricultural land?

Greenhouses can be built anywhere, so long as the plants within them are provided with sufficient nutrients and light. Depending on the crop, soil is not required if hydroponic methods are used.

The use of agricultural lands is not a requirement. In urban areas, disused warehouses and other buildings are used to establish vertical farms. Greenhouses are even placed on roofs of buildings.


Is there any correlation between La Niña/El Niño and seismic activity? I've read in the past that extreme precipitation levels may have an effect on seismic activity, and wondered if anyone has ever analysed the La Niña / El Niño cycles to see if there are any correlations with seismic activity in the area affected by the phenomena.

There is a case study from Environmental Science entitled El Niño: A Link among Atmospheric, Oceanic, and Crustal Circulation? that discusses  the correlations between seismic activity and El Niño cycles in certain areas of the world, that have been documented:

  A geophysicist, Daniel A. Walker, hypothesizes that a different sequence of events produces an El Niño event. Walker says that the thermal input to the oceans comes from Earth’s interior. This hypothesis is based on an intriguing correlation of seismic activity under the eastern portion of the Pacific Ocean near Easter Island and the onset of El Niño events. This correlation can be used to illustrate possible linkages among the physical systems of planet Earth and the difference between statistical correlation and physical causation.

  The East Pacific Rise is located west of Easter Island. Along this rise, tectonic plates move 160–170 mm (6.3–6.7 inches) per year. This rate is one of the most rapid in the world. As a result, seismic activity along the East Pacific Rise has been studied extensively for more than thirty years. During this period scientists have tracked the number of earthquakes and the amount of energy they release.

  Months with the greatest number of earthquakes or months with earthquakes that release the greatest amount of energy precede the onset of El Niño events. This hints at a relationship between seismic activity and El Niño events. A simple statistical analysis suggests that if the seismic events and the El Niño events occurred randomly, the probability of finding a sequence in which periods of heightened seismic activity precede El Niño events would be 1 out of 313. Such long odds imply that the sequence probably is not generated by random chance. Rather, there may be a physical connection between seismic activity and El Niño events.

  But statistical correlation should not be confused with physical causation. Correlation indicates that the timing of the two events is similar but does not mean that the two events are connected.

  Scientists have come up with several explanations for how seismic activity may trigger El Niño events. These hypotheses are based on the relationship between air temperature and pressure. In general, warm air has less pressure than cold air, so hypotheses seek to explain how seismic activity along the East Pacific Rise could affect air temperature.

There are several other articles that discuss the relationship between La Niña/El Niño cycles and seismic activity, such as this article published by RSTA entitled Statistical analysis of the El Niño–Southern Oscillation and sea-floor seismicity in the eastern tropical Pacific.

There also appears to be a relationship between El Niño cycles and volcanic activity, as published in Nature.


Tsunami - Mathematical classification I'm interested in knowing which type of wave a tsunami is, in a strictly mathematical sense.

I'm gonna cite from Wikipedia for this:

  As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its speed decreases below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously – in accord with Green's law.

and also

  Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore.

While the first property strongly hints at a tsunami being a shallow water wave, the latter fact, namely that they usually don't break would rather suggest for the tsunami being a soliton-solution of a Korteveeg-de-Vries-type equation.
I thought the latter property would not be inherent to shallow water waves although Wikipedia, again seems to argue differently, however only with a picture.

So maybe someone can clarify:

Which equations govern the surface profile of a tsunami? Has this model been tested against satellite measurments?

Short answer: Tsunami models use the shallow-wave mathematical approach, because their wavelength is usually much larger than the relevant water depth determining their propagation.

Long answer:
Tsunamis are often called 'tidal waves' to highlight the idea that their characteristic time response is closer to tydes than to the standard wind waves we are used to see in a beach. The full spectrum of water wave periods is shown in this figure (tsunamis being classified as long-period waves, together with waves caused by meteorological events):
Classification of the spectrum of ocean waves according to wave period. Redrawn from Figure 1 in: Walter H. Munk (1950) "Origin and generation of waves". Proceedings 1st International Conference on Coastal Engineering, Long Beach, California. ASCE, pp. 1–4.

