While 90% of tornadoes strike in the United States due to storms created by dry cool polar air from Canada which meets warm moist air from the Gulf of Mexico, tornadoes actually strike around the world. About 800-1200 tornadoes occur in the U.S. each year although most occur in uninhabited areas. A large proportion of United States tornadoes strike in an area known as Tornado Alley, which stretches from northwest Texas, across Oklahoma and Kansas.
Tornadoes form a circular column of low pressure (up to about 100 millibars lower than the surrounding air) which has very fast wind circling around it at speeds of 200 to 500 miles per hour. These 450-1800 foot wide columns can drive straw into wood and wood into metal as they create their path of destruction.
Tornadoes, also known as twisters, usually move at about 15 to 30 miles per hour towards the northeast and last from a few minutes to as long as eight hours. Tornadoes kill about 90 people each year in the United States and are the most destructive of any local atmospheric disturbance. The wind, not the low pressure, associated with a tornado is what causes the damage, which can made a city look like it was bombed (experts advise against opening windows when a tornado approaches).
Geography of TornadoesTornadoes occur in all fifty states. Texas and Oklahoma have tornadoes most frequently but Massachusetts has the highest tornado death rate, due to the state's much higher population density than Central Plains states and a 1953 tornado that killed 94 people.
Other regions of the world with frequent tornadoes include Canada, southern Brazil, South Africa, eastern and western Australia, Bangladesh, Europe, Japan, and New Zealand. The United Kingdom had weak but a very high incidence of tornadoes per acre, more than the United States as a whole.
The dark gray color commonly associated with tornadoes is due to the soil and other objects (as large as railroad cars) picked up and blown by the winds.
In the United States, the National Weather Service (NWS) issues tornado watches and warnings. A watch means that the conditions are conducive for a tornado to occur while a warning means that a tornado has occurred or is about to occur.