Wind speed data plot contains blank gaps I am relatively new to Python, and I am having trouble plotting some data. Specifically, I am plotting wind speed data on a 2-D plot that I grabbed from the Weather Research & Forecasting (WRF) weather model. Though I'm able to generate a plot, there are noticeable gaps in plot, particularly as the model height increases. The purpose of generating these plots is to compare WRF model data to observations collected from dropsondes during a field campaign. Below is the code that I've been working with as well as the plot that was generated.
import numpy as np
from netCDF4 import Dataset
import wrf
from wrf import getvar
import glob
import pandas as pd
import matplotlib.pyplot as plt

dir = glob.glob('~/wrfout_d04_2017-05-27_20:00:00')
dir.sort()
nf = range(0,len(dir),6)

for n in nf:
  data = Dataset(dir[n], 'r')
  windspeed = wrf.getvar(data, 'uvmet_wspd_wdir', meta = False)[0,:]
  winddir = wrf.getvar(data, 'wdir', meta = False)
  altitude = wrf.getvar(data, 'z', meta = False)
  lat = wrf.getvar(data, 'lat', meta = False)
  lon = wrf.getvar(data, 'lon', meta = False)

  for i in range(len(windspeed)):
    fig = plt.figure
    plt.plot(windspeed[i], altitude[i], color = 'blue')
    plt.axis('tight')
    plt.xlabel('Wind speed (m/s)')
    plt.ylabel('Height (m)')
    plt.title('Wind speed and model height for\nwrfout_d04_2017-05-27_20:00:00')
  i+=1
  plt.show()
  plt.savefig('wrf_windspeed.png')

I've tried checking to see if the WRF output file 'wrfout_d04_2017-05-27_20:00:00' contains masked data using ncdump -h wrfout_d04_2017-05-27_20:00:00, but I'm not seeing anything there. I really appreciate anyone taking the time to read this post and any recommendations would be greatly appreciated.

The answer is actually pretty easy. The WRF model lies on a grid. The vertical component of the grid is unevenly spaced, though you can manually space it. The vertical coordinate of WRF uses a sigma vertical coordinate (even though the namelist calls them eta): $$\sigma=\frac{P-P_{surface}}{P_{top}-P_{surface}}$$. So the values of sigma are specified (or just resorted to the default values). So when you get altitude levels for WRF, there is a space between the levels. You would probably find something similar if you change the y-axis to west-east or south-north.
The default vertical spacing for WRF has higher resolution near the surface, which is useful for convection and PBL modeling.


What is lava called if it has 56% to 64% silica? We've learnt in science at school that "Mafic" lava forms rocks with 45% to 55% silica, meanwhile "Felsic" lava forms rocks with >=65% silica. So what is lava called in between? Or is it impossible for any lava to cool into rocks with that range of silica? Why?

What is lava called if it has 56% to 64% silica?

It's called intermediate.

  Or is it impossible for any lava to cool into rocks with that range of silica?

Not impossible, but not common. The two most common lava types are indeed mafic (mostly basalts) and felsic (mostly rhyolite). A well known intermediate lava type is andesite.

  Why?

This question of why most lavas are either mafic or felsic has been bothering geologists for a very long time. This problem even has a name: the "Daly gap", named after the geologist who first identified the problem almost 100 years ago. There is no easy answer, and it's still being debated. A good (yet somewhat technical) read is available here in this ResearchGate question and following answers.


Are EF6 Tornadoes officially recognized Are EF6 Tornadoes officially recognized on the fujita scale or not, I have been quite confused on this and I have been very curious about it.

Basically the de facto best operational source would probably be the Storm Prediction Center website.  They provide this table:

It caps out at EF5.

However it also true to note that the scale was developed to mirror the Beaufort Scale into a range between the Hurricane Force on the Beaufort Scale and Mach 1.  And as such, indeed in Fujita's 1971 proposal of his scale, as well as in this 1981 journal article (which offers some additional interesting insight into the progression of understanding into tornadic wind speeds, plus alternative scales), Fujita lists the option:

However, in the same papers, likely as his dataset expanded, he evolved his initial wording

  ...the presumption that the occurrences of F6 or higher wind are
  extremely rare... [1971 paper]

into

  ... because F6 or stronger tornadoes are not expected to occur on the
  earth, they will be called Inconceivable tornadoes should they ever
  occur. [1981 paper]

Very importantly, the Fujita scales are damage scales, simply because it's very difficult to directly measure the speeds of a tornado. As such we base tornado ratings by analyzing the degree which it damaged objects; SPC's Fujita scale page lists the 28 damage indicators currently available as of 2017. The highest wind speed matched with any of them, found when significant structural deformation is done to a high-rise building (over 20 stories tall), would only indicate an upper bound of 290 mph.

That does not indicate that wind speeds aren't higher than that, only that we have no structures that can withstand any higher wind speeds.  Thus there would not be any potential for an F6 rating unless building methods are substantially changed to withstand stronger speeds, those buildings became prevalent enough that engineering testing were done and were able to verify its strength, and then such an extreme tornado ever hit them.  Not something we're going to see anytime soon it appears.

So back to the question of whether tornadoes have winds stronger than those listed for an (E)F5?  Measurement of the wind speed appears our only real hope of noting such.  In-situ instruments such as anemometers generally fail well before winds reach such speeds, and typical radars cannot offer the resolution to diagnose such speeds.  Only mobile Doppler radars offer much hope.  There are numerous issues with utilizing such measurements, such as the fact they scan significantly above the ground layer, their sampling areas\timescales that make analyzing suction vortices difficult, and they contain uncertainties due to Nyquist frequencies, variations in reflective objects, and statistical distributions.

Such tornadoes are very rare to encounter.  But in the past couple decades we have indeed managed to attain a handful of close-proximity estimated measurements of strong tornadoes.  But I am not aware of any peak estimated wind speed reaching beyond the (E)F5 range.  Here are the most notable ones I'm aware of:

May 31, 2013 El Reno, OK: 135 m/s (302 mph)
May 3, 1999 Bridge Creek, OK: 135 m/s (302 mph)
May 24, 2011 El Reno, OK: 132.1 m/s (295 mph)
April 26, 1991 Red Rock, OK: (268-280 mph)
May 30, 1998 Spencer, SD: (264 mph)
May 9, 2016 Sulphur, OK: EF4 or EF5 measurements (based on preliminary communication)

So, long story short:
While (E)F6 tornadoes may not be impossible, and the categorization does theoretically exist... in practicality, there is no need for such a value, as the damage would be complete whether in an (E)F5 or (E)F6+.


How to obtain the correlation between monthly streamflows based on physical concepts? I've been tasked with generating synthetic streamflow scenarios through disaggregation.

First, I will generate 50 years of streamflow data. For that, I will probably use a simple model as AR(1) or an ARMA(1,1) and am adopting some probabilistic parameters based on streamflow data I have. These data come from the historical record a gauge station.

From my yearly flow, I have to generate monthly streamflows. In the end, instead of 50 streamflow realizations, each one representing an annual flow; I'll have 600 streamflow realizations, each representing a monthly stream flow.

Now, to get from annual to monthly stream flow, I will need a correlation matrix.

I could simply estimate the monthly correlation from January to December by calculating the correlation among monthly streamflows from a set of historical streamflow.

One could say that the physical aspects involving the (auto)correlation are implicit in the data itself. However, the data I have for this gauge station - as for any other - is only one realization of the stochastic process from which the observed streamflows were generated.

How, then, could I enrich this monthly (auto)correlation with physical concepts stemming from the river where the gauge station is located? I should add that my question also applies to cross-correlation between two different time-series. How, in that case, could I enrich the cross-correlation matrix employing physical aspects between 2 observed time-series?

One suggestion on generating the monthly flows is to use a seasonal ARMA model directly (e.g. see course notes), since you are looking at ARMA model for stochastic generation anyway. That would allow the explicit estimation of seasonal parameters for synthetic generation, and let you skip the step of going annual --> monthly, instead go monthly and calculate annual flows from the monthly series. You may have other reasons for wanting to start with annual flows to verify those first, but that is one suggestion.

In terms of physical concepts at a monthly flow resolution, this is harder to comment on without knowing what physical processes are impacting your streamflows, e.g. snowmelt, river ice and freezing, spring freshet, depression storage, etc. One suggestion may be to check monthly volumes and peaks as well as timing of peaks (such as spring freshet), and ensure that any synthetic time series match these statistics within some acceptable boundary, throwing out any simulations that are deemed unacceptable.

This is a pretty broad (research level) question but hopefully that gives some ideas.


Does Earth risk losing its oceans to space due to global warming? I read that we are facing the very real possibility of Earth's average temperature rising by more that 2 degrees Celsius by the end of the century.

As I understand our planet is constantly loosing its atmosphere and oceans to space but at a very microscopic and thus safe levels. Will Global warming lead to accelerated evaporation converting Earth into a dead waterless desert in the observable future?

We know that terrestrial hydrogen reaches space because ultraviolet images reveal
  a halo of hydrogen atoms surrounding the Earth (Fig. 2). Some of these atoms are
  escaping while the rest are trapped. The temperature at Earth’s exobase is typically around
  1000 K, although it varies as the Sun’s ultraviolet output cycles up and down every 11
  years or so. At 1000 K, hydrogen atoms have an average speed of 5 km/sec, which is
  below Earth’s escape velocity of 10.8 km/sec at the exobase. Yet hydrogen atoms still
  manage to escape because many move faster than average. This loss of particles from the
  energetic tail of the speed distribution is called “Jeans’ escape” after James Jeans (from the
  University of Cambridge), who described it mathematically in the early twentieth century.

The escape of planetary atmospheres Catling & Zahnle, Scientific American,2009 1

The exobase is the lower boundary of the exosphere.

I guess that two degrees increase in surface temperature would not make much difference to the $~1000K$ at the exobase.


