Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: hot-air Baloney

Learn more physics!

Most recent answer: 01/19/2011
Q:
http://www.physlink.com/Education/AskExperts/ae670.cfm
Once again an "expert" site has some very non-expert answers. The site administrator did not repair the answer after being notified. Question: "If heat rises, why is it so cold in the upper atmosphere?"
- Mike W (age 61)
A:
The site gives two answers to the question of why the upper atmosphere is cooler than the lower atmosphere.

 The first answer is a little hard to follow, but includes "The reason it is "cold" in the upper atmosphere is actually because of a lower air density. "
This cannot be the reason, because there's an even higher layer of the atmosphere, much less dense, which is even hotter than the lower atmosphere.

The second answer is basically that, because of gravity, "When the warm air rises, the speed of those air molecules slows down just like a ball that is thrown into the air slows down. "
The second answer is also incorrect. It sounds sensible to say that as molecules rise in a gravitational field, they lose kinetic energy, and thus are colder. However, it also sounds sensible to say that only the ones that start with high energy would be able to go high up. Which effect wins? Basic equilibrium thermodynamics says it's an exact tie- in thermal equilibrium the temperature at every location is exactly the same, and so is the average kinetic energy of the ideal gas particles.

So why is the upper atmosphere colder? The answer is that the atmosphere is not in thermal equilibrium. Energy coming in from the Sun is mostly deposited at the surface of the earth, where sunlight is absorbed. Approximately equal amounts of energy pour out into space as infrared radiation, largely coming from the upper atmosphere.

That should sound familiar. It's like your body on a cold day. Energy (coming from burning fuel inside your body) pours out from the outside surface of a coat. The inside of the coat is, of course, warmer than the outside, just as the lower atmosphere is warmer than the upper atmosphere.

This effect turns out to be important for more than simple curiosity. Our atmospheric "coat" consists of the molecules that emit/absorb that infrared radiation. The more of them that are around, the better the earth's coat traps the heat and the hotter the surface gets. That's the basic physics of the greenhouse effect.

What about that very high layer I mentioned that's quite hot? That's where the UV component gets absorbed. For an extensive discussion, see

Mike W.

(published on 01/19/2011)

Follow-up on this answer.