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A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud. The word "tornado" comes from the Spanish verb tornar, meaning "to turn."
It is spawned by a supercell thunderstorm and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. Tornados are sometimes associated with lightning and hail. Many tornadoes are the tail end of a mesocyclone and they have a characteristic "hook echo" signature on a radar screen.
The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado winds range from a slow 40 mph at the low end to a possible 300 mph in the strongest storms. Tornado season in North America is typically March through August, although tornadoes can occur at any time of year. They tend to occur mostly in the afternoons and evenings: Over 80 % of all tornadoes strike between 5pm and midnight. They typically travel from a southwest to northeast direction.
April 10 2009 Good Friday Tornado
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- Sixty-nine percent of tornadoes have been rated F1 or F2. Population growth and warning coordination and awareness efforts have dramatically increased the number of documented tornadoes -- especially weak tornadoes -- in recent years, while simultaneously lowering the number of tornado-related fatalities.
- The most common hour of the day for tornadoes to touch down is between 1700 and 1759 CST. Fifty-eight percent of tornadoes touch down during the seven-hour period between 1400 and 2059 CST. Tornadoes are least common during the early daylight hours of 0700-0959 CST.
- Two-thirds (66%) of Middle Tennessee's tornadoes have occurred during the months of March, April, and May. Tornadoes are least likely to occur during September.
- The average path length for all tornadoes is approximately eight miles. However, this figure increases dramatically beyond F2-rated storms. The average path length for F3 tornadoes is 18.7 miles, and increases to 27.3 miles for F4 storms.
- Eighty-six percent of tornado fatalities have been caused by F3+ tornadoes, which in reality constitute just 17% of all tornado occurrences. Fifty-five percent have been caused by the twenty-two documented F4 tornadoes, which represent 5% of all tornado occurrences.
- During the first decade of the 1900's, tornadoes produced 109 fatalities (an average of seven fatalities per event). By comparison, a total of 102 tornado fatalities have occurred since 1950.
- The costliest tornado was the F3 twister which struck downtown Nashville during the afternoon of 16 April 1998. The $100+ million dollar storm greatly overshadowed a much larger supercell which produced the only F5 tornado in Tennessee's history later that day in Lawrence County.
- Nine counties across Middle Tennessee have documented at least 4.7 tornadoes per 100 mi2, and are largely clustered in two geographic locations. The first cluster encompasses five counties that include the Nashville Metropolitan Area and areas east (Davidson, Rutherford, Sumner, Trousdale, and Wilson Counties). The second cluster is formed in southern Middle Tennessee by the three counties of Giles, Lincoln, and Marshall.
- However, when only F3+ tornadoes are considered, there does not appear to be any one part of Middle Tennessee that can be considered most prone to strong tornadic activity.
Intensity of tornadoes: is given by the Fujita Tornado Scale. In 2007 this scale was changed to the Enhanced Fujita EF scale with ratings - 0 through 5. The ratings are based on the amount and type of wind damage.
EF-0. Light damage
Wind 65 to 85 mph. Causes some damage to siding and shingles
EF-1. Moderate damage
Wind 86 to 110 mph. Considerable roof damage. Winds can uproot trees and overturn single-wide mobile homes. Flagpoles bend.
EF-2. Considerable damage
Wind 111 to 135 mph. Most single-wide mobile homes destroyed. Permanent homes can shift off foundation. Flagpoles collapse. Softwood trees debarked.
EF-3. Severe damage
Wind 136 to 165 mph. Hardwood trees debarked. All but small portions of houses destroyed.
EF-4. Devastating damage
Wind 166 to 200 mph. Complete destruction of well-built residences, large sections of school buildings.
EF-5. Incredible damage
Wind above 200 mph. Significant structural deformation of mid- and high-rise buildings.
No two tornadoes look exactly alike. Nor have any two tornadoes behaved exactly the same. There are true incidents of tornadoes repeatedly hitting the same town several years in a row. But forecasting the exact position a tornado will strike at a certain time is nearly impossible. Also, anywhere that convection can occur, is a place where tornadoes can be formed.
Of all tornadoes formed in the US, EF0 and EF1 tornadoes account for a large percentage of occurrences. On the other end of the scale, the massively destructive EF5 tornadoes account for less than 2% of all tornadoes in the US.
Even though no two tornadoes are exactly alike, they always have the same general characteristics that classify them as tornadoes. First, a tornado is a microscale rotating area of wind. A thunderstorm can rotate, but that does not mean it is a tornado. Secondly, the vortex, rotating wind, must come from a convective cloud base. Some of those are thunderstorms embedded in squal lines, supercell thunderstorms, and also not to exclude the outer fringes of landfalling hurricanes. Third, a spinning vortex of air must have a wind speed above a certain rate to be classified by the Fujita scale as a tornado.
Walter Hill, May 2003