Is there a "submerged object" in Australia that causes a magnetic deviation of 20 degrees? This answer quotes this blogpost as an example of a magnetic anomaly:

  Basalt formation, “Mt. Jim”, in remote north-east Victoria, Australia, has a magnetic anomaly -20 degree magnetic compass variation - caused by a massive, yet unknown, submerged object.

Is there really a verified magnetic anomaly in north-east Victoria, Australia of 20 degrees? If so, is there a naturally occurring phenomenon (e.g. large sub-surface ferromagnetic rock formation) that causes it?

A link to some deviation map or other verifying, quantitative source is requested, thanks!

I'll disagree with Camilo Rada's answer, and show that there is definitely a magnetic anomaly.

We start with the interactive geophysics map of Geoscience Australia, Australia's federal geoscience agency. Our region is this:

Here's the same area with magnetic intensity layer turned on:

You can see a clear magnetic anomaly in this area, and it's actually much more widespread than noted on your map.

  If so, is there a naturally occurring phenomenon…that causes it?

In general, answers to questions like these are yes. But let's explore it further with the geological map of the area:

Overall, the rock type around is "OSn" and "Os". According to the legend, these are high grade metamorphic Ordovician and Silurian rocks, and neighbouring rocks are siliclastic which tells me they shouldn't be too magnetic (confirmed by the magnetic map). The area of Mt Jim has "Dg" (= Devonian granites) which might be slightly magnetic. But here's the thing you were looking for: "Czb", which is short for Cenozoic basalt. And basalts are magnetic.

So to answer your question, the source for the magnetic anomaly isn't a "submerged" (buried would more appropriate here btw), but a basalt fully exposed on the surface.

EDIT

Interestingly, the blog post mentions the presence of basalt, so it's not entirely clear why they claim that there's a "massive, yet unknown, submerged object". Most probably there's simply more basalt, or its intrusive analogue: dolerite or gabbro. These targets are attractive for the mining industries. I would be surprised if it hasn't been drilled at some point by an exploration company, and then the underground cause of the anomaly would be very well known (but probably not publicly).


What kind of mine pit in northeastern Minnesota could make residue that turns water turn bright red? Reading about turbidity, I came across this web page with the frightening image shown below.
Text immediately before reads:

Reasons for Natural Variation
Algal turbidity varies seasonally and with depth in a complex manner as discussed previously in response to physical, chemical and biological changes in the lake. Inorganic and detrital particles from the watershed vary largely in response to hydrological events such as storms and snowmelt.

The figure caption reads:

Even relatively small amounts of wave action can erode exposed lakeshore sediments, in this case a mine pit lake from northeastern Minnesota. Can you guess what mineral was mined here?

Any ideas? I know there are bright red oxides of heavy metals like lead and mercury, certainly hope that's not the right answer.

From the red stain in the water & the rocks in the lower left of the picture it looks like iron ore.

Also, the red stain in the water appears to be a colloidal suspension, consistent with minor erosion of a bank rather than an aqueous stain caused by water reacting with sulfide minerals in the bank.

The Iron Range occurs in north eastern Minnesota.


How much pollution would it take to stop global warming? The global cooling hypothesis of the 1970s included the theory that aerosols lead to global dimming. That is, that pollutants reduces the amount that the Sun warms the Earth, causing the Earth's surface to cool.

My understanding is that the theory itself was sound, though in the end the effects of greenhouse gases dominated the trend. Hence, climate change takes the form of global warming.

Setting aside the obvious biological effects from aerosol pollution (e.g. less plant growth), how much of that pollution would be physically required to offset global warming? Are the effects even in the same ballpark?

I found a decent review article by Zhang et al. (2015) covering various geo-engineering proposals that is worth reading. As for atmospheric pollution, they report that pollution should be injected at controlled altitudes into the stratosphere, where it is longer-lived. Using $\text{SO}_2$, apparently around $2-7 \text{Tg}$ of stratospheric injection would be needed, which is substantially lower than the global $\text{SO}_2$ emissions (e.g. Lee et al. (2011) report annual emissions around $50\text{Tg}$ $\text{S}$, probably the figure has decreased a bit since then).
So, as for your question, it is not about simply polluting more but about controlledly injecting pollutants into the correct atmospheric layers.
References
Lee et al., SO2 emissions and lifetimes: Estimates from inverse modeling using in situ and global, space‐based (SCIAMACHY and OMI) observations, Journal of Geophysical Research Atmospheres 116 (2011), https://doi.org/10.1029/2010JD014758
Zhang et al., Review of geoengineering approaches to mitigating climate change, Journal of Cleaner Production 103 (2015), pp. 898-907, https://doi.org/10.1016/j.jclepro.2014.09.076, available at http://www.homepages.ed.ac.uk/shs/Climatechange/Geo-politics/Comparisons.pdf


Water levels: What does "mBf" mean? I have seen the term MBf used in a couple of places (a ramp specification and a Hydrological Study) to describe height or depth of water. What is this unit? Can it be converted to meters?

For bonus points: how tall is the ramp at Baja?

It is "mean sea level above the Baltic Sea", in meters (source on page 109).

That explains why it is used mainly in East-European countries.

Martin Ekman - The Changing Level of the Baltic Sea during 300 Years - A clue to understanding the Earth shortly mentions some historical 'zero points' (page 111).

As mentioned in the comments:
mBf means méter Balti felett (Hungarian for meters above Baltic Sea). They also use meters above the Adriatic as a datum (https://en.wikipedia.org/wiki/Metres_above_the_Adriatic, mAf from méter Adria felett).


I'm still confused about what "probability of precipitation" means I'm trying to understand what weather forecasts mean more precisely. As I understand it from reading Wikipedia, blogs, etc., the percentage value for rain/precipitation that you see in a forecast is technically called the "probability of precipitation". To quote the National Weather Service webpage:

"Mathematically, PoP is defined as follows: PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.
So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)"

This definition does not seem well stated to me, for the reason that "confidence" is (presumably) not uniform across an area. For instance, the phrase "how much of the area will receive measurable rain" seems odd, since a forecast would (presumably) only be able to give a probabilistic estimate for this area.

Let's cook up an example. Consider a town (the forecast area) consisting of two parts of equal area (called north side and south side). Each point in the north side will be rained on with 100% probability tomorrow, and each point in the south side will be rained on with 50% probability (at every point) tomorrow. What is the PoP in this example? At face value, the definition could be interpreted as implying that the PoP is 100%, since precipitation will occur somewhere in the forecast area. However, this value seems intuitively unsatisfying, since some people might not get any rain.

Here's what I might expect a more precise definition to be. If $A$ is the area and $C(x)$ is the "pointwise confidence function" depending on a point (location) $x$, then define
$$PoP = \frac{1}{area(A)}\int_{A} C(x)\, dx.$$
In words, this is just the expected value of $C(x)$, or the probability that a randomly located person would see rain over the specified time interval. In practice, of course the integral would be estimated based on actual measurement sites. If such a formula is indeed an accurate definition, then I'd be satisfied. In the above example, the PoP would be 75%. (The official definition could in essence be viewed as a shorthand that is more useful for those without any background in calculus.) If this definition is not correct, then some explanation would be helpful.

I've read web articles with statements like the following: "As a student and observer of meteorology, it constantly bums me out that people do not understand what it means when someone says there’s an “X% chance of rain” tomorrow. A 50 percent chance of rain does not mean there’s a 1-in-2 chance that you’re going to get wet."

It's not clear to me why "A 50 percent chance of rain means there’s a 1-in-2 chance that you’re going to get wet" would be an inaccurate interpretation of PoP. If the definition I suggested above is correct, then it is perfectly correct to say that a stationary observer placed at a random location in this scenario would have a 50% chance of getting wet. Am I missing something, or is this author being careless?

I don't have any background in meteorology, and in particular I don't have much of a sense of how PoP is actually computed in practice.

I agree that the PoP = C x A that we see on a lot of websites leaves something to be desired.  It gets across the loose idea that the definition involves the area affected as well as the probability of occurrence, which is fine for most casual readers but can be frustrating for more inquisitive minds.  How much are the confidence and the area estimated from ensemble forecasts versus expert judgement?  How was it done before ensemble NWP started in the 1990s?

Anyway, there’s an interesting survey by Stewart et al (2016) that gives a broader overview of how different meteorologists use the phrase “probability of precipitation”.  They categorise the PoP = C x A usage as following the NWS Operations Manual (1984), which is nicely summarized in a comment by Schaefer and Livingstone (1990).  In short, your interpretation in terms of expectations is correct and, as you mention, they describe it in terms of a sum over a hypothetical network of points rather than a continuous integral:

$\textrm{PoP} = \frac{1}{N} \sum_{i=1}^N E[R_i]$,

where $R_i$ equals 1 if it rains at station $i$ and 0 if it doesn't.  In your Pluieville example, this could be approximated as a town with one rain gauge in the north and one in the south, giving PoP = (1.0 + 0.5)/2 = 0.75.

I don't know for sure, but it wouldn't surprise me if these things are still calculated by subsampling gridded NWP models at actual rain gauge network locations, which would allow for comparable long-term verification of the probabilistic forecasts.

Stewart, A.E., C.A. Williams, M.D. Phan, A.L. Horst, E.D. Knox, and J.A. Knox, 2016: Through the Eyes of the Experts: Meteorologists’ Perceptions of the Probability of Precipitation. Wea. Forecasting, 31, 5–17, doi:10.1175/WAF-D-15-0058.1

National Weather Service, 1984: Zone and local forecasts. NWS Operations Manual W/OM15.

Schaefer, J.T. and R.L. Livingston, 1990: Operational Implications of the “Probability of Precipitation”. Wea. Forecasting, 5, 354–356, doi:10.1175/1520-0434(1990)005<0354:OIOTOP>2.0.CO;2


Difference between the oxygen/$CO_2$ ratio during the period of the dinosaurs and the present Here's the base of my question: Was volcanic activity more prevalent when the dinosaurs roamed?

