Earth's Journal

Atmosphere Journal Entry

Twister Swarm Batters South (April 30, 2011)


Map shows huge storm system that triggered a deadly swarm of tornadoes across the southern United States. NASA Earth Observatory.

A massive storm system triggered a swarm of deadly tornadoes across the southern United States this week. The twisters were blamed for at least 340 deaths and more than 1,700 injuries in seven states. More than two-thirds of the victims died in Alabama, where much of the college town of Tuscaloosa was flattened. Thousands of people are homeless after tornadoes leveled homes in Alabama, Tennessee, Mississippi, Georgia, Virginia, Arkansas, and Louisiana. Hundreds of thousands of people were left without electrical power across the South.

There were reports of more than 200 twisters from the storm system. The one that struck Smithville, Mississippi was the most destructive category EF-5, with winds clocked at 205 miles per hour (328 kilometers/hour). It's the nation's deadliest tornado outbreak since 1925 and second deadliest in its history.

Tornadoes often result from collisions of two huge air masses. In the United States, cold, dry air from Canada or the Rockies slams into warm, humid air rising from the Gulf of Mexico. The collision can create funnel clouds of violent winds whipping around an area of extremely low barometric pressure. The cloud becomes a tornado when it touches the ground and can destroy everything in its path.

The United States gets struck with more tornadoes than anywhere else on the planet. Most occur in the spring and summer months. They most commonly strike in the region nicknamed Tornado Alley stretching from Texas and Oklahoma north through Kansas, Nebraska, and the Dakotas. On overage, about 800 tornadoes touch down in the U.S. every year. The most violent twisters have wind speeds of over 250 mph (400 km/h).