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greenhouse glass allows the short wavelengths of solar radia- This often leads to an increase of air pollution because the inver-
tion to pass into the greenhouse but does not allow all of the sion prevents dispersion of the pollutants.
longer infrared radiation to leave. This analogy is misleading, Temperature decreases with height at the observed lapse rate
however, because carbon dioxide and water vapor molecules do until an average altitude of about 11 km (about 6.7 mi), where
not “trap” infrared radiation, but they are involved in a dynamic it then begins to remain more or less constant with increasing
absorption and downward reemission process that increases altitude. The layer of the atmosphere from the surface up to
the surface temperature. The more carbon dioxide molecules where the temperature stops decreasing with height is called
that are involved in this dynamic process, the more infrared ra- the troposphere. Almost all weather occurs in the troposphere,
diation that will be redirected back to Earth and the more the which is derived from Greek words meaning “turning layer.” The
temperature will increase. More layers of glass on a greenhouse upper boundary of the troposphere is called the tropopause.
will not increase the temperature significantly. The significant The tro popause is identified by the altitude where the temper-
heating factor in a real greenhouse is the blockage of convection ature stops decreasing and remains constant with increasing
by the glass, a process that does not occur from the presence altitude. This altitude varies with latitude and with the season. In
of carbon dioxide and water vapor in the atmosphere (see also general, the tropopause is nearly one and one-half times higher
chapter 23). than the average over the equator and about one-half the average
altitude over the poles. It is also higher in the summer than in the
winter at a given latitude. Whatever its altitude, the tropopause
STRUCTURE OF THE ATMOSPHERE marks the upper boundary of the atmospheric turbulence and
Convection currents and the repeating absorption and reemission the weather that occurs in the troposphere. The average temper-
processes of the greenhouse effect tend to heat the atmo sphere ature at the tropopause is about –60°C (about –80°F).
from the ground up. In addition, the higher-altitude parts of the Above the tropopause is the second layer of the atmosphere
atmosphere lose radiation to space more readily than the lower- called the stratosphere. This layer’s name is derived from the
altitude parts. Thus, the lowest part of the atmosphere is warmer, Greek for “stratified layer.” It is stratified, or layered, because
and the temperature decreases with increasing altitude. On aver- the temperature increases with height. Cooler air below means
age, the temperature decreases about 6.5°C for each kilometer of that consecutive layers of air are denser on the bottom, which
altitude (3.5°F/1,000 ft). This change of temperature with altitude leads to a stable situation rather than the turning turbulence of
is called the observed lapse rate. The observed lapse rate applies the troposphere below. The stratosphere contains little moisture
only to air that is not rising or sinking, and the actual change or dust and lacks convective turbulence, making it a desirable
with altitude can be very different from this average value. For altitude at which to fly. Temperature in the lower stratosphere
example, a stagnant mass of very cold air may settle over an area, increases gradually with increasing altitude to a height of about
producing colder temperatures near the surface than in the air 48 km (about 30 mi), where it reaches a maximum of about 10°C
layers above. Such a layer where the temperature increases with (about 50°F). This altitude marks the upper boundary of the
height is called an inversion (Figure 22.7). Inversions often result stratosphere, the stratopause (Figure 22.8).
in a “cap” of cooler, denser air overlying the warmer air beneath. Above the stratopause, the temperature decreases again,
just as in the stratosphere, then increases with altitude. The ris-
ing temperature is caused by the absorption of solar radiation by
molecular fragments present at this altitude.
Layers above the stratopause are the mesosphere (Greek for
“middle layer”) and the thermosphere (Greek for “warm layer”).
6,000
The name thermosphere and the high-temperature readings of
5,000 Average observed the thermosphere would seem to indicate an environment that
lapse rate is actually not found at this altitude. The gas molecules here do
Altitude (m) 4,000 molecules are far apart. Thus, the average kinetic energy is very
have a high kinetic energy, but the air here is very thin and the
3,000
high, but the few molecules do not transfer much energy to a
thermometer. A thermometer here would show a temperature
2,000 far below zero for this reason, even though the same average ki-
netic energy back at the surface would result in a temperature
1,000 Layer of beyond any temperature ever recorded in the hottest climates.
cold air Inversion
The exosphere (Greek for “outer layer”) is the outermost
Surface
0 5 10 15 20 25 layer where the molecules merge with the diffuse vacuum of
space. Molecules of this layer that have sufficient kinetic energy
Air temperature (°C) are able to escape and move off into space. The thermosphere
and upper mesosphere are sometimes called the ionosphere
FIGURE 22.7 On average, the temperature decreases about
6.5°C/1,000 m, which is known as the observed lapse rate. An because of the free electrons and ions at this altitude. The elec-
inversion is a layer of air in which the temperature increases with trons and ions here are responsible for reflecting radio waves
height. around Earth and for the northern lights.

22-7 CHAPTER 22 The Atmosphere of Earth 547