There are distinct temperature regimes in the atmosphere from the troposphere (up to about 10 km though this can vary) to the exosphere (escape region) above the thermosphere. The temperature (T) decreases with height (z) in the tropopause which promotes convection, so the troposphere is generally considered well mixed. The opposite occurs in the stratosphere where the temperature increases with height (it has a positive lapse rate; lapse rate = Delta T/Delta z). Although some mixing does occur in the stratosphere it is a more stable, and stratified layer. It is also considerably drier than the troposphere because the temperatures in the tropopause are far below 0 degrees C. An important component of the stratosphere is the presence of ozone. Ultraviolet light is absorbed in the upper part of the atmosphere and by ozone and this causes the higher temperature at the top of the stratosphere where there is an ozone concentration maximum (this maximum is generally between 30 to 50 km).
from:
http://www.phas.ucalgary.ca/~annlisen/teaching/CHEM421/Chem421-lecture2a.html#Structure%20of%20the
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