rock, soil, and water, which radiate it back into the atmosphere as heat, further warming the
troposphere. The temperature of the troposphere is highest near the surface of the Earth
and declines with altitude. On average, the temperature gradient of the troposphere is 6.5oC
per 1,000 m (3.6oF per 1,000 feet) of altitude.
Notice that in the troposphere, warm air is beneath cold air. Since warm air is less dense
and tries to rise, this condition is unstable. So the warm air at the base of the troposphere
rises and cool air higher in the troposphere sinks. For this reason, air in the troposphere
does a lot of mixing. This mixing causes the temperature gradient to vary with time and
place. For reasons that will be discussed in the next section, rising air cannot rise above the
top of the troposphere. The rising and sinking of air in the troposphere means that all of
the planet’s weather takes place in the troposphere.
When there is a temperatureinversion, air temperature in the troposphereincreaseswith
altitude and warm air sits over cold air. This is called an inversion because the usual
situation is reversed or inverted. Inversions are very stable and they often last for several
days or even weeks. Inversions commonly form over land at night or in winter. At these
times, the ground is cold because there is little solar energy reaching it. At night, the Sun
isn’t out and in winter, the Sun is at a low angle, so little solar radiation reaches the ground.
This cold ground cools the air that sits above it, making this low layer of air denser than
the air above it. An inversion also forms on the coast where cold seawater cools the air
above it. When that denser air moves inland, it slides beneath the warmer air over the land.
Since temperature inversions are stable, they often trap pollutants and produce unhealthy
air conditions in cities (Figure15.5).
Figure 15.5: Smoke makes a temperature inversion visible. The smoke is trapped in cold
dense air that lies beneath a cap of warmer air. ( 22 )
At the top of the troposphere is a thin layer called thetropopause. Temperature in the
tropopause does not change with height. This means that the cooler, denser air of the tropo-