Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 25/03/2007 06:58:17

Title: heat rises, but.........
Post by: paul.fr on 25/03/2007 06:58:17

If heat rises, why is it that the higher in to the atmosphere we go, the colder it gets?

for example; at the foot of a mountain, it's warmer than at the top.

Title: heat rises, but.........
Post by: Karen W. on 25/03/2007 07:48:54

Maybe due to the lack of oxygen higher up in the atmosphere!That is only a guess as I have not a clue!

Title: heat rises, but.........
Post by: chris on 25/03/2007 15:35:18

We answered this recently:

"Why is it colder at altitude...?"

http://www.thenakedscientists.com/forum/index.php?topic=6239.0

Title: heat rises, but.........
Post by: paul.fr on 25/03/2007 16:35:36

Thanks, Chris.

Topic noted and read.  [:)]

Title: heat rises, but.........
Post by: lyner on 25/03/2007 20:40:48

The above link gives a good explanation why the atmosphere, as a whole, gets colder as you go up; it's not the sunlight that warms it up, it's the infra red radiation from the warmed-up ground and the upper atmosphere is screened by the more dense lower regions.
But it is not strictly true to say the "Heat rises".
What is true is that a volume of air will rise if is hotter than the surrounding air ; it is less dense, so it floats  upward (upthrust).
However, as the air rises,  it will expand because the pressure on the atmosphere gets less as you go up and it also gains Gravitational Potential energy. This energy must come from somewhere and, as the air expands, its temperature drops and reduces its lifting power.  The air will stop rising when its temperature has reduced to that of the surrounding air.
Cumulus clouds are formed by air, which has been warmed over the land, rising up and the water vapour is condensed as the temperature drops. They rise and rise until they run out of energy. You may notice that some clouds have decidedly flat tops because of the maximum height imposed by this effect.
There isn't enough energy in this mechanism to transfer heat to the upper atmosphere. I suspect that there may be a small increase in temperature due to this effect but it would run out of steam (no pun intended) above the clouds.

Title: heat rises, but.........
Post by: BillJx on 26/03/2007 01:31:15

Quote from: paul.fr on 25/03/2007 06:58:17

If heat rises, why is it that the higher in to the atmosphere we go, the colder it gets?

for example; at the foot of a mountain, it's warmer than at the top.

A very sensible question.  The ancients assumed that, since "fire" rises, there must be a region of fire above the air.  Which serves as a reminder that we can't just figure stuff out.  Science is much more than just observation and logic.