Tornadoes are narrow, circulating columns of air that originate from a supercell thunderstorm, from the cloud to the ground, and constantly become darker as they pick up dirt and debris from the ground. Tornadoes move at an average speed of 35 mph and have an average span of about 5 miles.
Tornadoes vary in diameter, from around 300 feet to more than 1 mile. They remain attached to the cloud they formed from and can develop over land or water.
Because scientists have not yet devised a way to measure a tornado's intensity while it is occurring, tornadoes are measured by the damage they cause on the ground. For example, for a tornado to be considered an F3 on the Fujita scale, it must tear off roofs and destroy walls of well-constructed homes and businesses.