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A tornado (or twister) is a violent storm extending from a thundestorm with very strong circular winds over a small area.
Where they mostly occur?
They mostly occur in the Southeast of Northamerica. Over the central area of the USA streches a stripe which is about 750 kilometres long and about 650 kilometres wide, and which is called “Tornado-Alley”. In this area form approximately 400 tornadoes every year, by far more than in any other part of the world. But they also occur in other locations such as Australia and Europe.
Time of the year when they mostly occur
Mostly in May and July. By increasing latitudes the time postpones to later months. Why the number of tornadoes variates from year to year has not been found out yet.
Warm, humid air rises from the golf of Mexico and moves north-westerly in the direction of the center of the United States.
When these different temperature and pressure systems collide above “Tornado-Alley” the atmosphere is already instable. Thunderstorms develop. Warm rising air (upcraft) from the ground fuels the thunderstrom so that they spread out one or more vortex.
The strongest tornadoes come from fierce thunderstorms called supercell. These are long lasting storms which sperate themselves into two zones: upwind ans downcraft areas. They produce in addition to tornadoes large hail and downburst. Whereas some bring heavy rain others are relatively dry. Not all tornadoes form from supercells but the strongest twisters usually have a supercell as a parent.
Tornadoes spin up beneath thunderstorms and thunderstorms need a good supply of humid air to stay alive themselves. That can easily be disturbed by other storms. They also die without the upcraft. During their life tornadoes run through five stages: forming, organising, mature, shrinking and decaying stage. The last stage is thought to occur when the burst of rain-cooled air on the storm’s back wraps around the storm and cuts off the warm, humid air flowing in.
The Fujita intensitiy scale
There is a scale used by researchers and forecasters which was created by T. THEODORE FUJITA. The F is for Fujita. F-0 and F-1 Tornadoes are considered „weak“ (broken windows and roofs);F-2 and F-3 are „strong“ (collapsing of walls) and F-4 and F-5 are „violent“ (ruined houses which are carried with the tornado).
F-0. Light damage. Wind up to 75 to 115 kmh.
F-1. Moderate damage. Wind 116 to 180 kmh.
F-2. Considerable damage. Wind 181 to 250 kmh.
F-3. Severe damage. Wind 251 to 330 kmh.
F-4. Devasting damage. Wind 331 to 420 kmh.
F-5. Incredible damage. Wind
421 to 510 kmh.
Facts and numbers
Tordanoes remain on the ground for just a few metres or for more than 161 kmh. On average, tornadoes move from the southeast to the northeast at about 50 kmh.They are also relatively small. They can last from a few seconds to several hours.A few monster tornadoes are a mile or more wide and can last for a few hours.
As one F-5 tornado in Texas showed in 1997, it can also move with only 16 km/h of speed. It killed 27 people.
A typical tornado contains 10 000 kilowatt-hours, its energy density is 6 times greater than a hurricane’s. In terms of energy densitiy a tornado is the strongest of nature’s storms.
In the Northern hemisphere they rotate to 99% anticlockwise. But as scientists, chasers and photographers have witnessed there are exceptions. Tornadoes sometimes have one or more extra vortexes which rotate in the opposite direction of the main vortex. The percentage is probably the same for clockwise spinning tornadoes in the Southern hemisphere.
Radio and TV stations send out warnings but one can not always rely on hearing them.
Often used is a weather radio that even sounds an alarm when it is not turned on, when a watch or warning is issued; it is not only for tornadoes but for any kind of dangerous weather. In the USA there are special safety-tip-events where people are shown the latest inventions and informations.
Of cours they cause great destruction along their paths, but this damage is usually confined to a small area.
They cause enviromental problems when they strike a facility that contains potential hazardous or toxic materials (e.g. farm chemicals or radioactive materials in storage). The tornado carries these chemicals for several kilometres before they are finally returned to earth’s surface.
They might also be responsible for the spread of certain kinds of small animals and plants across parts of the USA.
In May 1970 a tornado in Texas moved an empty fertilizer tank weighting 11 793 kilos about 118 kilometres. Investigators could not determine whether it was airboned all the way or whether it rolled and tumbled.
In Texas a post stick through a wall of bricks.
In Louisiana a tornado rammed a steal post which was 7 metres long two and a half metres into the ground.
The tornadoe’s speed is reckoned as the highest on earth. Damages are mainly caused by both speed and extremly low pressures inside the tornado.
Researchers tried to send a rocket into the tornado’s eye. But these tries failed.
In the movie „Twister“ producers copied the „Dorothy“ device from a real-life invention which was originally called „Toto“. Toto had rugged weather sensors sticking up on the outside to measure wind, pressure and temperature. It was carried out on tornadoes in the 1980s but scientists found it very difficult to deploy Toto in the uncalculatable path of the tornado.
Nowadays, forecasters and researchers use the „Doppler Radar“ (or Nexrad). It was discovered by the Austrian scientist CHRISTIAN DOPPLER in 1942. Doppler worked out his ideas by using sound waves, but the same principle applies to radar’s radio waves and to light arriving from stars. Doppler's Radar sends out waves which are reflected by raindrops, hail stones or even insects and make a picture of the weathersituation on a screen.
All weather radars electronically convert the reflected radio waves into pictures showing the location and intensity of precipitation. If the number of returning waves gets slower, the objects moves away from the antenna, if it gets faster it moves nearer. Scientists use these informations to make understand what will happen in the next few hours. By using Doppler's invention researchers have found out that most tornadoes consist of one main vortex and several smaller ones which rotate around the center vortex in the opposite direction.
Of course, also weather satelites are used.
The Storm Prediction Center (SPC) and the Severe Local Storms Unit (SELS) have the responsibility of forecasting tornadoes in the USA.
In 1994 and 1995 scientists tried to find out why some stroms produce tornadoes while similar storms do not. They used two portable “Doppler Radar“ to be able to draw three-dimensional wind-patterns. It was called „Verification of the Origins of Rotation in Tornadoes Experiment“ or VORTEX. The experiment was successful but it will take years to make sense of all the data.
Yet, researchers and forecasters still do not know why
tornadoes occur at all.