4.5 - Tornadoes

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Although smaller by far than the other atmospheric disturbances we have discussed, the tornado is the most destructive of all atmospheric disturbances.  It is a deep low pressure cell surrounded by a violently spinning cylinder of air.  These are small storms, usually less than 1/4 of a mile in diameter, but they have the most extreme pressure gradients known (as much as 100 mb from the center of the tornado to the air outside of the funnel). This extreme pressure difference produces high wind speeds. While we aren't sure what the maximum wind speeds that a tornado might be capable of, its conjectured that the top speed is about 300 mph. 

 

As with many other meteorological phenomena, the exact mechanism of tornado formation is not completely understood. They often develop in the warm, moist, unstable air associated with an Extratropical Cyclone, such as along a squall line that precedes the fast moving cold front or along the cold front itself.   Virtually all tornadoes are generated by severe thunderstorms. Lower level winds and upper level winds moving in opposite directions cause the air to rotate and updrafts cause the rotating air to lift (see the diagram above).  Once that occurs, a super cell or mesocyclone may forms, with a diameter of 2 to 6 miles. About half of these super cells produce tornadoes.

Spring and early summer are the most favorable for tornado development because there is such a difference between the temperatures of unlike air masses colliding at this time of the year - with cold air masses moving in from central Canada and increasingly warm water producing warm air masses moving in from the Gulf of Mexico.  But tornadoes can form in any month. Most occur in mid afternoon, at the time of maximum solar heating, though again they can and do form at any time of the day.  More than 90% of tornadoes occur in the U.S. where more than 1,000 are sighted each year. Tornadoes are common in the U.S. because warm, moist air masses from the Gulf of Mexico collide with  air masses moving south from central Canada.  At the start of the season, in early spring, tornadoes are most common in what is called "Dixie Alley". Dixie Alley includes most of the lower Mississippi Valley. It stretches from eastern Texas and Arkansas across Louisiana, Mississippi, Tennessee, Alabama, Georgia, far western Kentucky and into parts of the Carolinas.

As the season progresses, tornadic activity moves north, into the more familiar "tornado alley" (Oklahoma, Kansas, South Dakota, Iowa, Nebraska, North Dakota, N. Texas, Minnesota, Wisconsin, Illinois, Indiana, Missouri etc).  Later in summer, tornadoes extend all the way up into the Prairie Provinces of  Canada. Every state in the U.S. has tornadoes, including California. But tornadoes are most common in tornado  and Dixie alleys. Europe, southeastern South America, Australia, New Zealand, Japan and South Africa also experience tornadoes but they are much less common than in the U.S.

Tornadoes are ranked based on the enhanced Fujita Scale:

The vast majority of tornadoes are EF 0 - EF 1. The most devastating tornadoes, EF4 and EF5 are also the least common. They are responsible, however, for the greatest loss of life.