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LIGHTNING PROTECTION (page 2)
WHEN THE BIG CLOUDS GATHER
By David H. Levy
Reprinted with permission from PARADE, copyright 2003
It started
with the sky darkening—fast, as though by a dimmer switch—and sudden gusts
of wind on a recent evening in the Northeast. Instinctively, I raced
into the house for safety as bolt after bolt of lightning flashed in the
night sky, followed by deafening claps of thunder. When the rain poured,
it came not down but across, straight toward the house. A huge bolt
lit up the night like day, and I saw a tree split in two and crash to the
ground less than 100 feet away.
Thunderstorms,
among the most beautiful displays in Nature, are also a danger not to be taken
lightly. Those who ignore the warning signs, or don’t know what to do when
they appear, put themselves in harm’s way from lightning, high winds, hail and
sudden flooding. While they can develop at any time, thunderstorms are most
common between May and September in the U.S. The storm season varies with
the region: In the desert Southwest, storms can develop almost daily from July
to September but are virtually nonexistent the rest of the year. In general,
the higher the temperature, the more often thunderstorms occur.
You usually
will be safe to enjoy a storm’s beauty and power inside a house or an office building. You’re
decidedly more at risk by a lake or on open ground. Do you know what to do
if caught in a thunderstorm? Here is some basic science to explain these
spectacular events and a set of guidelines to help you to protect yourself and
your family.
Nature Rages to Restore Calm
Thunderstorms
are Nature’s way of maintaining stability in the atmosphere. The air
above us is stable or balanced when heavier, cooler air is near the surface
and lighter, warmer air is above it. Instability occurs when the air
at the ground becomes hot and humid while the higher air is cool and dry. The
atmosphere is then “upside down.” Nature will restore stability through
a process called convection: The warm, moist air begins producing
droplets of water that fall as rain in a thunderstorm, bringing cooler, drier
air down with them. The air has become stable. The storm has
done its job.
To understand
how droplets form, it is helpful to look inside a thundercloud. Imagine
a tiny speck of dust wandering about at the bottom of a rapidly growing cumulonimbus (mountain-shaped)
cloud. It is a humid day. The speck, carried along by winds, rises
through the cloud into cooler air. The air is cooling so rapidly that the
moisture in it is condensing. Particles of water adhere to the speck, making
it heavier and heavier until—along with millions of similar droplets—it falls as
rain. The droplets high-speed movement through the cloud builds up an electric
charge, released as lightning.
Simply put,
lightning is an enormous electric spark. When you walk on a carpet, the friction
you create builds up a static-electric charge in your body. If you then touch
an object that has an opposite charge (a wall, a doorknob, a person), a spark jumps
from you to that object, and you feel a shock.
Lightning is
basically the same thing—but on Nature’s grander scale, it can be devastating. As
water droplets race through the cloud, the friction they create builds up a huge
static-electric charge—mainly negative at the bottom of the cloud, positive at
the top. Much of a storm’s lightning remains within the cloud, leaping the
gap between top and bottom. But when an object on the ground, such as a building
or a tree, becomes positively charged, the lightning sparks to the ground as a
brilliant bolt.
When this happens,
enormous heat is generated, so that the air around the bolt virtually explodes—the
sound of thunder. You hear the thunder after you see the lightning
flash, because the speed of sound is much slower than that of light.
You can tell
how close you are to a lightning flash by counting the seconds that pass between
the lightning and the time you first hear thunder. Sound travels at 1100
feet a second. So, if the thunder began 5 seconds after the lightning, the
strike was 5500 feet away (1100 feet x 5 seconds), or just over a mile. If
the interval was 15 seconds, then the lightning bolt was about 3 miles away.