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SUMMARY

Earth’s atmosphere thins rapidly with increasing altitude. Pure, dry air The surface of Earth is not heated uniformly by sunlight. This
is mostly nitrogen, oxygen, and argon, with traces of carbon dioxide and results in a differential heating, which sets the stage for convection. The
other gases. Atmospheric air also contains a variable amount of water horizontal movement of air on the surface from convection is called
vapor. Water vapor cycles into and out of the atmosphere through the wind. A generalized model for understanding why the wind blows involves
hydrologic cycle of evaporation and precipitation. (1) the relationship between air temperature and air density and (2) the
Atmospheric pressure is measured with a mercury barometer. At relationship between air pressure and the movement of air. This model
sea level, the atmospheric pressure will support a column of mercury explains local wind patterns and wind patterns observed on a global scale.
about 76.00 cm (about 29.92 in) tall. This is the average pressure at sea The amount of water vapor in the air at a particular time is called
level, and it is called the standard atmospheric pressure, normal pressure, the absolute humidity. The relative humidity is a ratio between the
or one atmosphere of pressure. amount of water vapor that is in the air and the amount needed to
Sunlight is absorbed by the materials on Earth’s surface, which saturate the air at the present temperature.
then emit infrared radiation. Infrared is absorbed by carbon dioxide When the air is saturated, condensation can take place. The tem-
and water molecules in the atmosphere, which then reemit the energy perature at which this occurs is called the dew point temperature. If the
many times before it reaches outer space again. The overall effect dew point temperature is above freezing, dew will form. If the tempera-
warms the lower atmosphere from the bottom up in a process called ture is below freezing, frost will form. Both dew and frost form directly
the greenhouse effect. on objects and do not fall from the air.
The layer of the atmosphere from the surface up to where the Water vapor condenses in the air on condensation nuclei. If this
temperature stops decreasing with height is called the troposphere. happens near the ground, the accumulation of tiny water droplets is
The stratosphere is the layer above the troposphere. Temperatures in called a fog. Clouds are accumulations of tiny water droplets in the air
the stratosphere increase because of the interaction between ozone above the ground. In general, there are three basic shapes of clouds:
(O 3 ) and ultraviolet radiation from the Sun. Other layers of the atmo- cirrus, cumulus, and stratus. These basic cloud shapes have meaning about
sphere are the mesosphere, thermosphere, exosphere, and ionosphere. the atmo spheric conditions and about the coming weather conditions.

KEY TERMS APPLYING THE CONCEPTS

absolute humidity (p. 554) 1. The science that studies the atmosphere and weather
barometer (p. 544) phenomena is
Chinook (p. 552) a. astronomy.
condensation nuclei (p. 556) b. astrology.
dew (p. 556) c. meteorology.
dew point (p. 554) d. space science.
exosphere (p. 547) 2. Up from the surface, 99 percent of the mass of Earth’s
frost (p. 556) atmosphere is found within
greenhouse effect (p. 546) a. 12 km (7 mi).
hair hygrometer (p. 555) b. 16 km (10 mi).
humidity (p. 554) c. 24 km (15 mi).
hydrologic cycle (p. 544) d. 32 km (20 mi).
intertropical convergence zone (p. 552) 3. The most abundant gas in the atmosphere is
inversion (p. 547) a. oxygen.
ionosphere (p. 547) b. nitrogen.
jet stream (p. 552) c. argon.
mesosphere (p. 547) d. carbon dioxide.
meteorology (p. 542) 4. Approximately how much of the total volume of the atmosphere
observed lapse rate (p. 547) is oxygen?
psychrometer (p. 555) a. 21 percent
relative humidity (p. 554) b. 32 percent
saturation (p. 553) c. 78 percent
sea breeze (p. 550) d. 93 percent
standard atmospheric pressure (p. 545) 5. Fog and clouds are composed of
stratopause (p. 547) a. water vapor.
stratosphere (p. 547) b. tiny droplets of liquid water.
thermosphere (p. 547) c. pockets of rain or snow.
tropopause (p. 547) d. large crystals of water vapor.
troposphere (p. 547)
wind (p. 548)

560 CHAPTER 22 The Atmosphere of Earth 22-20