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masses of warm, moist air in front of the thunderstorm, lead- also adds to the rumbling sounds. The technique of calculat-
ing to the development of new cells. This stage in the life of a ing the distance to a lightning stroke by measuring the interval
thunderstorm has the most intense rainfall, winds, and possibly between the flash of the lightning and the boom of the thunder
hail. As the downdraft spreads throughout the cloud, the supply is discussed in chapter 5. Lightning can present a risk for people
of new moisture from the updrafts is cut off and the thunder - in the open, near bodies of water, or under a single, isolated tree
storm enters the final, dissipating stage. The entire life cycle, during a thunderstorm. The safest place to be during a thunder-
from cumulus cloud to the final stage, lasts for about an hour as storm is inside a car or a building with a metal frame.
the thunderstorm moves across the surface. During the mature Updrafts are also responsible for hail, a frozen form of pre-
stage of powerful updrafts, the top of the thunderstorm may cipitation that can be very destructive to crops, automobiles, and
reach all the way to the top of the troposphere, forming a cirrus other property. Hailstones can be irregular, somewhat spherical,
cloud that is spread into an anvil shape by the strong winds at or flattened forms of ice that range from the size of a BB to the
this high altitude. size of a softball (Figure 23.17). Most hailstones, however, are
The updrafts, downdrafts, and falling precipitation sepa- less than 2 cm (about 1 in) in diameter. The larger hailstones
rate tremendous amounts of electric charges that accumulate in have alternating layers of clear and opaque, cloudy ice. These
different parts of the thundercloud. Large drops of water tend layers are believed to form as the hailstone goes through cycles
to carry negative charges, and cloud droplets tend to lose them. of falling then being returned to the upper parts of the thunder-
The upper part of the thunderstorm develops an accumula- cloud by updrafts. The clear layers are believed to form as the
tion of positive charges as cloud droplets are uplifted, and the hailstone moves through heavy layers of supercooled water
middle portion develops an accumulation of negative charges droplets, which accumulate quickly on the hailstone but freeze
from larger drops that fall. There are many other charging pro- slowly because of the release of the latent heat of fusion. The
cesses at work, and the lower part of the thundercloud develops cloudy layers are believed to form as the hailstone accumulates
both negative and positive charges. The voltage of these charge snow crystals or moves through a part of the cloud with less
centers builds to the point that the electrical insulating abil- supercooled water droplets. In either case, rapid freezing traps
ity of the air between them is overcome and a giant electrical air bubbles, which result in the opaque, cloudy layer. Thunder-
discharge called lightning occurs (Figure 23.16). Lightning dis- storms with hail are most common during the month of May in
charges occur from the cloud to the ground, from the ground Colorado, Kansas, and Nebraska.
to a cloud, from one part of the cloud to another part, or be-
tween two different clouds. The discharge takes place in a frac- Tornadoes
tion of a second and may actually consist of a number of strokes
A tornado is the smallest, most violent weather disturbance
rather than one big discharge. An extremely high tem perature is
that occurs on Earth (Figure 23.18). Tornadoes occur with
produced around the path of the discharge, which may be only
intense thunderstorms; they resemble long, narrow funnel or
6 cm (about 2 in) or so wide. The air it travels through is heated
ropelike structures that drop down from a thundercloud and
quickly, expanding into a sudden pressure wave that you hear
may or may not touch the ground. This ropelike structure is a
as thunder. A nearby lightning strike produces a single, loud
rapidly whirling column of air, usually 100 to 400 m (about 330
crack. Farther away strikes sound more like a rumbling boom
to 1,300 ft) in diameter. An average tornado will travel 6 to 8 km
as the sounds from the separate strokes become separated over
distance. Echoing of the thunder produced at farther distances
+ +
+ + + + +
+ + + + +
+ + +
+ + + + +
+ – – –
– – – –
–
– – – – – – – – – – –
– – – – – – – –
– – – – – – – – + +
– – – – – – +
– – + – – + +
– – – + + – – – + +
– – + + + – – + +
– + +

+ + + + – – – –

FIGURE 23.16 Different parts of a thunderstorm cloud
develop centers of electric charge. Lightning is a giant electric FIGURE 23.17 These hailstones fell from a thunderstorm in
spark that discharges the accumulated charges. Iowa, damaging automobiles, structures, and crops.

576 CHAPTER 23 Weather and Climate 23-12