Is it true that during the time of the dinosaurs, both the oxygen requirements (by all living creatures on the planet) and the $CO_2$ released by volcanoes were higher than the same type of oxygen requirements today and the $CO_2$ pollution created by us and the currently active volcanoes?

I want to know if there's some simple argument against cars and factories being responsible for global warming (while methane production and deforestation are left behind) - that's what I believe in (for now).

Is it true that during the time of the dinosaurs, both the oxygen
  requirements (by all living creatures on the planet) and the CO2
  released by volcanoes were higher than the same type of oxygen
  requirements today and the CO2 pollution created by us and the
  currently active volcanoes?

I'm not quite sure what "Oxygen requirements" means.   It seems to me that atmospheric changes is a byproduct of life on earth more than a "requirement".  There's other smaller factors, such as
volcanic activity.

I think the best way to address this question is to start before the dinos, around 360 million years ago and the start of the Carboniferous Period
when oxygen increased to higher levels than it had ever been and CO2 levels fell significantly, perhaps as low as 300 PPM, though estimates vary.

These lower levels of CO2 played a key role in the Karoo Ice Age

Something rather drastic happened around 251-252 million years ago but CO2 levels had started to rise before then, and the simple termite may have played a key role in the end of the Karoo ice age and rise in atmospheric CO2 .

When the Dino's came about (230 million years ago), CO2 levels were too high to have glaciers.   Whether that was due to the massive mantle plume, or the termite or increased volcanic or some combination of all 3 is hard to say, but CO2 PPM has risen quite significantly by the time the Dinos came.

Source

I recommend reading this part as it's more relevant to your question:  https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere#Measuring_ancient-Earth_carbon_dioxide_concentration

  I want to know if there's some simple argument against cars and
  factories being responsible for global warming (while methane
  production and deforestation are left behind) - that's what I believe
  in (for now).

There's really no good argument at all.   it's worth pointing out that while some people don't like the man made climate change theory and a handful of scientists continue to question it, it has stood up reasonably well to the tests, and no alternate theory has even come close.

When the Koch brothers funded a study to prove the "urban heat island" effect, their study suggested the CO2 effect was even bigger than the IPCC's studies said it was.

There are a number of factors that can effect climate.   The Earth's orbital changes, ocean current changes, sunspots, volcanism, deforestation, snow cover and albedo, cow farts, land location, mountain ranges and their effect on rain and atmospheric circulation, but none of those effects are playing a significant role in recent decades. CO2 is the only driver that makes any sense at all and it's quite clear that the cause of the rise in CO2 is the burning of fossil fuels.


How is global greenhouse gas emission calculated? I am sure there are different methodologies to arrive at such a number. Can someone in layman terms describe some of the most widely used and trusted methods and what data are used? Sorry I am outside the field of climate modeling so the simpler the better.

I can imagine a satellite observation-based model that calculates emissions on a spatial basis but I am not sure if our technology is advanced enough to do that accurately. However, this method would not allow segregation of GHG by source/sectors (electricity generation, cement production, agriculture etc.)

Your question is simple enough, but the answer depends on what exactly you're looking for.

Who is emitting where right now?

Real-time global monitoring of greenhouse gas emissions with a high spatial resolution is an emerging technology.  We have very useful satellites (see Jean-Marie Privals answer), but they have limitations:

All existing public satellites with instruments for high-resolution GHG measurements are in low Earth orbit, so they only see a particular place when they pass over, not all the time.  So far only the Geostationary Interferometric Infrared Sounder (GIIRS) on the Chinese FengYung (FY)-4A satellite can in theory monitor CO₂ from a geostationary viewpoint, which allows for "continuous" monitoring (probably meaning hourly; it takes a while to scan the area of interest), but from what I've heard GIIRS is not performing very well.  Europe plans to launch the Infra-Red Sounder (IRS) in 2023.  Either way, their spatial resolution will be much worse than for the low Earth orbit satellites, because geostationary orbit is so far away.
They rely on visible radiation.  Although attempts to retrieve from infrared radiation exist, their information content is quite poor (for an Arctic methane example, see Holl et al. (2016)).  In Jean-Marie Privals answer you can see an illustration of how reflected sunlight is used as a retrieval.
They usually need clear skies.  We can't look below the clouds, but even retrieving above the cloud would require a very accurate characterisation of the cloud, and clouds are tricky.  So usually we just assume that the GHG concentration is the same with or without clouds, even when it probably isn't.
Some private satellites exist with a very high spatial resolution, but I can't find much verifiable information about them.  GHGSat claims a spatial resolution of less than 50 metre.  Inevitably, that comes at the cost of field of view (12×12 km² claimed), it will only view a spot when actively pointing, so although it may be able to point anywhere on Earth, it will only view very specific areas and is in that sense not global.  It appears many similar commercial instruments are planned in the near future.

  I can imagine a satellite observation-based model that calculates emissions on a spatial basis but I am not sure if our technology is advanced enough to do that accurately. However, this method would not allow segregation of GHG by source/sectors (electricity generation, cement production, agriculture etc.)

The spatial resolution of about 2 km may be good enough for that, unless the electricity plant is next to the cement producer, the emissions occur at night, the factory is switched off when the satellite happens to pass over, or it's cloudy (there are attempts to retrieve in the presence of clouds, but it's harder).

So while satellites are certainly very useful in GHG monitoring, it's difficult to get everything from satellites alone.

Where were emissions last month?

We can average daytime GHG contentration measurements over the period of a month.  Combined with chemistry and circulation models, we can then try to estimate in what regions of the Earth those emissions may have occurred, but not with a precision high enough to tell "electricity or cement".  This is an average of CO₂ measurements for July 2009:

The limitations are less serious now: the satellite has multiple attempts to capture a particular scene, and will usually see at least one clear-sky overpass per month, probably multiple.  In the image above, there was probably also some form of data fusion to combine with other sources or fill gaps using neighbouring pixels.  The longer the time period we average over, the smoother the distribution will look.

What were global emissions last year?

However, if you are looking for global emissions averaged over a long time period, we can make use of the observation that CO₂, and to a lesser degree CH₄, is a well-mixed gas.  Here, well-mixed means that it stays long enough in the atmosphere to reach pretty much everywhere given enough time.  That means that ultimately, it doesn't matter where you emit.  That's why "global" CO₂ concentrations may be measured at Mauna Kea (Hawaii, USA), even though this is far away from any emissions.  However, that also means that it doesn't tell us whether the CO₂ was emitted in India, Italy, or Idaho.  And much of the emitted CO₂ gets absorbed by the oceans, so the delta between this year and last is not enough to determine global emissions.


how high was mount vesuvius (mount somma?) before it erupted in AD 79? I guess the highest point of the Mount Vesuvius / Mount Somma complex is currently 1,281 metres (4,203 feet). My question is: how tall was it before it erupted in AD 79?

The AD 79 eruption is the last of four caldera forming events which reduced the heigth of Monte Somma. Their approximate timing is: ca. 18000 BP, ca. 8000 BP, ca. 3400 BP and AD 79.

Between 3400 BP and AD 79 it was about 1300 m high and height of the intact stratovulcano before the beginning of the caldera collapse was about 1900 m.
Note that while the original height can be extrapolated from the shape of the cone, the remaining height after individual events is not well constrained.

Reference: Cioni, R., Santacroce, R., & Sbrana, A. (1999). Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera. Bulletin of Volcanology, 61(4), 207-222.


How Hot was the Oligocene? When studying the paleoenvironment of the Oligocene epoch, a period in Earth's history spanning from 34 to 23 million years ago, I had a really hard time understanding what the climate was like in comparison to today:

  The Paleogene period general temperature decline is interrupted by an Oligocene 7-million-year stepwise climate change. A deeper 8.2°C, 400,000-year temperature depression leads the 2°C, seven-million-year stepwise climate change 33.5 Ma (million years ago). The stepwise climate change began 32.5 Ma and lasted through to 25.5 Ma, as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55 m (181 feet) decrease in sea level (35.7–33.5 Ma) with a closely related (25.5–32.5 Ma) temperature depression. The 7-million-year depression abruptly terminated within 1–2 million years of the La Garita Caldera eruption at 28–26 Ma. A deep 400,000-year glaciated Oligocene–Miocene boundary event is recorded at McMurdo Sound and King George Island.

This statement from Wikipedia struck me as vague and therefore confusing. So in plain, simple English and in general, what was the climate of the Oligocene like in comparison with today's climate?

Modified from Cramer et al. 2009 by adjusting to the Gradstein et al. 2012 timescale. In black, it is the Pacific trend; in blue the North Atlantic and in green the South Atlantic.

This graph shows the Cenozoic variations of $\delta\ce{^{18}O}_{Cibicidoides}$ according to Cramer et al. 2009. This proxy is thought to vary according to variations of deep sea temperature (to simplify) and thus should reflect the climate state at a given time. As negative values of $\delta\ce{^{18}O}_{Cibicidoides}$ corresponds to warmer temperature than positive ones, the graph is oriented accordingly. I highlighted the beginning (33.9Ma) and end (23.03Ma) of the Oligocene in red.

The Oligocene starts with a rapid cooling (linked to the East Antarctic glaciation, see for instance Kennett 1977, Zachos et al. 2001, De Conto & Pollard 2003). Yet as you can see on the graph the temperature is still warmer than the late Miocene to Recent period. Some regional trends then show a moderate warming at the end of the Oligocene that culminates in the middle Miocene climatic Optimum, ca. ten million years later.


Why doesn't the Earth's outer core dissolve into the mantle? It is well accepted that the outer core is made out of liquid iron and nickel, and as everything else it should tend to reach chemical equilibrium with its surrounding.

In particular, I would expect it to interact with the lower mantle through mixing and chemical reactions with the oxygen, silicates and other compounds. These interaction would result in the outer core slowly dissolving away into the mantle.

