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National Geographic Website Notes
Tornadoes
A tornado is a violently rotating column of air that extends from a
thunderstorm to the ground. Tornadoes are some of the Earth’s
most violent events.
Most tornadoes occur in the United States. Some have been
reported in Great Britain, India, Argentina, and a few other
countries, but most tornadoes occur in the United States. Most
tornadoes in the United States occur in Tornado Alley. Tornado
Alley stretches from Texas to Nebraska and also includes
Colorado, Iowa, Indiana, Illinois, Kansas, Missouri, and Arkansas.
Texas holds the record for the most tornadoes.
Waterspouts are weak twisters that form over warm water. They
sometimes move inland and become tornadoes.
Dust devils are small, rapidly rotating columns of air that are made
visible by the dust and dirt they pick up. Dust devils are not
associated with thunderstorms.
The most violent tornadoes come from supercells, large
thunderstorms that have winds already in rotations.
Tornado season begins in early spring for the states along the Gulf
of Mexico. The season follows the jet stream. May generally has
more tornadoes than any other month, but April’s twisters are
usually more violent. Tornadoes can occur at any time of the day
or night, but most tornadoes form in the late afternoon. By this
time the sun has heated the ground and the atmosphere enough to
produce thunderstorms.
Tornadoes form when warm, humid air collides with cold, dry air.
The denser cold air is pushed over the warm air, usually producing
thunderstorms. The warm air rises through the colder air, causing
an updraft. This updraft will begin to rotate if winds very sharply
in speed or direction. The updraft is called a mesocyclone. The
mesocyclone draws in more warm air from the moving
thunderstorm. Its rotation speed increases. Cool air fed by the jet
stream, a strong band of wind in the atmosphere provides even
more energy. Water droplets from the mesocyclone’s moist air
form a funnel cloud. The funnel continues to grow and eventually
it descends from the cloud. When it touches the ground, it
becomes a tornado.
Twisters are usually accompanied or preceded by severe
thunderstorms and high winds. Hail is also common. Winds can
reach up to 300 miles per hour.
The Fujita scale measures a tornado’s intensity by analyzing the
damage the tornado has done and then matching that to the wind
speeds estimated to produce comparable damage. The scale ranges
from F0 to F5.
Meteorologists at the U.S. National Weather Service use Doppler
radar, satellites, weather balloons, and computer modeling to
watch the skies for severe storms and tornadic activity. Doppler
radars record wind speeds and identify areas of rotation within
thunderstorms. Since Doppler radar has been in use, the warning
time for tornadoes has grown from fewer than five minutes in the
l980’s to an average of 13 minutes today.
The National Weather Service issues a tornado watch when
weather conditions are conducive for tornado formation. A
tornado warning is issued when a tornado has been sighted or
indicated on radar.
Conditions must be right for tornadoes to form.
Atmospheric conditions for a twister:
Cold, dry air mass
Falling Pressure
Warm, moist air mass
Variable Winds
National Geographic Website Notes
Tornadoes
A ______________is a violently rotating column of air that
extends from a ____________________to the ground. Tornadoes
are some of the Earth’s ___________________events.
Most ________________occur in the___________________.
Some have been reported in Great Britain, India, Argentina, and a
few other countries, but most tornadoes occur in the United States.
Most tornadoes in the United States occur in_________________.
Tornado Alley stretches from Texas to Nebraska and also includes
Colorado, Iowa, Indiana, Illinois, Kansas, Missouri, and Arkansas.
Texas holds the record for the most tornadoes.
________________________are weak twisters that form over
warm water. They sometimes move inland and become tornadoes.
_______________are small, rapidly ____________columns of air
that are made visible by the dust and dirt they pick up.
_____________________are not associated with thunderstorms.
The most violent tornadoes come from______________, large
thunderstorms that have winds already in rotations.
Tornado season begins in early spring for the states along the Gulf
of Mexico. The season follows the_____________________.
________________generally has more tornadoes than any other
month, but April’s twisters are usually more_____________.
Tornadoes can occur at any time of the day or night, but most
tornadoes form in the late afternoon. By this time the
__________has heated the ground and the atmosphere enough to
produce thunderstorms.
_________________form when warm, humid air collides with
cold, dry air. The denser ____________________is pushed over
the__________________, usually producing thunderstorms. The
warm air rises through the colder air, causing an____________.
This updraft will begin to rotate if winds very sharply in speed or
direction. The updraft is called a________________________.
The mesocyclone draws in more warm air from the moving
thunderstorm. Its rotation ____________increases. Cool air fed
by the jet stream, a strong band of wind in the atmosphere provides
even more_________________. Water droplets from the
mesocyclone’s moist air form a__________________. The funnel
continues to grow and eventually it descends from the cloud.
When it touches the ground, it becomes a_________________.
_____________________are usually accompanied or preceded by
severe ______________________and high winds.
____________is also common. _____________can reach up to
300 miles per hour.
The ____________scale measures a tornado’s intensity by
analyzing the damage the tornado has done and then matching that
to the wind speeds estimated to produce comparable damage. The
scale ranges from_____________________.
__________________________at the U.S. National Weather
Service us Doppler radar, satellites, weather balloons, and
computer modeling to watch the skies for severe storms and
tornadic activity. _____________________record wind speeds
and identifies areas of rotation within thunderstorms. Since
Doppler radar has been in use, the ______________________for
tornadoes has grown from fewer than five minutes in the l980’s to
an average of _______ minutes today.
The ___________________________issues a tornado
_____________when weather conditions are conducive for
tornado formation. A tornado __________________is issued
when a tornado has been sighted or indicated on radar.
Conditions must be right for tornadoes to form.
Atmospheric conditions for a twister:
Cold, dry air mass
Falling _____________
Warm, moist air mass
___________________