Tornadoes ripped across the Southern and Midwestern US on Thursday, killing at least 11 people, most of whom had been taking shelter in an Alabama high school when it collapsed.

Five people died when the school building was torn open by the twister in the town of Enterprise, according to Yasmie Richardson of the state's Emergency Management Agency. The agency had said earlier that 17 people had died in the town, but later lowered the state-wide toll to seven, blaming initial miscommunication among officials.

"Enterprise has suffered major and widespread damage," said governor Bob Riley, who declared a state of emergency and sent 100 National Guard troops to the town. Riley said rescue workers were still moving debris to search for survivors as night fell.

In Georgia, storms that swept through the southwestern part of the state killed at least three people and caused an unknown number of injuries, said Buzz Weiss of the Georgia Emergency Management Agency.

Two were killed in Americus, Georgia, when the Sumter Regional Hospital was hit by an apparent tornado. "Sumter County has apparently lost all of its ambulances and rescue vehicles," Weiss said. The third fatality was in Taylor County, he added.

In the Missouri town of Caulfield, a tornado killed a girl in a mobile home, damaging six other homes and two gasoline stations, officials said.
Peak time

A tornado is a violently rotating column of air extending from a thunderstorm down to the ground (see graphic). The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 miles per hour (400 kilometres per hour) or more. The trail of destruction can be in excess of a mile wide and 50 miles (80 km) long.

In an average year, about 1000 tornadoes are reported across the US, resulting in 80 deaths and more than 1500 injuries. Tornadoes can occur anywhere in the US at any time of year. In the southern states, however, peak tornado season is from March to May, while the peak months in the northern states are during the summer.

Tornadoes have been documented in every state in the US. The deadliest tornado recorded in the US touched down on 18 March 1925, killing 695 people and injuring 2027 in Missouri, Illinois and Indiana.

In global terms, the US leads the list, with an average of over 1000 tornadoes recorded each year. A distant second is Canada, with around 100 recorded per year. Other locations that experience frequent tornadoes include northern Europe, western Asia, Bangladesh, Japan, Australia and New Zealand.

The UK has more tornadoes, relative to its land area, than any other country, though most British tornadoes are relatively weak.
Exploding windows

In Enterprise, Alabama, television news footage showed the shredded school building surrounded by broken trees and overturned cars. Alabama state officials sent search and rescue teams, ambulances, generators and emergency lights to aid the search for survivors.

Phone services and electricity were out through much of the area. US president George W Bush called the Alabama and Missouri governors to extend condolences and offer aid, the White House said.

Doctors treated more than 50 people for lacerations, broken bones and other injuries, hospital CEO Jeff Brannon said. Hospital workers had rushed to move patients away from the windows as sirens screamed out a warning moments before a dark funnel cloud roared past. Hospital windows burst and cars were pummelled in the hospital parking lot.

Parts of several Midwestern states and regions as far south as the Gulf Coast to the Florida Panhandle had been under tornado watches or warnings most of the day. In Chicago, more than 400 flights were cancelled at O'Hare International Airport as gusty storms and fog rolled in.

"To the best of our knowledge there was at least one significant tornado that went through the city of Enterprise but we won't know if there was more than one before tomorrow when we send a team to conduct a storm survey of the affected area," said Tom Bradshaw, a meteorologist in Fort Worth, Texas, with the National Weather Service headquarters for the southern region.

"Several counties in Alabama and also Georgia are currently under tornado warnings," he said. Thursday's storms came just a month after a tornado killed about 20 people in central Florida.