This post is obsolete and wrong in many respects. Please see this more recent post for links to the answer.
14/Jan/05: This post was updated in the light of my further education in radiation physics.
25/Feb/05: Groan…and again.
Recent discussions of climate change (MSU Temperature Record, ACIA) have highlighted the fact that the stratosphere is cooling while the lower atmosphere (troposphere) and surface appear to be warming. The stratosphere lies roughly 12 to 50 km above the surface and is marked by a temperature profile that increases with height. This is due to the absorbtion by ozone of the sun’s UV radiation and is in sharp contrast to the lower atmosphere. There it generally gets colder as you go higher due to the expansion of gases as the pressure decreases. Technically, the stratosphere has a negative ‘lapse rate’ (temperature increases with height), while the lower atmosphere’s lapse rate is positive.
[Read more…] about Why does the stratosphere cool when the troposphere warms?