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SUMMARY

The cloud-forming process begins when something gives a parcel of air changes take place when the front, a thin transition zone between two
an upward push. The three major causes of upward air movement are air masses, passes a location.
(1) convection, (2) barriers to moving air masses, and (3) the meeting of A stationary front often develops a bulge, or wave, that forms
moving air masses. As a parcel of air is pushed upward, it comes under into a moving cold front and a moving warm front. The faster-moving
less atmospheric pressure and expands, becoming cooler. The relation- cold front overtakes the warm front, lifting it into the air to form an
ship between the temperature of the parcel of air cooled by expansion occluded front. The lifting process forms a low-pressure center called a
and the temperature of the surrounding air determines whether a state cyclone. Cyclones are associated with heavy clouds, precipitation, and
of atmospheric stability or atmospheric instability exists. stormy conditions because of the lifting action.
Condensation nuclei act as centers of condensation as water vapor A thunderstorm is a brief, intense storm with rain, lightning and
forms tiny droplets around the microscopic particles. The accumula- thunder, gusty and strong winds, and sometimes hail. A tornado is the
tion of large numbers of tiny droplets is what you see as a cloud. smallest, most violent weather disturbance that occurs on Earth. A hur-
Water that returns to Earth in liquid or solid form falls from ricane is a tropical cyclone, a large, violent circular storm that is born
the clouds as precipitation. Precipitation forms in clouds through two over warm tropical waters near the equator.
processes: (1) the coalescence of cloud droplets or (2) the growth of ice The general pattern of the weather that occurs for a region over
crystals at the expense of water droplets. a number of years is called climate. The three principal climate zones
Weather changes are associated with the movement of large are (1) the tropical climate zone, (2) the polar climate zone, and (3) the
bodies of air called air masses, the leading fronts of air masses when temperate climate zone. The climate in these zones is influenced by four
they move, and local high- and low-pressure patterns that accompany factors that determine the local climate: (1) altitude, (2) mountains,
air masses or fronts. Examples of air masses include (1) continental (3) large bodies of water, and (4) ocean currents. The climate for a given
polar, (2) maritime polar, (3) continental tropical, and (4) maritime location is described by first considering the principal climate zone, then
tropical. looking at subdivisions within each that result from local influences.
When a location is under the influence of an air mass, the loca- Earth’s climate has undergone major changes in the past, caused by
tion is having air mass weather with slow, gradual changes. More rapid changes in the Sun, changes in Earth’s orbit, and changes in the atmosphere.

KEY TERMS APPLYING THE CONCEPTS

air mass (p. 570) 1. Condensation of water vapor into clouds or fog requires
anticyclone (p. 574) a. high temperatures.
arid (p. 583) b. storms.
climate (p. 580) c. condensation nuclei.
climate change (p. 587) d. no wind.
cold front (p. 571) 2. Clouds that are between you and the Sun will appear gray because
continental air mass (p. 570) a. the clouds filter sunlight.
b. the clouds reflect sunlight.
continental climate (p. 583)
c. the clouds refract sunlight.
cyclone (p. 574)
d. the Sun’s shadow makes the clouds dark.
front (p. 570)
3. Which is not an example of precipitation?
high (p. 574)
a. Rain
humid (p. 583)
b. Dew
hurricane (p. 577)
c. Snow
hydrologic cycle (p. 566) d. Ice
marine climate (p. 583)
4. Cloud droplets merge and fuse with millions of other droplets to
maritime air mass (p. 570) form large raindrops. This process is called
microclimate (p. 584) a. seeding.
occluded front (p. 574) b. precipitation.
polar air mass (p. 570) c. coalescence.
precipitation (p. 569) d. combination.
proxy data (p. 587) 5. What type of clouds will usually produce a long, cold winter
semiarid (p. 584) storm with drizzle, rain, ice, and snow?
stationary front (p. 573) a. Cirrus
storm (p. 575) b. Cumulus
c. Stratus
thunderstorm (p. 575)
d. Storm
tornado (p. 576)
tropical air mass (p. 570)
warm front (p. 573)

23-27 CHAPTER 23 Weather and Climate 591