A tornado is a violently rotating column of air that extends from a thunderstorm cloud to the earths surface. They are sometimes nicknamed twisters because of their shape and because of what they do. The winds in the tornadoes are usually 100mph or less. In +F4’s they can exceed 250mph. They usually stay on a track of a few miles or less and are less than 100 yds. wide. For the development of tornadoes there are a few conditions required. An abundant low-level of moisture and unstable atmosphere is required not only for the tornado, but for a thunderstorm also. A trigger, which is a cold front or low level zone of converging winds, is needed to lift the moisture and the air. When the air rises , it becomes saturated and continue to rise higher and higher.
They then form in areas where winds at all levels of the atmosphere are strong and turn clockwise with height.Some tornadoes appear as a funnel shape and some have a churning smoky look . Some contain multiple vortices , which are small individual tornadoes rotating around a common center. Some can be invisible , with only swirling dust or debris at ground level as the only indication of the tornados presence. Tornadoes can occur at any time of the year, and anywhere in the world. The unique geography of the US is what helps us produce some of the most violent tornadoes because of the favorable conditions for their development.
The months with the greatest amount of tornadoes are April, May and June . Tornado seasons vary in different parts of the US. In the Southeast the peak season is February through April. In the northern plains it is June through August. Tornado frequency is high in the South during late winter and in the Plains, mid-west, and Ohio Valley it is high during early spring through summer. They can occur at any time of day but are more likely to occur 2-7pm. The most tornadoes occur at around 5pm. In Tornado Alley, very few tornadoes occur in the morning . In the southeast , most of the tornadoes occur during the night and early morning hours. Most tornadoes form in conjunction with thunderstorms.
Forecasters therefore have to determine which areas are most at risk with severe thunderstorms. It is very difficult to tell the difference between a severe thunderstorm with hail and strong winds without a tornado and a severe thunderstorm with light rain and a severe tornado. The computers that forecasters use to determine tornado-severe locations observe and locate areas with strong instability and wind shear. Recent research begins to recover that tornado formation is an extraordinary series of rapidly evolving storm scale processes . Forecasters can determine areas where instability and wind shear coexist , and NEXRAD Doppler Radar is one of the best indicators of impending tornado formation.
Doppler radar displays swirling cloud-level winds that often precede a tornado and can help forecasters pinpoint areas where tornado formation may be occurring or is imminent . This gives meteorologists valuable lead time for the issuance of watches and warnings. The Super OutbreakThe super outbreak is one of the most infamous events that occurred in 1974. It was one of the most violent tornadic events ever on record in the United States .The 1974 Super Outbreak spawned 148 tornadoes, the largest number of tornadoes ever produced by one storm system.
Thirty of the 148 tornadoes from this system were reported to be classified as either an F4 or F5 (on the Fujita-Pearson scale) Before the fourteen state rampage was over 300 people had been killed in 48 killer tornadoes. On the morning of April 3, an area of low pressure was located in central Kansas. A warm front extended east-northeastward through the lower Ohio River Valley. South of this front, extremely unstable air had gathered during the overnight hours and was rapidly spreading north. A cold front stretched from the area of low pressure south through Texas . At the upper levels of the atmosphere, a powerful trough was spreading strong winds aloft over much of the eastern half of the country.
With the warm air in place and a cold front approaching, along with the favorable upper air dynamics , intense thunderstorms developed rapidly in the afternoon of April 3. Those thunderstorms spawned nearly 150 tornadoes across parts of the Midwest , ohio and tennessee river valleys and southern states from the afternoon of the 3rd to the early morning of the 4th . Six F5’s were spawned. One hit Guin, Alabama destroying the entire town and killed 20. The tornado lifted back just before reaching the city limits and entering Huntsville. In Brandenburg, Kentucky, 30 people were killed and the town was left in ruins by an F5 tornado.
And an f5 in Xenia, Ohio +300 homes were destroyed and 2100 were damaged and 34 people were killed.
FUJITA-PEARSON SCALEF0Wind: 40-72mph
Light Damage: Some chimneys damaged, twigs and branches broken off trees, shallow rooted trees pushed over, signboards damaged some windows broken.F1Wind: 73-112mph
Moderate Damage: Surface of roofs peeled off, mobile homes pushed off foundations or overturned out buildings demolished moving autos pushed off the roads, trees snapped or broken beginning of hurricane speed winds.F2Wind: 113-157mph
Considerable Damage: Roofs torn off frame houses, mobile homes demolished, frame houses with weak foundations lifted and moved , large trees snapped or uprooted light object missiles generated.
F3Wind: 158-206mphSevere Damage :Roofs and some walls torn off well constructed houses trains overturned most trees in forecast uprooted heavy cars lifted off the ground and thrown weak pavement blown off the roads.
Devastating Damage: Well constructed houses leveled structures with weak foundations blown off the distance cars thrown and disintegrated trees in forest uprooted and carried some distance awayF5Wind: 261-318mphIncredible Damage : Strong frame houses lifted off foundations and carried considerable distance to disintegrate automobile sized missiles fly through the air in excess of 300 feet trees debarked incredible phenomena will occur.