I would also expect this to happen in spite of the density gradient, that anyway should be less relevant down there, where the acceleration of gravity is much smaller than in the surface.

Why this dissolution doesn't happen? Why is the outer core stable?

As a bonus question inspired by this one: would any other blob of molten iron be stable anywhere in the mantle?

From the comments:

And indeed, after browsing trough a few sections they seem to assume that "Liquid metal separates rapidly from liquid silicate", like oil and water, and they don't mix later. Very interesting. I wonder what's @Gimelist opinion about this.

You are very correct.
The answer is simple: the core and mantle are immiscible. The core doesn't dissolve into the mantle because it's not soluble in the mantle.
This is precisely why we have the core and mantle to begin with.
My answer here is very relevant: https://worldbuilding.stackexchange.com/a/123951/39999

Let's take a hypothetical very simple core and mantle composition:
The core is made of pure iron ($\ce{Fe}$), and the mantle is fayalite ($\ce{Fe2SiO4}$).
What happens when you chemically react the two by $\ce{Fe + Fe2SiO4}$? Absolutely nothing. There is no intermediate $\ce{Fe3SiO4}$. Does not exist. So they just stay as separate iron and fayalite. And because iron is much denser than fayalite, it stays down at the core, whereas the fayalite is the mantle above it.

Edit:
Here's a recent paper describing the overall redox conditions of Earth compared to its neighbours:
https://doi.org/10.1038/s41467-020-15757-0
May be of relevance.


Are there any seasonal or diurnal trends in earthquakes? Though I imagine it's unlikely there is any significant correlation because daily and annual temperature variation really only impact a fairly shallow layer of the crust, I wondered whether the actual data backs that up.  I could envision a few unlikely causes, such as the expansion\contraction, rainfall amounts, or crazy things like the little differences in earth-sun distance.  So what say you?

There is a slight tendency for earthquakes in coastal areas to coincide with higher gravitational attraction. In one study in Greece, earthquakes were 15% more likely to occur when the sun's gravitational pull was at a maximum. In general though, seismic-tidal correlations are very weak (but not zero).


Why are there no snakes in New Zealand when ecosystem needs them I was trying to understand why there are no snakes in New Zealand and at same time I see snakes are important for ecosystems.

So my question is how New Zealand's ecosystem is surviving without snakes?

Snakes are important in some ecosystems because they keep down vermin, but the natives of these places pay a high price for this service. Why does new Zealand not have any? Maybe like Ireland there was a water barrier preventing snakes from reaching the islands. Australia is swarming with snakes, most of them very poisonous, but maybe it wasn't when New Zealand broke free many millions of years ago.

Another possibility is that there were once a few snakes in New Zealand, but predators like the Giant Moa and its relatives, and snake-eating birds of prey, might have hunted them to extinction. As for Ireland, snakes were killed off or driven out by the Ice Age, and when the ice receded the snakes from Europe were too slow to get there before the Irish Sea was formed. Only three species manged to reach England before it was cut off by the Channel. Snakes are not important for British ecosystems, but not many people would like to see them become extinct here. Only one, the adder, is poisonous, and kills about one person every fifty years. Just as British ecosystems could survive quite happily without snakes, so can New Zealand. To introduce even harmless ones there could risk a similar eco-disaster to the one in Australia caused by the introduction of the cane toad.


Can anyone explain the torque that causes the Earth's axial precession? Looking around on the web, I frequently see explanations like "precession is caused by the action of the Sun's gravity on the Earth's equatorial bulge." That's nice, but I'm interested in a little more detail.

Can anyone give a somewhat more detailed explanation of where the Torque comes from? I'm looking for something like "at thus and such a time of year, you have more gravity here and less there; hence the torque acts in thus and such a way"

Any object with a non-spherical moment of inertia tensor that is subject to a non-uniform gravitational field will experience a torque due to the gradient of the gravitational field. This can result in a benefit or a challenge for artificial satellites.

The Earth has a non-spherical moment of inertia tensor due to its equatorial bulge, and the Moon's and the Sun's gravitational field are non-uniform (gravitational acceleration more or less follows an inverse square law). This means the Earth is subject to instantaneous gravity gradient torques from the Moon and the Sun.

These torques would average out to zero if the Earth's equator was aligned with the Earth's orbital plane about the Sun and the Moon's orbital plane about the Earth and if the orbits were circular. Since this isn't the case, residual torques exist after averaging (better: integrating) the instantaneous gravity gradient torques over time. It is these averaged / integrated torques that result in the axial precession.


If a very huge Earthquake occured anywhere on Earth could waves emerge to come together again on the opposite side? Suppose that a super-powerful earthquake occurred anywhere on Earth, say one with the value 10 on Richter's scale. The quake can have any value but as can be read in a comment below the highest value ever measured was 32 on a superdense star. In that case, it's much more difficult to tear the star apart. The Earth,  in contrast, could be torn apart by a quake with value 10 because she is highly less massive.

Suppose the quake was mainly transversal (in a vertical direction). Could it be that correspondingly waves emerged from the center of the quake, traveling the Earth around to come together and reinforced again on the opposite side of the center, with the effect that the quake was felt more strongly on the opposite side of the center than at places halfway from the center (or halfway to the opposite side of the center), to say it in one long breath? Or would too much energy be absorbed from the waves by the Earth to reach the opposite side?

It is called "antipodal focusing". See for example Antipodal focusing of seismic waves observed with the USArray.

  We present an analysis of the M-w = 5.3 earthquake that occurred in the Southeast Indian Ridge on 2010 February 11 using USArray data. The epicentre of this event is antipodal to the USArray, providing us with an opportunity to observe in details the antipodal focusing of seismic waves in space and time. We compare the observed signals with synthetic seismograms computed for a spherically symmetric earth model

The above paper deals with "body waves" that travel through the interior of the Earth.

There are also Rayleigh waves that travel on the surface and can travel around the Earth several times before dissipating (Wikipedia). Antipodal focusing of seismic waves due to large meteorite impacts on Earth does numerical simulations of surface waves at the antipode of the Chicxulub impact. The waves do not arrive at the antipode at the same time because of Earth’s ellipsoidal shape and different rock properties along their paths.

  Isosurfaces of the norm of the peak displacement vector after a vertical impact for the impact hemisphere (left-hand side) and antipodal hemisphere (right-hand side).

(They say: "Note that contours of the continents are today’s, not their end Cretaceous positions, and therefore do not show the original Chicxulub antipode 65 Ma.")


How can I account for rainfall effects on air pollution? How can I adjust black carbon surface mass air pollution concentrations (ug/m3) for rainfall effects?

Specifically, I have monthly and daily averaged black carbon pollution data (i.e. 2.0ug/m3 Jan 2016, Feb 2016, etc) obtained from MERRA-2 reanalysis dataset. I also have averaged monthly rainfall data (i.e. cm per month).

Generally, the air pollution decreases in the higher rainfall months and increases during dry months. I'm interested to remove the rainfall effect from the air pollution values. The end goal is to compare the different months to explore biomass burning effects.

Any suggestions would be greatly appreciated.

Sounds like you want a dataset that is unadjusted for rainfall.  I'm not sure what you have to gain from that, but there are ways to do it.  One way to do this would be to find the inverse correlation between rain and black carbon concentrations, and extrapolate to zero rainfall.  Or, if you have any months that have zero rainfall, you could use that for other months where temperature is similar (thus the mixing layer should be similar).  This would all need to be done for each location separately, so that source contributions weren't mixed up.


Will a volcano erupt in Australia soon? A Science Daily article Volcanic eruptions long overdue in SE Australia (University of Melbourne, 2011), suggests that a volcanic eruption is potentially "long overdue" in south-east Australia (south-west Victoria and the south-eastern part of South Australia).  As per the map below:

Image source

Bernie Joyce, of Melbourne University, has studied the late Palaeogene-Quaternary volcanoes in this region, the last eruption believed to have been at Mt. Gambier in South Australia, erupting about 5000 years ago (image below). His study revealed that the frequency of eruptions is about 1 every 2000 years.

Image source

It is clearly an example of intraplate volcanic activity, as the plate boundaries are a considerable distance away, as shown below.

Image source

What evidence, other than the periodicity mentioned in the article, is there that a future volcanic eruption may occur in south-east Australia?

There are some well established means of detection of an imminent volcanic eruption; however, having said that, I must mention the caveat that not all the signs occur and in some cases, such as the recent Mt. Ontake eruption in Japan - volcanoes can erupt without warning.

Another thing to consider is that the notion of a volcanic eruption anywhere on the Australian mainland is enough to get the media all excited and to latch on any piece of sensationalism, as suggested by one of the commenters in this volcanism blog entry, stating in regards to quotes and information the media attribute to Professor Bernie Joyce of Melbourne University (A person I have met in the past and I can vouch for his non-alarmism):

  This nonsense runs in the media here about once a year. What Dr Joyce is really trying to say is that Southwest Victoria has a history of monogenetic volcanism, and should have some contingency plans in place similar to Auckland.

So, the signs of an impending volcanic eruption may include the following (in summary from the Oregon State University page What are the signs that a volcano is about to erupt?:

First and foremost, increased or anomalous seismicity (as suggested by Gary Kindel in the comments to the question).  Earthquakes are not uncommon in Australia, and not uncommon in the area in the volcanic field of Victoria and South Australia.  Some earthquakes have been reasonably severe, as shown in the map below, from the University of Western Australia's webpage Volcanoes & Earthquakes in SE Australia:

The red triangles are a handful of the more 'significant' of the 300+ volcanoes in the region.  The purple represent earthquakes with magnitudes greater than 4.0 on the Richter Scale and the blue are all earthquakes recorded between 1995-2004.  A word of caution with this map, it has been only since the 1960's that earthquakes have been recorded.  The largest earthquake in the region was 6.5 on the Richter Scale in 1897 (north-west of Mt. Gambier).

