A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes can produce massive destruction with wind speeds of 250 miles per hour or more. The typical tornado moves from southwest to northeast, but they have been known to move in any direction. The average forward speed of a tornado is 30 miles per hour but it may vary from stationary to 70 miles per hour. Although tornadoes occur in many parts of the world, they are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries.
All thunderstorms are characterized by updrafts, rising air currents which supply the warm, humid air that fuels thunderstorms; sometimes, however, the column of rising air becomes a vortex—a funnel cloud, or if it reaches the ground, a tornado. A tornado is often located at the edge of an updraft, next to air coming down from the thunderstorm with falling rain or hail. This explains why a burst of heavy rain or hail sometimes announces a tornado’s arrival. As air rises from the ground in the tornado’s vortex, a low-pressure area is created near the ground. Air rushes to fill this area, causing additional damaging to areas not directly hit by the tornado. As air rushes into the vortex, its pressure lowers, cooling the air. This cooling condenses water vapor in the air into the tornado’s familiar funnel-shaped cloud. As the swirling winds picks up dust, dirt, and debris from the ground, the funnel turns even darker.
Experts once thought tornado winds exceeded 500 miles per hour. Research in recent years, however, has shown that winds rarely exceed 250 miles per hour and most tornadoes have winds of less 112 miles per hour. An average tornado will be 400 to 500 feet wide and travel four and five miles on the ground, lasting only a few minutes. A mile-wide tornado is extremely large, and tornadoes like these are very rare. Many tornadoes are small, less than 100 feet wide, and last only a few minutes. A few monster tornadoes are a mile wide or larger, and can last for an hour or more. Tornadoes are measured by the Fujita-Pearson Tornado Scale. They range anywhere from a F-0, being the weakest to a F-5 being the strongest.
The strongest tornadoes come from the kind of long-lasting fierce thunderstorms known as supercells. As the name implies, these are intense thunderstorms, which can produce large hail and downbursts in addition to tornadoes. Supercells are most common on the Plains in the Southeast and across the Midwest, but do occur elsewhere.
Tornadoes can be very dangerous and destructive. Because of this fact, the National Weather Service...