It is estimated that at any given moment nearly 2,000
thunderstorms are active on the earth. All thunderstorms have the
potential for the violence to make them one of nature's great
destroyers and killers.
Thunderstorms begin when relatively warm, humid air is forced
upward. This can occur three different ways...
- Mechanical - A wedge of dense air undercutting warm, moist
air (for example, a cold front).
- Orographic - Moist air that is forced over higher terrain
such as the Olympics or Cascades.
- Thermal - Rising air caused by the sun's heating of the
earth's surface.
As the air rises it expands and cools, and the water vapor it
contains begins to condense into cloud droplets. Continued upward
movement can produce a cumulonimbus or thunderstorm cloud. Once an
individual thunderstorm reaches maturity, downdrafts caused by
falling precipitation begin to destroy it.
Most thunderstorms consist of several cells, each lasting about 20
minutes. New cells may form, replacing older ones, giving
thunderstorms the potential to last several hours or more. Strong
gusts of relatively cool wind occur beneath many mature
thunderstorm cells. Lightning always accompanies thunderstorms.
Hail (ranging from pea size to that of a softball) and strong wind
sometimes occur and cause considerable damage. Flash floods
(produced by heavy rains), tornados and/or funnel clouds may also
occur in thunderstorms.
Lightning strikes the earth about 100 times per second with and
more than 100 Americans are killed each year by lightning strikes.
Annual property losses--fire and other damage to structures,
aircraft damage, livestock deaths and injuries, forest fires and
other effects--are estimated to cost hundreds of millions of
dollars.
Lightning is a huge electrical spark resulting from the generation
and separation of electrical charges in a thunderstorm. The
accumulation of an electrical charge in the lower portion of the
cloud can induce the opposite charge on the earth's surface.
Electrical charges accumulate until the air, a poor conductor of
electricity, can no longer prevent a flow of current. When the
discharge occurs, it is along a lightning channel that typically
connects elevated ground objects (such as buildings and trees) and
the cloud base.
Thunder is the sound produced by an explosive expansion of air
heated by a lightning stroke. When lightning is near, the thunder
sounds like a sharp crack. More distant strokes produce growling
and rumbling noises, a result of the sound being refracted and
modified by the turbulent environment of the thunderstorm.
Because the speed of light is about a million times that of sound,
the lightning bolt is visible before the thunder is heard. This
makes it possible to estimate the distance (in miles) of the
lightning stroke by counting the number of seconds between
lightning and thunder and dividing by five.
When weather conditions in an area are expected to become severe,
the Storm prediction Center (SPC) in Norman, Oklahoma issues a
severe thunderstorm or tornado watch. This watch defines the area
that severe weather may occur during the watch period.
A severe thunderstorm watch means the potential exists for
thunderstorms to reach the severe threshold but the threat of
tornados is limited. A tornado watch means there is a potential
for severe thunderstorms and tornados.
When severe weather is occurring or imminent, a warning is issued
by the local NWS office with warning responsibility for that area.
The warning consists of the type of event, location affected,
duration of warning, and a call-to-action statement. It contains
limited information and is designed for rapid dissemination to the
public. Shortly after the warning is disseminated, a severe
weather statement is usually issued. The severe weather statement
updates the initial warning and provides additional information.
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