Friday, November 4, 2016

Is Our Atmosphere Dynamic?


It may surprise readers to learn that the American Geophysical Union has divisions named Atmospheric and Space Electricity, Global Environmental Change, and Atmospheric Sciences. In short, the geosciences world include a very large element of atmospheric science.  The following query has several different elements, all of which suggest an awareness of how dynamic our atmosphere really is.

Q: Hello. I wasn't sure which category to inquire within but this seemed appropriate. Since Earth is not a perfectly spherical object, nor any other planetary bodies we know of, how does that affect the gaseous layers of atmosphere surrounding us? My question stems from an uneducated assumption that our atmosphere is not a perfect bubble around us but must be dynamic given the amount of energy factors associated with it, the terrain beneath it and which locations have the greatest gravitational pull. Can the sphere of air around us ever dissipate into space? Are there higher or lower points that exist because of geography that make our categories of layers more ambiguous?
-Joe A

A: As you suggest, the atmosphere is indeed a very dynamic thing, and yes all rotating solar system bodies are oblate spheroids because of centrifugal force at the equators (and none at the poles). Jupiter rotates at a phenomenal rate (it has 9.8 hour days!) and is thus is the most oblate planet of all.

If you think about oceans, however (the ocean surface is at the same elevation above the spheroid datum over the Marianas Trench as it is in Pamlico Sound), then mountain ranges will similarly have little to do with atmospheric height over the globe (there IS a small amount of isostacy - high mountains with deep roots that locally distort the acceleration of gravity). The most common exception to this are called storm surges - the low-pressure cores of hurricanes and typhoons will literally lift up the (warmed and expanded) ocean water. With Hurricane Katrina, the storm surge reached an astonishing 8.5 meters (27.8 feet!) at Pass Christian, Mississippi. That's 'WAY above the normal high tide!

There is atmospheric thinning with altitude, however, and the upper reaches can still be detected at 100+ kilometers, which is why satellites must fly at 250+ kilometers. Even at those altitudes there is measurable drag that over time will bring down low-flying satellites and launch vehicle debris. Most of the upper atmospheric variation has to do with solar wind and solar heating activity, however. Because of Earth’s gravity (and to a surprising degree, Earth's magnetic field), most of our original atmosphere remains - unlike Mars, where the original atmosphere and water were stripped over time by solar winds that weren't diverted by a magnetic field. 

When you see clouds over mountain tops (pretty common over our volcanoes in the Pacific Northwest), it is because winds trying to get around the mountain send some of their components up and OVER the mountain. This leads to a drop in temperature with increasing altitude, which contributes to dissolved moisture precipitating out into what we call orographic clouds - cloud caps. As the air moves past and back down to lower elevations the water re-dissolves back into the atmosphere and the trailing edges of the clouds disappear... but the same AMOUNT of water remains in the air.

Q: I had to do a little research to understand a few of the terms you used but I definitely feel like I came away with a better understanding. Thank you for your insightful response. I shouldn't be surprised I guess that avenues of inquiry like this are out there given the ubiquity of websites, but I never tried something like this before. I had a thought, did some googling and found you. It's awesome to get answers from professionals as if I was back in school and could pick the brains of my professors after hours. So thanks again, despite my questions being kind of convoluted!
- Joe

A: I'm glad I could help. I suppose I am technically a professional, in that I get paid to do research in geophysics, but I'm just a very ordinary person with the same level of curiosity that you have. I personally don't divide the world into professional vs. non-professional, but instead into interested vs. non-interested. I plumber who asked some really deep questions about the lithosphere and upper mantle told me he spends a lot of "windshield time" thinking about the physical world as he drives from job to job. 

THAT meets my definition of a scientist. You and I fit in there also. That goes for anyone reading this chapter, too.



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