This image shows that the wave frequency depends on the cause behind the perturbation of the water surface.

Oceanographers generally treat ocean waves as free surface waves in an ideal fluid [2].

Mathematically, tsunamis are surface gravity waves approached as shallow-water waves (see chapter 4.1.5 in here), meaning that the wavelength λ > 20H (H being the water depth). The involved shallow-water-wave mathematical approximations oppose to the deep-water waves (λ < H, see section 4.1.4 in the same [ref 3]).

Whereas for shallow waves the wave frequency and the wave speed are (ref. 3):

$ω = \sqrt {g k 2H}$    with $k = 2\pi / \lambda$

$c = \sqrt {g H}$

for deep-water waves, instead:

$ω = \sqrt {g k}$

$c = \sqrt {\frac{g \lambda}{2\pi}}$

As the wave approaches the shore and H decreases, the wave velocity decreases and energy is preserved by increasing the wave amplitude.

Detailed information here: 3.


What the humidity metric is hiding? So I have been spending time in Moscow Russia which essentially records very similar climatic humidity to my home city of London. However in Moscow my throat is permanently dry, I need to consume more water, many people have humidifiers in their bedrooms and the government spray the roads with water to prevent citizens choking on the dry dusty air.

Clearly the climate is 'drier' (or another more accurate technical adjective). What metric will show this difference between Moscow and London ?

And why is the humidity metric the same ?

Weather services rarely state the absolute humidity because it is not easy to determine. Instead, they state the relative humidity.

  Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Absolute humidity is expressed as grams of moisture per cubic meter of air ($\mathsf{g/m^3}$).

  Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.

A better measure of how atmospheric moisture will affect people and the environment is the dew point.

  The dew point is, in short, the point at which dew droplets form on objects like grass – in other words, when a relative humidity of 100 percent is achieved.

  If you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

Edit 31 January 2020

Taken from Oppressive, humid conditions for southern Australia as heatwave combines with tropical airmass

  Dew point is the indicator of the amount of moisture in the air ...
  it's the temperature that the air needs to cool to in order to form cloud.

  When temperatures are very high, relative humidity can under represent how much moisture there is in the air, making dew point the better measure.


What influences the movement (speed and direction) of tropical cyclones? The recent cyclone to impact the Queensland coast of Australia (Cyclone Debbie, Category 4) tracked towards the coast at what was reported to be a very slow speed (between 6 and 7 km/hr).

Is that a particularly slow speed for a cyclone to move at, and second, what governs the speed and direction of cyclone movement?

Cyclones generally drift in a westerly/north westerly direction due to the Beta Drift  and Beta Effect

For atlantic hurricanes the forward speeds are summarized here by latitudinal range -Average forward speed of Atlantic Hurricanes and  if there is recurve(turn more northerly) they can slow down or if they cross mid latitudes they may pick up speed by interacting with mid latitude troughs.

For the latitude of Midge Point, Queensland you can expect forward speeds of about 17-18 kms/hr. So this cyclone is certainly slow moving.

TC forward speed

There are other factors influencing cyclone tracks which include steering winds, presence of high pressure systems,etc and more recent studies have studied the influence of SST on cyclone tracks Response of  cyclone tracks to SST.

From this peer reviewed reference Effect of forward speed on flooding modeling studies reveal that increasing forward speed decreases flooded volumes while increasing peak surge

Back to Cyclone Debbie it looks like from this blog at least Cyclone Debbie Progress the forward speed of the cyclone is probably one of the reasons for the water to be present for more than one day and making sure storm surge occurs at the same time as the high tide.


Extratropical transition of tropical cyclone I am a beginner of learning Tropical Cyclone. I would like to represent the extratropical transition of a tropical cyclone. But I can't understand on what actually in need to focus.

For a tropical cyclone, I can calculate time series of rainfall, maximum wind speed, minimum sea level pressure and discuss its features. Similarly, what are the quantities I should calculate to represent the extratropical transitions of the same tropical cyclone? and What actually I need to show?

Is it just simple discuss how a tropical cyclone changes its behavior when it moves toward the higher latitudes?

