A tornado is a funnel shaped column of air that rotates violently, extends from a thunderstorm, and touches the ground. The word tornado comes from two Spanish words tornar, which means to turn, and tronada, which means thunderstorm. A tornado can be between a few meters, and a kilometer wide where it touches the ground. Usually they're a few hundred yards wide. Like a hurricane, the winds spin different ways in different hemispheres. In the southern hemisphere, the winds rotate clockwise, and in the northern hemisphere, the winds rotate counter-clockwise. In a few rare cases, the winds do the opposite, rotating counter clockwise in the southern hemisphere, and clockwise in the northern.
Although the air in a tornado is rising, a tornado grows towards the ground. They form from a thunderstorm (occasionally from a hurricane) when cool air prevails over a layer of warm air and forces the warm air to rise rapidly. Tornadoes develop inside a low pressure area that has high winds. The center of the vortex (something that resembles a whirlpool) of the tornado is a very low pressure area. As more air comes in, the pressure lowers, which cools the air down. The air cooling helps condense the water vapor that is in the air, and form the tornado into the funnel shaped column tornadoes are associated with. The as the rotating winds pick up dirt, dust, and debris from the ground, the funnel becomes a darker color. If a tornado picks up red dirt, the funnel turns red.
The funnel speed is usually estimated at a little more than three hundred miles per hour. Scientists have estimated that a few extremely strong storms have had speeds at up to five hundred miles per hour. In the United States, tornadoes usually occur in the spring. A sudden burst of heavy rain or hail can sometimes tell a place that a tornado is coming because often, tornadoes are located on the edge of an updraft, next to the air that is coming down from the thunderstorm, with rain or hail.The damage inflicted by a hurricane is usually results from the high velocity of the wind, and the debris that is being blown around.
In the United States, the peak tornado season in the southern states is from March to May, and in the northern states, the season is during the summer. Most tornadoes happen in the afternoons and evenings; eighty percent of them occur between noon and midnight. In an average year, one thousand tornadoes form, eighty people die, and one thousand five hundred people are injured in the United States alone. A tornado that occurs over water is called a waterspout.
tornadoes are sorted into three categories, weak, strong, and violent. A weak tornado has winds that are no stronger than one hundred ten miles per hour, and has a thin rope-like appearance. About seven tornadoes out of ten are classified as weak. A strong tornado has the "classic" funnel shaped cloud, the wind speed is between one hundred ten and one hundred twenty miles per hour; almost three of every ten tornadoes are strong. A violent tornado has wind speeds that are greater than two hundred miles per hour, and less than two percent of all tornadoes are violent.
The first picture is an aerial view of a tornado, courtesy of http://www.viktoria-herxheim.de/cards/ tornado.jpg The second two photos are courtesy of http://ww2010.atmos.uiuc.edu/(Gh)/guides/ mtr/svr/torn/home.rxml A weak tornado is pictured on the left, and a strong one on the right.