When a funnel cloud touches the ground, it becomes a tornado. Often, however, apparent funnel clouds are already tornadoes. But the part nearest the ground is still invisible because cloud hasn't formed there and little dirt is being picked up. Experts once thought tornado winds exceeded 500 mph. But research in recent years, including detailed analysis of movies and video tapes, shows that winds rarely exceed 250 mph and most tornadoes have winds of less than 112 mph.
Tornadoes are also relatively small.
An average tornado will be 400 to 500 feet wide and travel four or five miles on the ground, lasting only a few minutes. A mile-wide tornado is an extremely large one and tornadoes this big are rare. Many tornadoes are small, less than 100 feet wide, and last only a few minutes. A few monster tornadoes are a mile or more wide and can last for an hour or more. As the parent thunderstorm travels along, tornadoes can come down from the cloud, run along the ground and lift back up to be followed by other tornadoes.