The diameter of a tornado can vary between a few feet and a mile, and its track can extend from less than a mile to several hundred miles. These powerful, highly organized storms form when warm, moist air along the ground rushes upward, meeting cooler, drier air. As the rising warm air cools, the moisture it carries condenses, forming a massive thundercloud, sometimes growing to as much as 50,000 ft. in height. Variable winds at different levels of the atmosphere feed the updraft and cause the formation of the tornado's characteristic funnel shape. The conditions that lead to the formation of tornadoes are most often met in the central and southern U.S., where warm, humid air from the Gulf of Mexico collides with cool, dry air from the Rockies and Canada.
Tornadoes can also occur elsewhere, though, including all U.S. states, Europe, Asia, and Australia.
The Fujita scale classifies tornadoes according to the damage they cause. These tornadoes reach speeds of 73-112 m.p.h. and can overturn automobiles and mobile homes, rip off the roofs of houses, and uproot trees. With wind speeds in excess of 261 m.p.h., these tornadoes are capable of lifting houses off their foundations and hurling them considerable distances. The most populous city in the world is Seoul, South Korea, with a population over 10 million.