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Why is it colder at higher elevations?


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sort of, there's nothing much to absorb thermal radiation but plenty of places to radiate it to(space pretty much). the heat in the upper atmosphere mainly comes from convection mixing with the lower atmosphere although some of it will come from direct absorbtion from the sun and earth.

 

its not really anything to do with solid mass though, its to do with how much of the radiation passing through it the mass absorbs, the mass can be solid, liquid, gas, plasma or any of the more exotic states of matter that have been observed.

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What is the reason that it's colder at higher elevations? Is it because there is less solid mass to heat up at higher elevations?

 

You should google lapse rate. It's really neat. There are several reasons that work together to determine the rate at which the temp falls off with altitude, some interesting physics. Elegant even, in a nice classic way.

 

BTW I noticed you asked some good classic physics questions in the past few weeks which is one of the most helpful contributions to this kind of site, so thanks.

And one question was why reducing pressure on a gas can chill it down.

So you are either prepared by your own curiosity to understand the lapse rate. Or you have already been reading up on atmospheric temp structure and that prompted the question. Or you already know about the lapse rate, which either way is fine!

 

Anyway here's a thumbnail sketch:

 

1. Transparency

Except for greenhouse-type molecules, most air molecules don't resonate to visible and infrared. They dont stop it, or scatter it, or interact with it very much. BTW a lot of sunlight energy is in the infrared, its not all visible. A detail to keep in mind.

 

2. Conduction

If the only vertical heat transport were conduction, the drop-off would be extreme. All the warm air would be near the ground. Sunlight would pass down thru the transparent air and be absorbed and warm the ground, and the ground would warm the air in contact with it. And that layer would warm the next higher layer by conduction. The extreme gradient of a good insulator.

 

3. Convection and the Lapse Rate

So you would expect convection to start! All the warm air would want to go up. Leading to an inversion of what we expect----a paradoxical situation with all the cold air down low and the warm air up high.

 

But in fact this does not happen because of a clever thing called the lapse rate which depends on gravity. With gravity and the pressure gradient, convection has a threshold. Convection will not start merely because lower air is warmer and wants to rise. Convection will not start until the lower air is enough warmer.

 

In a gas in a gravity field there is a temp gradient (a drop-off rate) which is required for convection to start. We can calculate what it is. It is less for moist air---it is easier for moist air to start convection because it has an energyreserve in the form of water vapor. So consider the dry air case.

 

4. The Reason for the Lapse Rate Threshold

The reason is that question you asked in some other thread about air getting colder as pressure is reduced. Consider a bag of air, or a cube of air, near the ground. It is warm so it wants to rise up into the colder air above. But the stupid air finds that when it has risen up it is so much colder that it wants to sink back down again! Convection is getting nowhere!

 

Convection will only work if the bag of air near the ground is enough warmer that when it has risen up into some higher colder air it is STILL a little bit warmer than this higher colder, even though it has cooled off some because of reduced pressure.

 

5. The Beauty of Physics

The beauty of basic classic physics is that you can calculate what this threshold for convection lapse rate---this critical temp gradient---is, just using basic information like the Ancient and Honorable Gas Law, and perhaps a knowledge of how much a typical air molecule weighs. And the lapse rate actually does tell you roughly how fast temperature falls off as you go up a mountain, or up in an airplane. Because if the temp gradient ever gets above the lapse rate, then convection kicks in and begins to mix the air and share the energy which starts to bring the temp gradient back down to the lapse rate.

Edited by Martin
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That's interesting about the lapse rate... I always thought it was a simple case of pressure being lower - so T is lower. That and the fact that the earth has some warming effect on the air nearer the ground. :embarass:

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Because if the temp gradient ever gets above the lapse rate, then convection kicks in and begins to mix the air and share the energy which starts to bring the temp gradient back down to the lapse rate.

 

This part I didn't understand completely. What do you mean when the temperature gradient gets above the lapse rate then covenction kicks in? What do you mean by 'above'?

 

Also what is the different between the lapse rate and the temperature gradient?

Edited by Uri
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... What do you mean by 'above'?

 

Also what is the different between the lapse rate and the temperature gradient?

 

By "above" I meant steeper.

Maybe someone else should explain the terminology, if I'm not being clear enough.

 

Did you google "lapse rate"? Or did you look up lapse rate in Wikipedia?

==========================

 

I'll try again. Lapse is an antique word for falling. Lapse rate is technical jargon for a certain ideal rate that temperature falls off with altitude. To be more correrct I should say "adiabatic" lapse rate.

 

In other words, the lapse rate (say for dry air) is a certain specific temperature gradient. It is a theoretical gradient that you can calculate, knowing nothing but the avg. weight of an air molecule, the earth's gravity, some laws.

 

If the real temperature gradient on a certain day in a certain place happens to be steeper than the lapse rate, then convection will start.

 

This will tend to mix the air and make the real gradient less steep, so convection will tend to bring the gradient more into line with the theoretical gradient you calculate----more in line with the ideal lapse rate.

 

The lapse rate on Mars would be different from the lapse rate on Earth, because Mars atmosphere has a different avg. molecule weight and because Mars has a different gravity. But you could calculate it. So you could predict the temperature structure of Mars atmosphere. And predict under what conditions convection will start, and cause winds. And how rapidly it would get colder as you climbed up a mountain.

 

Please ask more questions if you don't understand---but I also would suggest checking Wikipedia, it should have something, or if not then google.

======================

 

I checked Wikipedia myself! The real actual temp gradient they call the "environmental lapse rate". It varies by time and place.

The theoretical gradient that you calculate they call the "dry adiabatic lapse rate" (DALR)

And there is also the "moist adiabatic lapse rate" (MALR). And so on.

I had forgotten some of the weatherman's beautiful technese jargon.

 

But the basic thing is that for dry air the theoretical ideal lapse rate, here on earth, is about 10 celsius per kilometer. That's the DALR.

 

So warm air does indeed stay closer to the ground, but if it is TOO warm downstairs and too cold upstairs then the air feels that the gradient or drop-off is too steep and the old "warm air rises" thing takes over. Which mixes and makes the too-steep gradient more gradual.

 

This only applies up to a certain altitude called the "limit of convection" or "tropopause" IIRC where the stratosphere begins. The temp structure is different above the highest clouds.

 

I think atmosphere structure is a beautiful subject (tho I've forgotten most of what I ever learned about it). How did you get interested? By climbing mountains?

Edited by Martin
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