Geoscience Reference
In-Depth Information
heard as thunder. Have you ever noticed that thunder seems to
rumble? This occurs because sound waves travel from differ-
ent distances within individual lightning bolts, from lightning
strikes, and from sound bouncing off of clouds. If you want to
approximate your distance to a lightning strike, count the num-
ber of seconds between when you see the lightning flash and
hear the thunder. A good rule of thumb is that a lightning strike is
about 1.6 km (1 mi) away for every 5 sec it takes between when
you see the flash and subsequently hear the thunder.
Another indicator that a thunderstorm is strong is hail,
which consists of fragments of ice that plummet from the sky
to Earth. Hail indicates that strong
updrafts
are present at the
rear of the storm, which repeatedly pull ice crystals back into
the upper part of the storm, where they grow larger by adding
layers of ice through condensation. Once these hailstones grow
sufficiently large that they cannot be pulled up anymore, they
fall to Earth by the force of gravity, with large hailstones reach-
ing speeds of 160 km (100 mi) an hour. Usually, hail ranges
in size from
pea-sized
to
golf-ball-sized
. Occasionally, updrafts
are sufficiently strong to produce
softball-sized
hail. The largest
hailstone ever recovered in the United States fell in south-cen-
tral Nebraska during a severe thunderstorm in June 2003. This
hailstone was 17.8 cm (7 in.) wide, which is almost as large as a
soccer ball! A severe hailstorm can be very destructive, causing
millions of dollars in damage to agricultural crops, buildings,
and automobiles (Figure 8.14).
Figure 8.14 Hail damage.
Hail can cause extensive damage to
homes, crops, and automobiles, as this photo clearly demonstrates.
velocity of about 57,910 m/sec (190,000 ft/sec) and can reach
altitudes of about 5 km to 6.4 km (
∙
3 mi to 4 mi). Once the re-
turn stroke reaches the cloud, it drains the cloud of its excess
negative charge and the cycle begins again.
Thunder occurs in association with lightning because light-
ning heats the air in the path of the stroke to above 45,000°F.
This incredible heat creates a shock wave, which disintegrates
within a few meters of the stroke but generates sound waves
Tornadoes
When certain atmospheric conditions are present, very strong
supercell thunderstorms
can develop. These extremely vio-
lent storms are unique because they contain large rotating up-
Tropopause
Anvil
Uplift of
warm air
Downdraft
of cold air,
precipitation
Spiral updraft of
high-speed air
(mesocyclone)
Cloud deck
Tornado
Gust front
Wall cloud
(a)
(b)
Figure 8.15 Supercell thunderstorms.
(a) Typical development of a supercell thunderstorm. (b) Seen from the outside, a supercell thun-
derstorm is an ominous sight. The lower (circular) part of the cloud complex is rotating cyclonically.
Updrafts
An area of rapidly flowing air that is moving up-
ward within a thunderstorm.
Supercell thunderstorms
Large thunderstorms that contain
winds moving in opposing directions and that are associated
with strong winds, lightning, thunder, and sometimes hail and
tornadoes.