No, it is not sufficient to just discuss the behavior as it moves into higher latitudes. ET occurs because of environmental reasons, not geographical reasons.

How much of a beginner are you? My answer below may be a bit over your head if you do not know much about tropical cyclones. If that is the case, I suggest looking back at the theory and basics of tropical cyclones (structure, maximum potential intensity, etc.) to get a good grasp of what separates tropical cyclones from extratropical cyclones.

If you are specifically looking at quantities, I suggest looking at cyclone phase space diagrams. The variables in these diagrams include the thermal wind for different levels, as well as baroclinicity. FSU has examples of cyclone phase space diagrams for different storms.

Edit: I remembered two recent articles that review ET. They are Evans et al. (2017) and Keller et al. (2019).


Global tide calculation I am familiar with the concept of harmonic tide coefficients and was wondering about the simplest software solution to calculate tides anywhere, worldwide.

Are there any available, open source packages to achieve this?

Thanks,
EL

Science-quality tools for global and regional tidal heights and transports are available at
http://volkov.oce.orst.edu/tides/global.html.


List some common metrics to analyze the atmospheric vertical motion? Could you give me some basic understanding of how to analyze the atmospheric vertical motion?
For example, could you list some basic metrics to diagnose the vertical motion? Like divergence, vertical velocity (Omega). I got these two proxies from the published papers. Are there other common metrics to analyze vertical motion?
I am a hydrological guy and have not so much meteorological background.

Thank you so much.

BTW, I am sorry for my too general question, but I just wanna some common metrics to do vertical motion analysis. My research focus on extreme precipitation.

First of all, I am unsure if your question relates to Numerical Weather Prediction (NWP) or field-experiments. I would say divergence is one of the best variables to look at since vertical motion is both difficult to measure and to model. Also, some nowcasting models which includes a nudging method to increase precipitation intensity in the model, based on the observed intensity does this by nudging the divergence field directly, to generate more or less vertical motion.


How does one interpret negative geopotential height values? I am using NCEP Reanalysis Geopotential height data and this is at 1000 mb and downloading it from here - 1000 Mb Geopotential height values and some of these values are negative for a particular date - May 12th 2014 00Z. I am aware of geopotential height anomalies - Geopotential height anomalies  and that these can be negative i.e. the average of a height surface is lower than than what it should be. But the negative values I am referring to are "raw" geopotential height values and not anomalies.

When I sent them a note and asked them what that meant this is the explanation I got - "1000mb geopotential height could be negative if the contour line drawn around the surface of the earth connecting all 1000mb pressure intersected with the earth's surface. At the point where the height was negative, the 1000mb height is actually below the ground and pressure is low."

Can someone explain what that means as doing a google on  negative geopotential does not reveal anything meaningful ?

This likely means that the surface MSLP is less than 1000 mb.

If you are looking at a 1000 mb geopotential height, and the MSLP at a point is 995 mb, then the height of the 1000 mb geopotential surface would be below the ground, i.e. negative height.

Effectively the question was what does a negative difference of distance from a pressure surface mean? The answer is that the surface must lie below the frame of reference. In the example, the frame of reference is the ground and the surface is the 1000 mb surface.


How much would weather data from ocean-going vessels improve climate research? I would like to know how good the weather data coverage is on the surface of the world’s oceans, and how much it would improve climate research/models if weather observations from ocean-going vessels were made publicly available. As I understand the current situation, publicly available ocean weather data is mostly measured via stationary floats and satellites, where the satellites only measure temperature.
Many ocean-going vessels measure sea temperature, air pressure + temperature and wind speed + direction. These vessels cover a fair bit of the ocean surface during their trips, and some vessels log these observations every hour or even more frequently.
I have two questions relating to this:

Are there more publicly available sources of oceanic weather data
than the ones I list in the beginning?
Would weather data from ocean-going vessels be beneficial to
climate research, by e.g. giving many additional calibration points for climate
models relating to ocean weather? Can you give examples of which specific
areas of research it would apply to? I’m thinking things like
ocean heat absorption and emission, hurricane formation and maybe
El Niño.