Additionally, as earthquakes in Australia are intraplate, they tend to be considerably shallower than those from plate margins, with, according to the Seismology Research Centre, depths usually less than 20km.

According to the USGS earthquake track page for Mt. Gambier, there has been some seismic activity in the past 25 years, but not much at all - a total of 5 earthquakes, ranging in magnitude from 2.8 to 4.7 and at depths from 10-22km - not particularly anomalous, especially when compared by the literally dozens of quakes occurring in the state of Victoria (including non-volcanic areas) of similar magnitude and depths in 2014 alone.

However, as more seismographs have been established over the area, there have been some anomalies, including (from the University of Western Australia):

  Three earthquakes of magnitude approximately 2.5 occurred in the vicinity of Mount Gambier in January and February of 2003, with greater than usual depths ( ie, approximately 40 km deep). Some scientists have suggested that they may be generically related to the volcanoes in the area.

According to the Geoscience Australia report Australian Seismological
Report 2008 - addendum, there is still significant seismic activity around Mt. Gambier and comparatively less in western Victoria (as seen in the map above).

The Oregon State University document also states that the presence of fumaroles and ground deformation are also signs of potential volcanic activity.  In terms of western Victoria and south-east South Australia, a conclusion regarding this is given by the University of Western Australia:

  Many of the volcanoes have preserved their classical volcanic shapes, and the region is considered by some to be volcanically dormant rather than extinct. However, note the conclusion by Cliff Ollier ( in "Landform Studies from Australia & New Guinea, by Jennings & Mabbutt, 1967, p 338) - " Perhaps the most remarkable feature of Victorian vulcanicity is its complete cessation. Older basalts were erupted in Victoria through long periods in the Tertiary: the newer basalts were erupted throughout the Pleistocene and extended into the Recent. The last eruption probably took place only 5000 years ago, and yet there are no fumaroles, no hot springs, no anomalous temperature gradients or seismic effects, in fact nothing to indicate how recently the volcanic activity ceased."

However, according to the Romsey Australia blog-page Is there a risk of a volcanic eruption in Australia?, Professor Bernie Joyce has developed a volcanic hazard map, regarding the potential risk according to his research and observations:

And notes that with an eruption that formed Mt. Gambier

  an eruption could either be an explosion like the one at Mt Gambier, creating a big hole in the ground

with little or no warning.  But another scientist Dr. Wally Johnson disagrees in part, stating:

  "If Mt Gambier did erupt it would impact on local communities," Johnson said. "But any changes to the state of these volcanoes would be noticed early on, either through earthquakes, or in the case of Mt Gambier an increase in the temperature of the water.

This leads to one final point, recent studies and observation reported in the Romsey blog section Geologists suspect an earthquake that originated 50 kilometres from King Island in February 2002 signalled the reawakening of the hot spot - Bass Strait is the waterway separating Victoria and Tasmania - this is known as the East Australian Hotspot which is a considerable distance from Mt Gambier and the other Victorian/South Australian volcanic centres.  Shown on the map below:

Image Source:

  Rapid change in drift of the Australian plate records collision with Ontong Java plateau
  Kurt M. Knesel, Benjamin E. Cohen, Paulo M. Vasconcelos & David S. Thiede
  Nature 454, 754-757(7 August 2008)
  doi:10.1038/nature07138


Does earth itself emit electromagnetic waves? (.... and if so, of what frequency and amplitude)?

This Skeptics.SE question about nonsense called 'geopathic stress' made me wonder if and how earth itself emits electromagnetic waves.

We need moving charges for that, and I can hardly imagine that things like moving magma or 'moving' radioactive minerals would 'emit' anything, but maybe I'm missing something?

I'm not asking about atmospheric phenomena, or about externally introduced electrical currents (man made, lightning).

Yes, the earth emits electromagnetic radiation.

It emits infrared radiation with wavelengths of about 1 µm to 1 mm. This is heat, essentially, and much of it radiates into space:

Most if not all of the earth's rocks are radioactive (e.g. because of minerals containing isotopes in the uranium decay chain, such as potassium-40) and emit gamma radiation with wavelengths around $10^{-12}$ m. I'm not sure how much of this radiation would make it through the atmosphere.

You said you're not interested in atmospheric phenomena, but I wonder what the relative contributions of subsurface and atmosphere (e.g. the auroral kilometric radiation), as well as manmade sources, would be to the total signature. The planets emit radio waves (ask a radio astronomer), but I suspect most of that energy is from their atmospheres, since Jupiter and Saturn are especially bright. This book by Vázquez, Pallé and Montañés Rodríguez (2010; Springer) looks like it might address some of this, e.g. in Chapter 4.


2018 Swan Islands earthquake tsunami How come the strike-slip 7.6 M earthquake, occurring January 9th, 2018, produced no tsunami? Has there ever been any produced in this region?

This is the centroid moment tensor published by the USGS as of 01/10/18 (it will likely be updated in the future). As you can see this was an almost pure strike-slip mechanism on a near vertical plane.

Tsunamis are generated when the ocean floor is displaced upwards by an earthquake, displacing the water above it in the same pattern. This displaced water then spread out laterally to form the tsunami. Thrust faults tend to produce large seafloor uplifts and so most (maybe all?) tsunamis are associated with thrust faulting earthquakes.

There was no tsunami in this case because a strike slip fault like this does not produce a significant uplift of the seafloor. Instead the seafloor slides past each other.

With regard to the second part of the question; yes there have been tsunamis in the Caribbean before. This website provides a good list of known ones.


Is the temperature (even a bit) warmer in a forest? Is the temperature (even a bit) warmer in a forest?

For example, there no frost on the ground in a forest when there is some in a field just outside the forest. Is it because the temperature is a bit warmer? Or simply because there is no dew on the ground in a forest?

It will depend on the time of year and the time of day. As you noted, you often see more frost in an open field than in a forest. A comment indicated the forest insulated the surface but I believe it will also reduce radiant heat loss at night, compared to the open field. During the day, the ground surface is more shaded and the trees reduce the advective movement of warmer air into the forest floor so the forest will often be cooler than the open field.

Another factor is evapotranspiration. Forests will tend to be cooler in drier summer conditions because, in general, you get more evapotranspiration from trees than from shallow-rooted vegetation and annual grasses die off in summer. The evapotranspiration cools the area.


Introduction to Geology Books I am a high school student who is thinking about going to university for Geological Sciences. I've had an interest in rocks and such since I was little, but I want to get an idea of what studying Geology at a post-secondary level would be like. I am looking for recommendations for high school/undergrad level books that would be accessible for a 16 year old, specifically on Mineralogy and Physical Geography (the areas I am most interested in). Thanks!

I believe Manual of Mineralogy (based on Dana) is a good start for mineralogy and crystallography subjects. It is a good manual to understand the basics behind crystalls and minerals. It is a bit theoric, but it explain crystallography laws you need to know to face microscope. It also list the minerals by groups following Dana classification.

I would recommend to chose recent introduction books to get a view about Earth Sciences in general, as modern tectonics has changed some concepts. That will cost you some chips; if that's a trouble, university libraries use to be open to public. You can go there and read any book published recently whose title look like "Introduction to Earth Sciences". There are some books that specifically have a geophysics approach.

Talking about studying geology at a post-secondary level would be like, you need to know in geology field is the lab. In firsts courses you might find difficult to get it, but once you got the basic concepts, you will start to enjoy the Friday trips.

I am going to recommend, if you have the opportunity, to join a geology summer camp. You can ask at the university you want to join if they have field introductory courses. You will need good shoes, a geology hammer, a compass, a magnifying glass and good attitude to learn new things.


What causes the aurora borealis I heard of the aurora borealis in North America and I've also seen the picture which made me to wonder that what causes the aurora borealis and does it have any advantages or disadvantages?

When charged particles from the sun strike air molecules in Earth's magnetic field, they cause those molecules' atoms to become excited. The molecules give off light as they calm down.                         At the center of our solar system lies the sun, the yellow star that sustains life on our planet. The sun's many magnetic fields distort and twist as our parent star rotates on its axis. When these fields become knotted together, they burst and create so-called sunspots. Usually, these sunspots occur in pairs; the largest can be several times the size of Earth's diameter.                                                                       At the center of the sun, the temperature is 27 million degrees Fahrenheit (15 million degrees Celsius). As the temperature on its surface rises and falls, the sun boils and bubbles. Particles escape from the star from the sunspot regionson the surface, hurtling particles of plasma, known as solar wind, into space. It takes these winds around 40 hours to reach Earth. When they do, they can cause the dramatic displays known as the aurora borealis.


What is the chasm in the Arctic on Google satellite map? In the image below, taken from Google Maps satellite view, between Svalbard and Greenland there is what looks like a massive crevasse, chasm, crack, &c.

What is it? Should I be worried?

You probably realise this, but you're not seeing that directly from satellite photos. Google Earth (or the "satellite" view in Google Maps) shows direct imagery on land, but underwater it shows a hillshaded view of bathymetry. So you're looking at the shape of the seabed.

With that said, I think this is the nothernmost part of the Mid-Atlantic Ridge, possibly the part known as the "Mohns Ridge". You'll see a similar line, coming and going, all the way from here down to the middle of the South Atlantic. There's a deep valley with mountains on either side. It's the boundary between two tectonic plates, where new seabed is slowly created as the Atlantic widens.

No, you should not be worried about it :-)


Himalayas are currently rising. What will be the highest point they can reach? Himalayas are currently rising about 5 mm/year. What will be the highest point they can reach?

