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Published on: February 11, 2012, by: STW Editor

Severe Weather – The Tornado

The United Kingdom has around 33 tornados per year, which is the second highest amount per land area in the world.

Although still made up of wind, a tornado is a completely different kettle of fish from a hurricane.  A tornado forms over land, it is a violent rotating column of air extending from a thunderstorm to the ground, and in some cases can destroy anything in its path.  Tornados can have windspeeds reaching 300mph, and can often form with very little warning.  Most tornadoes form from thunderstorms. You need warm, moist air  and cool, dry air. When these two air masses meet, they create instability in the atmosphere. A change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.  A funnel cloud is a rotating cone-shaped column of air extending downward from the base of a thunderstorm, but not touching the ground. When it reaches the ground it is called a tornado.

Twisters, as they are often known, are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year. “Tornado Alley,” a region that includes eastern South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.  The meteorological factors that drive tornadoes make them more likely at some times than at others. They occur more often in late afternoon, when thunderstorms are common, and are more prevalent in spring and summer. However, tornadoes can and do form at any time of the day and year.

Tornadoes’ distinctive funnel clouds are actually transparent. They become visible when water droplets pulled from a storm’s moist air condense or when dust and debris are taken up. Funnels typically grow about 660 feet (200 meters) wide.

Tornadoes move at speeds of about 10 to 20 miles (16 to 32 kilometers) per hour, although they’ve been clocked in bursts up to 70 miles (113 kilometers) per hour. Most don’t get very far though. They rarely travel more than about six miles (ten kilometers) in their short lifetimes.  Like hurricanes, tornados are classed by windspeed, as hurricanes have the saffir simpson scale, tornados haveThe Fujita Tornado Scale:

Category FO – Gale Tornado Category 40 – 72 mph
Light damage: some damage to chimneys, breaks branches off trees, pushes over shallow-rooted trees, and damages sign boards.

Category F1 –  Moderate Tornado Category 73 – 112 mph
Moderate damage: The lower limit Category 73 mph– is the beginning of hurricane wind speed, peels surfaces of roofs, mobile homes pushed off foundations or overturned, and moving autos pushed off roads.

Category F2 –  Significant Tornado Category 112 – 157 mph
Considerable damage: Roofs torn off the frames of houses, mobile homes demolished, boxcars pushed over, large trees snapped or uprooted, and heavy cars lifted off ground and thrown

Category F3 –  Severe Tornado Category 158 – 206 mph
Severe damage: Roofs and some walls torn off well-constructed houses, trains overturned, most trees in forest uprooted, and heavy cars lifted off ground and thrown.

Category F4 – Devastating Tornado Category 207 – 260 mph
Devastating damage: Well-constructed houses leveled, structures blown off weak foundations, and cars and other large objects thrown about.

Category F5 – Incredible Tornado Category 261 – 318 mph
Incredible damage: Strong frame houses are lifted off foundations and carried a considerable distance and disintegrated, automobile sized missiles fly through the air in excess of 100 meters, and trees debarked.

Category F6+ –  Inconceivable Tornado Category 319 – 379 mph
The maximum wind speed of tornadoes is not expected to reach the F6 wind speeds.

Tornadoes can be classified into one of three types:

Weak Tornadoes Category F0/F1
These tornadoes account for 74% of all tornadoes. They cause less than 5% of tornado deaths. Their lifetime is usually 1 – 10+ minutes with wind speeds less than 113 mph.

Strong Tornadoes Category F2/F3
These tornadoes account for 25% of all tornadoes. They cause nearly 30% of all tornado deaths and may last 20 minutes or longer. Their wind speeds are clocked between 113 and 206 mph.

Violent Tornadoes Category F4/F5
These rare tornadoes account for less than 2% of all tornadoes. However, they cause 67% of all tornado deaths nationwide. They may last for one hour or more with wind speeds greater than 206 mph.

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