Tornadoes

Tornadoes

Introduction What can lift roofs from buildings and sweep houses into the air? Tornadoes can! Tornadoes come in many sizes. Some tornadoes measure only a few feet (1 meter) across. Others are more than a mile (1. 6 km) wide. Some tornadoes touch down for a short period of time. Others travel for hundreds of miles.

How Tornadoes Form Why do tornadoes happen? Scientists aren't sure. They know that strong tornadoes come from huge thunderstorms called supercells. These storms form when warm, moist air rises in the atmosphere and mixes with cold, dry air. Scientists think that in some supercells, the mixture of air at different temperatures causes the air to spin. The spinning air forms a cloud in a funnel shape. When the funnel touches the ground, it becomes a tornado.

Some scientists think that tornadoes happen in order to balance the temperature and moisture in the air.

Measuring Tornadoes Scientists are still studying the causes of tornadoes, but they have a system to estimate their strength. They use a special scale called the Enhanced Fujita (EF) Scale. It uses wind speed to rate a tornado's strength. Scientists look at the damage caused by a tornado to estimate wind speed. EF-5 tornadoes, the most powerful tornadoes, have winds greater than 200 miles per hour (322 kmph).

Where Tornadoes Form Tornadoes may be hard to measure, but scientists have a good idea where they will strike. It's possible for a tornado to happen anywhere in the world. However, most tornadoes occur in the central part of the United States, also known as Tornado Alley. More than one thousand tornadoes develop there each year.