The Himalayas are indeed rising, but they are also being eroded at a comparable rate. It will come as no surprise to anyone that the maximum possible height of a mountain on Earth is only marginally higher than Mt Everest, that is probably about 9000 to 10,000 metres high. There are, of course compressive / convergent plate boundaries where there is a potential  tendencey to build higher mountains, but non-linear counter processes kick in to counteract the mountain building. There are three processes, glacial erosion, shear collapse and lower crust / upper mantle plasticity. The latter is the most important. As lateral compression builds higher mountains, the mountain roots are depressed into the hot plastic region such that the mountain literally sinks under its own weight, deforming the root zone, sometimes in spectacular fashion. Go to Scotland, or Norway, where you can see the eroded remains of the long-gone Caledonides, and you can see the exposed and metamorphosed root zone of mountains that were once as big as the Himalayas.

In principle, the mountain would eventually collapse by shear failure, simply because the rock isn't strong enough to support the weight of gigantic mountains. In practice, deformation takes place before the shear failure is reached.

Then there are glaciers. The higher the mountain gets, the more rain  turns to snow, then to ice, which is a phenomenally effective erosive agent (just look at any of the worlds recently glaciated terrains - European Alps, Patagonian Alps, Norway, Alaska, New Zealand, etc., to see the spectacular erosion). This erosion peaks at a high altitude, but not at the very highest altitude, where the atmosphere is so cold that it cannot deliver much moisture - so very little snow. Result: glacial erosion takes bites out of the mountain at middle-upper elevations, thereby keeping the slopes steep, whence gravity collapse along pressure release joints, and other related processes. Hence occasional peaks get close to 9000 metres but, in geological terms, they don't last long before erosion brings them down.


Causes of fluctuations in atmospheric oxygen in past 300 Mya I have two questions based on the history of atmospheric oxygen levels depicted below.

Source: https://commons.wikimedia.org/wiki/File:Sauerstoffgehalt-1000mj2.png.

The notes below the chart explain most of the changes in the past Gya.

What they don't explain, however, is why oxygen has declined for
the last ~100 Mya. For example, is this explicable in terms of volcanism?
They do explain the Permian decline in terms of volcanic activity,
but why do volcanoes affect oxygen levels?

Here's a somewhat different looking graph, from Oxygen and Evolution,
Robert A. Berner et al., Science 316, 557 (2007):

The graph shows three marked drops in O2 levels, each corresponding to an extinction event. These are intervals 4, 9, and 11. Interval 9, the largest such drop, represents the end Permian event about 252 million years ago. Interval 11 corresponds to the end Triassic event, 201 million years ago. The unsourced wikipedia graph linked in the question has at most a superficial resemblance to this sourced graph.

Now for your two questions.

  What they don't explain, however, is why oxygen has declined for the last ~100 Mya. For example, is this explicable in terms of volcanism?

I would take even the sourced graph that I used with a grain of salt. That unsourced wikipedia graph: A very large block of salt is needed.

  They do explain the Permian decline in terms of volcanic activity, but why do volcanoes affect oxygen levels?

It is very widely conjectured that the end Permian extinction event was caused by the Siberian Traps. This extreme volcanism may have triggered coal fires, thereby reducing oxygen levels. This remains conjecture.

What isn't conjecture is that the end Permian event killed off almost all life extant at that time, including much of the algae and plants that produce oxygen. Without a continuous supply of new oxygen, oxygen sinks such as unweathered rock will inevitably result in a decline in oxygen levels.


Could evidence of a signal from an exocivilization be found in the geologic record? We know that the current SETI efforts have not found evidence for the existence of intelligent exo-civilizations (XCIV). There also has not been any evidence found of ancient non-human civilizations in the fossil record. One possibility, among many, is that XCIVs only exist for a geologically short amount of time before collapsing/extinction. Then, it could be possible that, even though none exist now, one or more XCIV could have existed in the past, and it was capable of transmitting electromagnetic (EM) radiation signals.

So, consider the following hypothesis:

In the last 100 million years, at least one XCIV capable of emitting directed EM radiation existed within 100 light-years of our solar system.
The CIV sent directed EM beams towards all the planets in the star systems surrounding it.
Based on what it could detect about each planet's composition, it designed the beams (frequency, signaling pattern, modulation, etc...) such that they would leave as large of a mark on the targeted planet's geologic record as possible (the mark).

Question: Is there any form of patterned, extra-solar EM radiation that could theoretically leave a mark in the geologic record that could be recognizable to us as such up to 100 million years later? And could a human-level civilization be capable of projecting such beams?

No, it is not possible.

The only thing I could think about is re/de/magnetisation of magnetic minerals. Even then I'm not sure it's feasible, because you to form a magnetic field you need an electric current in motion, but that's a question for physics, not here.

So far, the major magnetic signals recorded in the geological history are well understood. There are many small scale signals that are less well understood, and are believed to be the result of internal processes within the Earth. You can look at my answer here for examples of these signals. Anyway, even if an extraterrestrial civilisation could somehow affect the geological record, it would have to be sustained for very long times (hundreds to thousands of years) for us to be able to see something, and even then we probably wouldn't be able to distinguish it from noise.

As an example, think of a river with rocks in it, constantly getting moved and tumbled. Then you go and throw a rock inside. There would be absolutely no way to know which rock was it (and even that someone threw a rock inside), even a day later, let alone 100 million years. The only way to distinguish something would be a huge amount of rocks, not just one. And then there's the question, of why would someone spend this enormous amount of energy of throwing rocks in the river or sending EM beams towards planets?


Do river fluxes help contribute to long-term carbon sequestration? Rivers help transport sediment from higher regions into the oceans (where it's hard to get back into the atmosphere).

And this sediment could be carbon-rich.

Rivers generally include dissolved oxygen. Therefore any organic content in suspension will tend to dissolve. The movement of water (open to the atmosphere, etc) will tend to entrain some oxygen from the atmosphere.

For carbon to be preserved in sediment, it has to be buried before it can break down. Typically this would occur where water movement is minimal so that it can precipitate out and be buried by mud/sand.

Areas of water may be anoxic (eg. ocean bottom waters, or the sea bottom waters that led to the deposition of the English Kimmeridge Clay), but these areas tend to have restricted currents so that what oxygen that is available is "quickly used up" oxidising some of the organic material.

Yes you can capture carbon in sediments - that is how all fossil fuels are formed, but you have to have the right conditions. Lakes, swamps, seas are much better than running rivers themselves. A deltaic swamp with lots of organic stuff growing in situ but experiencing periodic river floods could be a good way to preserve organic material...  (cf. the Carboniferous in Europe and the US).

Further references:

Factors influencing organic carbon preservation in marine sediments.

"Sedimentary organic matter preservation: an assessment and speculative synthesis" in Marine Chemistry Volume 49, Issues 2–3, April 1995, Pages 81–115

Interesting research on the influence of iron's oxidation states on the oxidation of organic matter:  Preservation of organic matter in sediments promoted by iron, Lalonde et al,  Nature v483, pp198–200


To delineate the drainage basin for a lake, would the pour point be the inlet or the outlet? I am interested in the area draining into and affecting a particular lake. Delineating this drainage basin, should I use the lake's inlet or outlet to best capture the relevant drainage basin? I think including the lake is valuable in this exercise, as events on or in the lake itself also affect the lake.

I'd think outlet, but want to double check with you all. To complicate things, most lakes have more than one inlet and outlet. I guess I'd go with the lowest outlet in order to avoid missing any land which may drain into the lake.

I think there are a few things to consider. First, a drainage basin is defined as the area upstream of the point to which all precipitation converges. Flow does not converge at the outlet - flow converges at the lake. Outlets don't contain any additional information of the upstream area draining into the lake. This implies that to find the area draining into a lake, you would necessarily have to proceed beginning from each and every inlet.

Here's another way to see this. Approximately 20% of all land drains to lakes with no outlets. These are referred to as endorheic lakes or endorheic basins. Even though there is no outlet, there is absolutely still a definable drainage basin area.


What is the technical term for the amount of water within the surrounding enviornment? This may seem an obvious question but I am unable to understand what the technical term for the surrounding water of an environment is, specifically surrounding a person/animal within that environment.

Humidity: This seems like the most obvious answer but 100% humidity still refers to air and as far as I understand doesn't apply to environments like the bottom of the ocean or a pond.

Is there a single term that describes the water content surrounding a person or animal?

What is capped at 100% is the relative humidity, that describes the degree of water saturation of air. Relative humidity only consider the water that is dissolved in the air, not all the water that exists in a given volume of air.

You are interested in the mixing ratio between air and water, which is referred to as specific humidity. Specific humidity is the ratio of the mass of water to the total mass of the moist air parcel.

That water, usually refers only to the one dissolved in the air parcel. However, it can easily be extended to include water droplets and other bodies of water. But that approach will require that you define an arbitrary volume over to which compute the mixing ratio, because you won't be able to treat this as a continuum problem.

For instance you can compute the mean mixing ratio between air and water on a 10 meters sphere around your object of interest. Then, the more water there is within that volume, the higher will be your mixing ratio. For example, if a person is swimming 10 m deep, the mixing ratio would be almost 1. And if the same person is in the desert more than 10 m away from any water body, the mixing ration will be close to zero (not cero because all the water in the person's body and what is always in the air).


How good is K-Ar dating? I don't doubt that K-Ar dating of rocks is a sound method, but in discussions with skeptics it would be good to know just how reliable exactly? For example, has K-Ar been cross-checked with other rock dating methods, such as radiocarbon, sediment varves, etc. If so, yielding what level of correlation?

The reliability is dependent on the quality, (for instance is the mineral reworked or porous and thus prone to contamination),  and size of the sample, (small samples result in poorer measurement). There is also of course the date range issue, use a sample that is too old or too new and the amount of each element is so tiny it can't accurately be measured. Thats why multiple measurements are always taken, if they vary wildly you know something is wrong. A good general rule for modern studies is expect a date to vary by ±5%.

Potassium-argon can be compared with relative dating methods like fossils and bedding as well as absolute dating methods like Uranium-Thorium, Uranium Lead, Rubidium Strontium, and even it's own dates to check for consistency. This is always being improved upon and multiple dates from different methods are sought whenever possible so there are tens of thousands of comparisons.

