Page 274 - NS-2 Textbook

METEOROLOGY 269

around the storm. If a pilot must fly through one, it
should be penetrated at an altitude of about one-third its
40,000 ---====~-7. height.
lS,OOO --------/
TORNADOES
30,000 --;==-----1.
The most intense and violent of localized storms is the
tornado. It is usually associated with violent thunder-
20,000
storm activity and heavy rain. Tornadoes are V\Tind
15,000 whirlpools of such violence that houses in their path will
disintegrate like matchsticks, brick buildings will be de-
10,000---\;'.
stroyed, and trains will be derailed. Feathers and straws
5.000--',.!:......-A:
RAIN oEtiU1SING may be propelled by the swirling wind with the force of
I I AT SURFACE \\
power-driven nails.
C\)IoIVLUS "''''TURE AlNl... OR
STAGE STAGE OtSS1PATIHG STAGE Tornadoes are of very small diameter, usually
300-400 feet; but they may continue on an erratic path for
life cycle of a thunderstorm cell.
more than 100 miles. Winds in the vortex-the whirlwind
causing the ftnmel-often exceed 300 mph. But the speed
not completely understood, but it is generally believed of the storm moving over the Earth's surface is compar-
that lightning is caused by the breaking-up of large water atively slow, usually 25-40 mph. The duration over any
droplets into positively and negatively charged particles. given spot may be only seconds-but in that short time
Positive charges develop near the top of the cloud, and the devastation can be almost total.
negative particles accumulate in the lower reaches. An Tornadoes build up only during severe thunder-
electrical discharge occurs when these charges reach suf- storms. Fortunately, only about one thunderstorm out of
ficient strength to overcome the electrical resistance be- a thousand develops a tornado. A tornado forms as a
tween them. The lightning, then, is nature's way of neu- nmnel cloud on the fOlward edge of a fully developed
tralizing the charges behveen the hvo electrical regions. cumulonimbus cloud. Rising air causes a swirling at the
TIle buildup of electricity in a thunderhead may base of the parent cloud. As the swirl increases in size
reach millions of volts. The lightning may flash within and speed, the funnel drops out of the cloud, like an ele-
the cloud, jump to other clouds, jump from the clouds to phant's trunk dangling toward the surface. When it
the grolUld, or even jump from the ground up to the touches ground the ftnmel is called a tornado; if it forms
cloud. over watel, it is called a waterspout.
Lighhling occurs in two steps. First, a leader of elec- Tornadoes are most common in the temperate zone
trified (ionized) air runs behveen the two oppositely (the midlatitudes between 23.5 degrees and 66.5 de-
charged regions. This establishes the" circuit" for the sec- grees north and south), probably because of the greater
ond stroke, which leaps along the leader to complete the ahllospheric-temperature contrasts there. The midwest-
circuit. nus second stroke is the one you see, and conse- ern United States is the most tornado-ravaged area of the
quently, the one that causes the thunder you hear. The world. Usually these storms hit in the late spring or early
lightning generates terrific heat, causing an explosive ex- summer. But they can occur at almost any time.
pansion of glowing hot air and producing the audible TIle extreme low pressure in the vortex of a tornado
thunder. causes closed homes and barns to explode outward from
Lighhling follows the shortest route between a cloud the normal pressure of air trapped inside. There is a
and the ground. Thus, high points such as trees, tele- 100--200 mph updraft in the center of the funnel, which
phone poles, TV antennas, ship and boat masts, and the can suck up all manner of dust and debris/ houses, cars,
like are the places most likely to be struck by lighhling. animals, and even people.
Lightning also follows the easiest route to ground
after striking, so it will follow electrical wires, plumbing TROPICAL CYCLONES
pipes, sailboat rigging, and even drafts of air in its at-
tempt to reach the ground. It is very lUlwise to be on or A cyclolle is a circular area of low ahllospheric pressure
near bodies of water during a thunderstonn; never be around which winds in excess of 74 mph blow. These
out in an open boat. Mountainous areas also should be winds are counterclockwise in the Northern Hemisphere
avoided during an electrical thunderstorm, especially and clockwise in the Southern Hemisphere. The so-called
crevices or rushing mountain streams. tropical cyclones are subdivided into three categories:
A fundamental rule for airplane pilots is never to fly (1) tropical depression-maxinlum wind less than 34
under or through a thlUlderstorm. It is safest to fly knots, (2) tropical storm-maxinlum wind 34-63 knots,