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for increasing pressure, which means a high and its associated or over flatlands from differential heating can move miles after
fair weather are coming. Consulting a daily weather map makes they form, sometimes appearing to wander aimlessly across the
such projections a much easier job, however. land. These storms are not just one big rain cloud but are some-
times made up of cells that are born, grow to maturity, then die
out in less than an hour. The thunderstorm, however, may last
MAJOR STORMS
longer than an hour because new cells are formed as old ones
A wide range of weather changes can take place as a front passes
die out. Each cell is about 2 to 8 km (about 1 to 5 mi) in diam-
because there is a wide range of possible temperature, moisture,
eter and goes through three main stages in its life: (1) cumulus,
stability, and other conditions between the new air mass and the
(2) mature, and (3) final (Figure 23.15).
air mass that it is displacing. The changes that accompany some
Damage from a thunderstorm is usually caused by the
fronts may be so mild that they go unnoticed. Others are no-
associated lightning, strong winds, or hail. As illustrated in
ticed only as a day with breezes or gusty winds. Still other fronts
Figure 23.15, the first stage of a thunderstorm begins as con-
are accompanied by a rapid and violent weather change called
vection, mountains, or a dense air mass slightly lifts a mass of
a storm. A snowstorm, for example, is a rapid weather change
warm, moist air in an unstable atmosphere. The lifted air mass
that may happen as a cyclonic storm moves over a location. The
expands and cools to the dew point temperature, and a cumu-
most rapid and violent changes occur with three kinds of major
lus cloud forms. The latent heat of vaporization released by the
storms: (1) thunderstorms, (2) tornadoes, and (3) hurricanes.
condensation process accelerates the upward air motion, called
an updraft, and the cumulus cloud continues to grow to tower-
ing heights. Soon the upward-moving, saturated air reaches the
Thunderstorms
freezing level, and ice crystals and snowflakes begin to form.
A thunderstorm is a brief but intense storm with rain, light-
When they become too large to be supported by the updraft,
ning and thunder, gusty and often strong winds, and sometimes
they begin to fall toward the surface, melting into raindrops in
hail. Thunderstorms usually develop in warm, very moist, and
the warmer air they fall through. When they reach the surface,
unstable air. These conditions set the stage for a thunderstorm
this marks the beginning of the mature stage. As the raindrops
to develop when something lifts a parcel of air, starting it mov-
fall through the air, friction between the falling drops and the
ing upward. This is usually accomplished by the same three
cool air produces a downdraft in the region of the precipita-
general causes that produce cumulus clouds: (1) differential
tion. The cool air accelerates toward the surface at speeds up to
heating, (2) mountain barriers, or (3) an occluded or cold front.
90 km/h (about 55 mi/h), spreading out on the ground when it
Thunderstorms that occur from differential heating usually
reaches the surface. In regions where dust is raised by the winds,
occur during warm, humid afternoons after the Sun has had
this spreading mass of cold air from the thunderstorm has the
time to establish convective thermals. In the Northern Hemi-
appearance of a small cold front with a steep, bulging leading
sphere, most of these convective thunderstorms occur during
edge. This miniature cold front may play a role in lifting other
the month of July. Frontal thunderstorms, on the other hand,
can occur any month and any time of the day or night that a
front moves through warm, moist, and unstable air. 15
Frontal thunderstorms generally move with the front that
produced them. Thunderstorms that developed in mountains
10
Altitude (km)
5

0
A B C
FIGURE 23.15 Three stages in the life of a thunderstorm cell.
High Low (A) The cumulus stage begins as warm, moist air is lifted in an
unstable atmosphere. All the air movement is upward in this stage.
(B) The mature stage begins when precipitation reaches the ground.
FIGURE 23.14 Air sinks over a high-pressure center and This stage has updrafts and downdrafts side by side, which create
moves away from the center on the surface, veering to the right violent turbulence. (C) The final stage begins when all the updrafts
in the Northern Hemisphere to create a clockwise circulation have been cut off and only downdrafts exist. This cuts off the supply
pattern. Air moves toward a low-pressure center on the surface, of moisture, and the rain decreases as the thunderstorm dissipates.
rising over the center. The anvil-shaped top is a characteristic sign of this stage.

23-11 CHAPTER 23 Weather and Climate 575