There is a good article about it here. They also provide a good example study (under the title a good example) which compares several methods for dating the K-T boundary and yields an error of about ~1 million years between them . Beware of older studies (1970's or older) which can have much larger error bars.

For other studies comparing lots of dates your best bet is large scale timescale reconstructions like this one which will compare dozens if not hundreds of individual dates.


Cleaning fossil samples with acetone Can I clean fossil samples with acetone, or will this damage them?

It depends on what you want to use them for.

If you might someday want to extract DNA from them, or do some other chemical analysis, then it's probably a bad idea.

If you only want to display them, and you like the way they look after acetone treatment, then it should be OK.

Test on some samples you don't care about first.


Why do rainbows appear to have a fixed structure? Why do rainbows appear to have a fixed structure when a bunch of raindrops are 1. moving, and 2. granular?

In short, this is because the location of a rainbow is not dependent on water droplets. The location is only dependent on the location of the Sun and of your eyes. The visibility of the rainbow is what depends on the presence of condensed water.

As each drop passes through a location that corresponds to that of the rainbow your eye will see the reflected photons originating at the Sun. Before and after the rainbow, that individual drop is not in a location that allows you to see these reflected photons. The rainbow appears stationary because it is (as long as you aren't moving) and droplets are falling through it often enough that it always appears visible.


Eclipse, Hurricanes, Earthquake....Related Events? Forgive my ignorance, I'm out of my depth.

My wife has pointed to the upsurge climate and geological activity post Eclipse.  My immediate response, at least inwardly, was to reject her suspicion.  However, the tectonic plate shift yesterday, in Mexico was substantial and the constant interplay of nature's  habits seems more robust than usual.

Is there substance to her hypothesis / theory, that the Eclipse and recent events on Earth are interrelated?

The occurrences of Hurricanes, Harvey, Irma, Katia and Jose, east of Mexico and a magnitude 8.1 earthquake southwest of Mexico within a few weeks of the American solar eclipse of August 21, 2017 are purely co-incidental.

Hurricane Katrina, which devastated New Orleans in 2005 was not associated with any type of eclipse - solar or lunar.

Likewise the eruption of Mt St Helens in May 1980 was not associated with any type of eclipse, or were  the San Francisco earthquake of April 1906 or the Loma Prieta earthquake of October 1989.

There is a list of every solar eclipse that occurred globally during the 20th Century. From the list, you will see there were one to two solar eclipses every year during the past century. None of these have ever been associated with any natural event such as earthquakes or hurricane/cyclones/typhoons.

Co-incidences in nature occur all the time.


Could an earthquake create a design in a sand pendulum? Is it possible for an earthquake to generate such a design in a sand pendulum during an earthquake? I saw an article online describing the story of a shop owner finding one of his pendulums had created this image during an earthquake.

I researched seismographs and they all seem to be in a linear fashion. Obviously a pendulum is not a scientific seismograph which raises the question: is this possible?

I could not find any research debunking this concept nor confirming. All the articles I read were different news groups reiterating the same story.

Interesting images, thanks for sharing them.

I think its quite likely that a sand pendulum would act as a type of seismograph.  I'm more in geochemistry than geophysics side of things but this kind of pattern does not seem at all inconsistent with seismic shaking.  The linear nature of a recording seismograph is because it is designed to record against a time scale which is perpendicular to the "movement" scale.  Even if movement had a stronger direction in the ground, the table and building this sand pendulum is in would translate the shaking movement into additional directions.  There were multiple movements to make the pattern shown.  It defiantly looks like more than just a truck or subway going by!


If two adjacent tectonic plates are moving in the same direction, is there significant interaction at the boundary? If two adjacent tectonic plates are moving in the same direction, is there significant interaction at the boundary? Is it just a convergent boundary if one moves faster and "catches up" with the other? The boundary types typically shown include Transform, Convergent, Divergent, and "Other" (for when there are scattered microplates, or when other complex/compound interactions are occurring)

It’s relative motion that matters. So in your case, I assume one side is moving faster than another, so you would have a right lateral or left lateral transform fault depending on which side is moving faster. It’s “significance” would depend on the amount of relative difference in motion and, as usually defined, its potential impact on man.

Note that if they are moving at the same rate there would be no fault (and therefore no boundary.)

There has to be relative movement of the two plates towards one another for it to be “convergent.” Convergent plate boundaries are where one plate is being subducted (e.g. Japan) or pushed against (e.g. Himalayas) another. Having a combination of movements is not really uncommon, but typically of secondary importance. Good example this situation exist along the San Andreas Fault where it locally bends a bit, adding localized convergent components to what is an overall transform fault system.


Atmospheric visibility measuring device I have this project at university where I have to make an atmospheric visibility measuring device using Arduino. I talked with my teacher and he said that I should do something like this, using an emitter to transmit a laser beam and a receptor. This is all the information he gave me. I have never used Arduino before and I found almost nothing on the internet to help me.

Can someone provide me with a starting point and explain what kind of components I should use (I have to buy everything myself) and how to build this device?

Thank you!

The transmissometer setup should be quite easy to build in principle, but it could be hard to get accurate values. I did a very similar instrument but to measure water transsmisivity (a.k.a. turbidity).

If you are new to Arduino you can start with the Arduino UNO board, and this is a good guide on how to get started. I got started myself with the video tutorial Arduino Course for Absolute Beginners.

Practice with a couple simple programs. In particular "Blink" to get used to switch pins on and off and "Analog Read Serial" to learn how to output data to the computer.

You need a laser diode like this one. It should be powered (or at least turned on/off) by the Arduino. Because you will need to do measurements with the laser on and off to know how much light is ambient light and how much light is coming from your laser.

To sense the light intensity of the laser after it travelled trough the air, you can use a photoresistor to make it simple and cheap, or a luminosity sensor like this one. You can find here a tutorial and example code to wire and read data from that sensor into your Arduino.

Then you need to calibrate the sensor, and it will be challenging for this project. You will need to put the system in the outdoors, measure the visibility by finding out the distance to the further object you can see, and record the reading of your Arduino. The reading would be the difference between the luminosity with the laser on and off.

After you have many calibration points over a wide range of visibilities, you can plot visibility vs reading and hopefully the points will follow a nice pattern that you can then fit with a suitable function (a line, a polynomial or logarithmic function), and then code that function into the Arduino so it can output the actual visibility.

If the pattern isn't nice, you will need to find ways to make the reading more accurate and sensible to changes in visibility. Some options for that (that you should consider also in your initial design) are:

Increase the travel length of the laser through the air.
Use a blue laser, as blue light is absorbed quickly in low visibility conditions.
Use a hood with black interior (like the photographic lens hoods) around the sensor to minimize the contribution to ambient light.
Use a luminosity sensor with larger resolutions (12 or 16 bits)
Make sure the laser voltage is stable.
Make sure the laser (and mirror if you use one) have a good and stable alignment.
Make sure readings are not affected by temperature or compensate otherwise.
Get multiple readings and take the median.

There is an Arduino Stack Exchange where you can find help if you need help to make the sensors to work or problems with the coding. However, there are tons of online resources and tutorials, so I'm sure you will find answers to all your questions.


Extent of desert area influence in local climate With reference to this image...:

...from this recent tweet, I was wondering to what extent would Mediterranean regions be affected by the large desert areas to its South?

"Affected", here meaning that there would be higher correlation between temperature measurements between the Sahara and Malta (for example), than Malta and Iceland.

I suppose that measurements are correlated but that this correlation reduces with distance to some lower value (but not zero). The question here is, how quickly does this correlation tend to its lower value and is the distance between the "centroid" (?) of the Sahara desert to points in the Mediterranean short enough so that it can be considered an "influence"?

The mediterranian do in a way create its own climate and is partly protected from the north saharan desert by the dominating wind direction in the area.

The dominating wind direction is from west and northwest,The wind blows into the saharan desert in the direction of the equator in the winter,This creates the mild winters the mediterranian is known for.

In the summer more warm and dry air moves from the north saharan desert and into the mediterranian area,this creates the dry summer climate the mediterranan is known for.

So the north saharan desert have the strongest influence on the mediterranian climate in the summer.

you can read about it here https://en.wikipedia.org/wiki/Hadley_cell if you click on the animation you can see how the precipitation changes over the year and thereby also the temparature.

and here https://en.wikipedia.org/wiki/Mediterranean_climate


What is the definition of a continent? Is there a scientific definition of a continent? Is there an academic reference for such a definition? There's a tendency to correlate with tectonic plates, but of course lots of plates don't have continents.

I keep running across "by convention" and "landmass" but I don't see anything really definitive, or anything academic. I've got the whole Pangaea/Gondwana/craton/shield/continental crust/oceanic crust/plate tectonics fine, but this simple question is pretty vague.

According to the National Geographic Education Encyclopedia webpage about continents, defines these as the main divisions of continuous land, extending out to the edge of the continental shelf - this also includes much of the surrounding islands. In the case of the division between Europe and Asia, this division is largely due to cultural differences.

The exceptions to this are what are referred to as 'microcontinents', that National Geographic site describes as being landmasses that are 'geologically distinct' from the neighbouring continents.

There seems to not be a consensus on a tectonics based definition of a continent; however, given that continents change over time, it is suggested in the article Continents as lithological icebergs: the importance of buoyant
lithospheric roots  (Abbott et al. 1997) as implying that the particular lithosphere is "unsubductble' over two 0.25 billion year time periods - which, according to the authors, include mid-level and lower crustal levels, and that timeframe would also include Precambrian cratons as their core.


Calculating Solar Noon from Sunrise and Sunset I've seen the solar noon formula described as snoon = 720 – 4*longitude – eqtime. I'm curious if it is equally reliable within a relatively small margin of error (+-60 seconds) to simply get the midpoint between sunrise and sunset, given that data is already calculated and available.

