Introduction to Tornadoes

In the Northen Hemisphere (north of the equator), most tornadoes rotate in a counterclockwise direction.

In the Southern Hemisphere (south of the equator), most tornadoes rotate in a clockwise direction.

Tornadoes form in thunderstorms, when unstable hot air near the ground rises and meets cooler air above in thunder clouds. Tornadoes can form at any time of the year, but most form from March to August (in the Northern Hemisphere).

Tornado Watch and Warning
Meteorologists (scientists who study the weather) can sometimes warn people when conditions are right for tornadoes in a particular area, but the ability to predict tornadoes is very limited. People usually only have a few minutes warning (if that much). When you hear a tornado warning, find shelter IMMEDIATELY!

The following are tornado warning terms:
Tornado Watch -- This means that tornadoes may form in the area, because conditions are favorable for their formation. People should stay tuned to radio or TV weather and listen for local warning sirens (but these are only available in some areas).
Tornado Warning -- A tornado has formed in the area! Find shelter immediately! A basement is best, but if that isn't available, go to an interior room or closet in a house, and stay far away from windows. If there aren't any buildings around, lie face-down on the ground in a low lying area (like a ditch) and cover your head with your hands for protection.

How Long Does a Tornado Last?
Tornadoes usually last just a few minutes. The shortest last for only a few seconds, but the longest (and most dangerous) can last for over an hour (but these are extremely rare)!

Wind Speed in a Tornado
The average wind speeds in most tornadoes are about 112 mph (180 km/h) or less. Winds in the most extreme tornadoes can be over 300 mph (500 km/h).

Tornadoes are ranked using a scale called the Fujita Scale, five categories of wind speed that are estimated the damage left behond (these are not wind speed measurements, since most wind-speed measuring devices are destroyed during tornadoes, and since the tornadoes die out so quickly, Doppler radar wind measurements are not usually done).

F-5 tornadoes are the most dangerous type (and the rarest type).

• F-0 40-72 mph, Light damage, chimney damage, tree branches broken
• F-1 73-112 mph, Moderate damage, mobile homes pushed off foundation or flipped over
• F-2 113-157 mph, Considerable damage, mobile homes demolished, trees uprooted
• F-3 158-205 mph, Severe damage, roofs and walls torn down, trains overturned, cars thrown around
• F-4 207-260 mph, Devastating damage, well-constructed walls leveled
• F-5 261-318 mph, Violent damage, homes lifted off foundation and carried considerable distances, autos thrown as far as 100 meters.

Tornado Alley extends from central Texas northward to Illinois and Indiana. The heart of Tornado Alley includes parts of Texas, Oklahoma, Kansas, Nebraska, eastern Colorado, and South Dakota. Less intense areas of Tornado Alley incude parts of Arkansas, Louisiana, Mississippi, Iowa, Tennessee, Kentucky, Wisconsin, and Minnesota.

The Most Deadly US Tornado:
The Tri-state Tornado of March 18, 1925 was the deadliest tornado in U.S. history (according to the National Weather Service Storm Prediction Center). 689 people were killed by this tornado (it may have been a series of tornadoes, and not a single giant tornado).

Web Links:
FEMA for kids, http://www.fema.gov/kids/tornado.htm.
NOAA current US tornado activity map - http://www.spc.noaa.gov/index.html.
American Red Cross, Materials for Children: http://www.redcross.org/pubs/dspubs/childmatls.html.

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