Why does the atmosphere get colder as we climb to higher
elevations? The atmosphere is warmer near Earth’s surface
because the air is in contact with the sunlight-heated ground,
which warms the air by its infrared radiation. The atmosphere
is cooler at very high altitudes because there the atmosphere
freely radiates its thermal energy into space. In fact, it would
get colder with increasing altitude even faster if not for convection.
Figure 9.8 (on page 270) illustrates how convection
carries thermal energy upward through Earth’s atmosphere.
At a given pressure, cold air is denser than warm air. So when
cold air encounters warm
air, the denser cold air slips
under the less dense warm
air, pushing the warm air
upward. This convection
sets up air circulation between the lower and upper levels of
the atmosphere, and tends to diminish the extremes caused
by heating at the bottom and cooling at the top.