Is there any occasion when solar noon wouldn't be equal time from sunrise and sunset, give or take the aforementioned margin of error?

Received an answer on the astronomy stack. https://astronomy.stackexchange.com/questions/22239/getting-solar-noon-from-sunset-and-sunrise/22241#22241

The short of it is that yes, this is reasonably accurate for most geographies, but can vary significantly in polar regions. For most cases, middling the difference of sunrise and sunset is sufficient.


How much energy released by the Theia-Gaia impact? I read somewhere that the Chicxulub impact released the energy equivalent of anywhere from 100-300 teratons of TNT. That's an impressive amount of energy.

Assuming a head on collision (according to newer research), how would it compare to the impact that created our moon?

Here's a quick-and-dirty estimate.
The gravitational self-energy of a uniform-density sphere is
$$
U = \frac35 \frac{GM^2}R
$$
Let's assume Theia had the same mass and density as Mars, and that Gaia contained the rest of the mass of the Earth-Moon system.
The binding energies for the four bodies are then

theia/mars 4.82e+30 joules
gaia       1.90e+32 joules
earth      2.24e+32 joules
moon       1.24e+29 joules

You can see that Theia and the Moon contribute to the binding energy starting the third significant figure: moving the 90% of Theia's mass to Gaia, leaving us with the Earth and the Moon, must have released something like $0.3\times10^{32}\rm\,J$ of gravitational binding energy as heat.

Kinetic energies due to Earth's rotation and the Moon's orbit are irrelevant compared to Earth's binding energy --- the biggest contributor there is $0.25\times10^{30}\rm\,J$ associated with Earth's daily rotation.
It's probably safe to assume the same about the progenitors.

Apparently a teraton of TNT is $4\times10^{21}\rm\,J$, if you insist on that comparison.

Note that I've assumed nothing about the geometry of the collision, whether it was head-on or glancing. I'm only making assumptions about the (well-known) final state and the (poorly constrained) initial state.


Do Siberia and Canada have a rasputitsa? The rasputitsa is a muddy season that happens twice per year in West Russia, Ukraine, and Belarus. It happens in mid-October to late November due to rains, and mid-March to late April due to thaws (according to the Russian wiki page with google translate). These conditions make travel by dirt road almost impossible, as made famous during WW2.

What I'd want to know is, does Siberia and Canada also have rasputitsas? If so, what times of the year do they occur? and are they comparable to the West Russian one?

Edit: I should have specified this, but Siberia and Canada are large places. I would guess there's an arctic limit somewhere, to the north of which never gets muddy since it's always too cold. So I guess what I'm asking is, what regions of Siberia and Canada have rasputitsas (if any)?

In Canada there is definitely a rasputitsa period, though it does not occur everywhere or necessarily in the fall, and it is not considered a rasputitsa by name.  Every spring in most parts of Canada there is a period of time when many of the soils are saturated with water from the recent snow melt.  The various forest services and companies will often close roads or reduce the load limit during the spring thaw specifically because the roadbeds are saturated to the point that large equipment can punch out the roads and if bad enough get stuck.  Even the major highways in parts of the western prairies and in many mountainous areas have reduced load limits just after spring melt because of issues with the roadbed being saturated.  This issue mainly affects the tertiary road systems because modern road construction methods are developed to reduce the effect of the spring thaw on the stability of the roads.  The post here describes the highway load reductions.


Why did the carboniferous period have so much atmospheric oxygen? Even if all the carbon dioxide (which makes up less than 1% of the atmosphere) in the air were sequestered by plants, would the atmosphere not remain about 21% oxygen? Why did the carboniferous period have 35% atmospheric oxygen?

The Carboniferous was when the growth of woody plants took off. Non-plant life had not yet evolved the ability to consume lignins, the key chemical components that makes woody plants "woody". Lignins are rather hard to decompose. Despite high volcanic activity, carbon dioxide levels fell by a factor of over four during the Carboniferous, from over sixteen times preindustrial levels at the start of the period to less than four times preindustrial levels at the end of the period.

The end result was a gradual increase in oxygen levels and huge deposits of then non-digestible materials that eventually became coal.


Is it possible to detect Coriolis effect using a Wind Rose? I once learned in high school that the area where I used to live experiences the Coriolis effects of the air movement above the city. I never paid attention to this before, but I recently heard about it again, and I have no idea if it is true or not. There is absolutely no way I can validate that theory by using the wider definition of the Coriolis effect, but I do have access to a wind rose which looks like in the graph below:

Is there any way to understand if there are any Coriolis-like winds by using this graph or a similar one? I found a nice sketch of this effect online, but I fail to understand how to correlate these two.

Is there anyone who could help me to understand if one can get this effect from a wind-rose?

Yes, your figure is correct: in the mid-latitudes we have cyclones and anticyclones. They can't exist on equator because the Coriolis force there is too small to keep them stable. But the cyclones or anticyclones are present all around your country and their effect cancels each other. So, you can't determine the presence of Coriolis force. Otherwise, the effect of spining anticyclones and cyclones isn't specific to your country, but is (as previously stated) present over mid-latitudes. Also, the Coriolis effect isn't acting on microswirls because the Coriolis parameter is so small (due to too slow Earth's rotation).


Earth's ocean basins Was Earth's sea-floor always carved with the numerous geological features that we see today, or was it at some point flat and boring as Mars is today?

Yes, in terms of appearances (as in your question), the Earth's ocean basins have more than likely have been carved with features of some kind or another throughout Earth's history; however, the Martian surface is not featureless - not even close.

The reason being that the geological evidence suggests that plate tectonics has been going on for much of the Earth's history, a mechanism that seems not to be present on Mars - many observations and subsequent models show that the plates have been in motion for much of the history of the planet - resulting in trenches and underwater (and above water) volcanoes (subduction ones), ridges (sea floor spreading). Additionally, hot spot activity has given rise to numerous sea mounts, such as the Emperor-Hawaii seamount chain.  These features often taken millions of years to fully develop.

Even though oceanic crust is recycled through plate tectonics, it could be assumed that these processes have formed similar ocean-floor-scapes on occasions over a long period of the Earth's history.

One of the main differences is that the processes that form these features on Earth have been, overall, semi-continuous on Earth - whereas on Mars, these processes, if started, died out a considerable time ago.


What goes into a tide power prediction for every fifteen minutes over the next twenty five years? I was watching a segment about tidal power generation on a PBS NewsHour broadcast. I've set the time code at about 11:45 to capture a little background. This bit starts at 12:36:

As the tide ebs and flows, the turbines spin between seven and fifteen times a minute, generating power similar to a wind turbine. Cables cary the energy back to shore; first underwater, then underground, where it’s then fed into the national grid.
The tides are so predictable that Atlantis says it can tell how much energy these turbines will generate every fifteen minutes for the next twenty-five years.

What kind of information goes into such a long-term prediction? Is it just the relative positions of the Sun, Earth, and Moon, or something more? I ask because those positions can be calculated for thousands of years, there wouldn't be a cut off at only 25 years, so I assume there's more that goes into it.

Tide prediction at some locale is more of an empirical art rather than an analytic science. It essentially is a reduction of decades or centuries of historical tide levels at the locale to Fourier-like coefficients. Tides at a locale are modeled as a sum of various frequency components, each with a magnitude and a phase offset from some reference. The frequency components are based on the Earth's rotation about its axis, the orbit of the Moon about the Earth, and the orbit of the Earth-Moon system about the Sun. Given a sufficiently long historical record at some locale, mathematical techniques enable teasing the magnitudes and phases for each frequency component from the record for that locale.

There are issues with this approach that limit the usefulness of these tidal coefficients for future predictions. The utility of the computed coefficients to predict future tides declines as time passes. The coefficients occasionally need to be recomputed. One issue is that there's a lot of noise in the historical records; a decent storm will temporarily make the tides behave differently than the record would suggest. This noisiness alone means that the computed tidal coefficients are not perfect. This makes the accuracy of predicted tides decline over time.

Another issue is that neither the Earth's rotation rate nor the Moon's orbital rate is constant; the tidal models assume they are. Even without the noise, a set of tidal coefficients from a hundred years ago would be worthless today because of the small changes in those rates.

Yet another issue is that the oceans themselves change over time because of sedimentation, changes in mean sea level, and changes in the shapes of the oceans due to plate tectonics. An estuary filling with sediment will drastically change the high frequency overtones that result from interactions between the driving frequencies and the coast. The oceans' responses to each of the driving frequencies is a set of amphidromic systems. Each amphidromic system comprises a central point around which water waves rotate at the driving frequency. The nature of these amphidromic systems slowly change over time as the oceans change.


How deep does the frost have to go before it becomes permafrost? Supposing you had an unusually cold winter and a region that does not normally have permafrost. How deep would the frost have to go before it was too deep for it to thaw fully over summer?

Permafrost is permanently frozen ground. However, most definition qualify it as permafrost if it remains frozen for at least two years.

The speed at which a temperature change travels down the ground depends on the thermal conductivity, and it varies between soils. As a thumb rule, the temperature at 10 m depth is pretty much constant and equal to the mean yearly temperature at the surface. This means that 10m is roughly the distance the heat wave travels in a year.

That means that if you manage to freeze the ground below 10m the next summer won't be able to thaw it within a year no matter how hot it is, so it will stay frozen for at least one and a half years. How much longer it will stay frozen will depend on the exact temperatures and lengths of the winter that created the frost layer, the winters after that and the summers in between. Without more details it is impossible to give a number.

Said that, a single cold snap is unlikely to significantly affect the temperature very deep into the ground, you need an extended period of cold weather and many cold winters in a row to do that.

An exceptionally cold winter followed by a mild summer could create a layer of frost that survive two years, but its depth will depend of the actual temperature profile over that period.

Anyway, if that frost layer thaw is not going to be on the next summer, but much later, because the heat will take a long time to travel down there. More on these delay on this answer.