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Storms
Lesson 15
eteorologists are studying weather patterns daily using
satellite images and computer drawings of the Earth to forecast the weather.
Weather forecasters predict hurricanes, tornadoes, blizzards and floods with
greater accuracy using these tools. More and more people are moving to areas
where these hazards occur on a regular basis. Scientists have created a number
of new weather instruments used to track storms. Their weather predictions alert
pilots to dangerous weather conditions so they can fly around dangerous areas.
Farmers also use daily weather forecasts when planting and harvesting their
crops.
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Satellite picture of Hurricane Elena, was a category
3 hurricane that occurred in 1985.—
Photo courtesy NASA |
Hurricanes
Hurricane season in the Atlantic Ocean lasts from June through
November. During these months, giant storms can strike the coasts bringing
devastation and destruction to large areas. The storms begin as violent
whirlwinds caused by differences in atmospheric pressure. Hurricane-force winds
can cover thousands of square miles. Tropical storm force winds blowing between
34 and 64 knots per hour cover an area ten times larger than the hurricane as it
moves over the oceans. Torrential rains fall along the contours of the storm in
spiral rainbands of dense clouds. Spiral rainbands swirl upward in decks of
cumulus and cumulonimbus clouds up to 40,000 feet. At this height, the wind
pattern reverses and turns in a clockwise motion in the Northern Hemisphere. The
winds accelerate as they enter the central vortex of the storm. The chimney like
structure is the eye of the storm. The wind in the eye of a hurricane may be
less than 15 knots. The strongest winds of the storm are just outside the eye.
Water beneath the storm’s center is pulled upward like water in a straw. The
mound of water can be up to a meter in height. These mounds may translate into
coastal storm surges of six or more meters. Storm surges cause the most death
and destruction in a hurricane. Hundreds of thousands of people died in
Bangladesh in 1970 due to storm tides. In an average year, only six tropical
storms mature into hurricanes and only one or two strike the United States.
Tornadoes
Tornadoes form in thunderstorms when a change wind direction and an
increase in the wind speed create an invisible horizontal spinning effect in the
air in the lower atmosphere. The spinning air begins to tilt from the horizontal
to the vertical as updrafts in the thunderstorm lift the air. The rotating air
can be up to six miles across in the thunderstorm. Most tornadoes form within
this areas of strong rotation inside the clouds. The
spinning air of the tornado extends from the thunderstorm to the ground. At
first, the tornadoes often appear transparent. They lose their transparency as
the swirling winds pick up debris from the ground or a cloud forms inside the
funnel. Tornadoes travel at an average speed of 30 mph. Some tornadoes remain
stationary while others travel up to 70 mph. Tornados often accompany tropical
storms and hurricanes as they move onto the land.
Waterspouts are tornados that form over warm water. They sometimes move on shore
and cause great damage to coastal areas.
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People should immediately seek a safe
place when they
see or hear that a tornado has touched down near them.
Photo courtesy of NOAA |
Meteorologists use Doppler
radar to detect air movement coming toward or away from stations located
across the country. With these devices meteorologists are able to detect
increasing rotation within the clouds of a thunderstorm. Television and radio
stations warn the public by transmitting this information in broadcasts that
warn people a tornado is forming. People driving in automobiles that see a
tornado should seek the best available shelter immediately. If they remain in
their cars, there is the danger of being injured or killed. People inside
buildings should also seek a safe shelter immediately when a tornado is moving
in their direction.
Thunderstorms
Thunderstorms produce tornadoes, lightning, strong winds, flash
flooding and hail. These storms are much smaller than hurricanes and winter
storms but they can still be very dangerous. The average size of a thunderstorm
is about 15 miles in diameter and it will last about 30 minutes. In the United
States each year, we have about 100,000 thunderstorms. Around the world, about
1,800 are occurring at any minute, which translates into over 16 million each
year.
A thunderstorm begins to develop when towering cumulus
clouds form. These clouds indicate that the air is rising. This stage of a
thunderstorm lasts only about 10 minutes and there is little rain although there
may be occasional lightning. During the mature stage of a thunderstorm hail,
heavy rain, frequent lightning, strong winds and tornadoes may occur. The storm
will sometimes appear black or dark green in appearance. The mature stage of a
thunderstorm generally lasts between 10 and 20 minutes although in some storms
they may last much longer. As the storm begins to dissipate the rainfall
decreases in intensity, there are occasional gusts of strong winds and lightning
remains a danger. The conditions that produce a thunderstorm are the following:
sufficient moisture to form clouds and rain, unstable warm air that rises
rapidly, cold or warm fronts, sea breezes, and the sun’s heat causing the air to
lift.
Blizzards
Blizzards are snowstorms that have blowing winds. The winds must blow
at least 35 mph reducing visibility for at least three hours for the snowstorm
becomes a blizzard. Visibility during this time is less than one quarter of a
mile. Snowstorms can immobilize a region, strand commuters, close airports and
disrupt emergency and medical services. The world record for seasonal snowfall
occurred at Mt. Baker, Washington during the snowfall season from 1998-1999. A
record 1,140 inches of snow fell that year. Avalanches are common occurrences
after a large snowfall in the mountains. More than 80% of the avalanches occur
in mid-winter. They are due to a rapid accumulation of snow. Over 90% of the
avalanches will occur within 24 hours of a snowfall. Millions of tons of
material can travel down the slopes of a mountain at speeds up to 200 mph.
People should stay out of wilderness areas when they are skiing or snowboarding
right after a snowstorm if there is any danger of avalanches in the area where
they might be traveling.
Lesson summary
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Meteorologists are
studying weather patterns daily using satellite images and computer drawings
of the Earth to forecast the weather.
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In an average year,
only six tropical storms mature into hurricanes and only one or two strike
the United States.
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Tornadoes form in
thunderstorms when a change wind direction and an increase in the wind speed
create an invisible horizontal spinning effect in the air in the lower
atmosphere.
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Thunderstorms produce tornadoes, lightning, strong winds, flash
flooding and hail.
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Snowstorms can
immobilize a region stranding commuters, closing airports and disrupting
emergency and medical services
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