According to The Weather Channel, a tornado is "a violently rotating column of air that stretches from a cloud to the Earth's surface." The source also states that tornadoes are "the most destructive of all storm-scale atmospheric phenomena." Often forming from a thunderstorm, tornadoes also result from hurricanes.
Wind velocity inside the tornado ranges from less than 100 miles per hour to more than 250 miles per hour. More than two-thirds of tornadoes are considered weak, with wind speeds slower than 115 miles per hour and lasting 10 minutes or less. About 29 percent of tornadoes are classified as strong, with wind speeds of 110 to 205 miles per hour and lasting up to 20 minutes or longer. Only 2 percent of tornadoes bear the label of violent. These are capable of lasting an hour or even longer and are responsible for 70 percent of all deaths due to tornadoes.