"Ship of opportunity" weather observations have contributed to global data collection exercises since the dawn of meteorology as a science. There are many national and international initiatives that collect data and the data already inform weather forecasts and climate models. The practice is summarised in https://www.frontiersin.org/articles/10.3389/fmars.2019.00434/full
In recent years the number of ships contributing has declined, but over 2000 ships that do participate are collecting higher quality data.


Name of majorite endmember $\ce{Mg3(FeSi)Si3O12}$ Does the majorite garnet $\ce{Mg3(FeSi)Si3O12}$ have a name?

Yes, it's called (unsuprisingly) majorite.
This is an IMA-approved name, first described from a meteorite.

More information: https://www.mindat.org/min-2546.html


Thermal expansion of Earth Matter tends to change its volume in response to changes in temperature--or, hot things tend to be expanded. Is the thermal expansion of Earth significant?

That is, is it significant when compared against a cooler or perhaps internally-inert Earth?

Here's a take on calculating earth's thermal expansion and shrinking, under certain assumptions mentioned in the text. It seems to me, from a short overview, that the core is assumed to be constant over the time. It focuses on the mantle part of the earth, its mineral phases and convetion. Take it with a grain of salt:

https://www.sciencedirect.com/science/article/pii/S167498711200148X

tl,dr: after consolidation, earth's radius was 120km larger than today. It shrank by 110km in a few million years, then expanded a bit (11km), then contracted by 8km, and subsequently contracted by around 12km to the present day.

If they say so :-)


How do spectral methods work in the context of numerical weather prediction models? I am broadly familiar with the concept of numerical weather prediction models using various finite methods to solve the primitive equations used in these models, but how do spectral methods work, generally speaking?

What are the basic differences between NWP models using "traditional" finite-difference or finite volume methods and those that employ spectral method approaches?

Presume that the atmosphere can be modeled with waves of different frequencies. Therefore, the atmosphere can be written as a series of sine and cosine functions with different frequencies and coefficients. Consider the linear advection equation: $$\frac{\partial u}{\partial t}= -c \frac{\partial u}{\partial x}$$

Consider the assumption that $u$ can be modeled as $$u=\sum^\infty_{|n|=0} A_n(t)\exp(i\omega_n x) $$ But since we have limited computational resources, $$u=\sum^N_{|n|=0} A_n(t)\exp(i\omega_n x) $$ Therefore, our advection equation becomes $$\sum^\infty_{|n|=0} \frac{d A_n(t)}{dt}\exp(i\omega_n x)=-c\sum^\infty_{|n|=0} i \omega_n A_n(t)\exp(i\omega_n x) $$

If we integrate this equation and leverage the orthogonal properties of sine and cosine, $$\int^L_0 \sum^\infty_{|n|=0} \frac{d A_n(t)}{dt}\exp(i\omega_n x)\exp(i\omega_m x)dx=-c\int^L_0 \sum^\infty_{|n|=0} i \omega_n A_n(t)\exp(i\omega_n x)\exp(i\omega_m x)dx$$ Then the summation notation can be removed. $$\frac{d A_m(t)}{dt}=-ic \omega_m A_m(t)$$ Now this can be solved analytically, and as such we do have an analytic solution to the advection equation, but what we can do to model this numerically, is combine the finite difference notation and the spectral solution to numerically integrate the advection equation.

For example, $$u_{n+1}= u_n -c\Delta t\sum^\infty_{|l|=0} i \omega_l A_l(t)\exp(i\omega_l x) $$

So to summarize,

Assume that the variable can be expressed as a sum of basis functions, commonly sine and cosine functions.
Create a truncated series (fourier series), creating an analytic expression for said quantity.
Evaluate the derivative of the truncated series, and replace the derivative (often computed with finite difference approximations) with the analytic representation of the derivative.
Numerically integrate in time.

Now there are plenty of limitations, such as the nonlinear version of that equation. If I had my notes on hand, then I could address those problems more. I'll edit this when I can. Also, model physics cannot be computed in spectral space, so there has to be conversion from spectral to physical space and vice-versa.

I'll check my notes and correct any errors I can find later.