The Science Behind Tornadoes

Nov. 12 -- The tornado outbreak that killed at least 35 people in five states Sunday was created by a rare and potent mix of conditions that meteorologists call the "perfect" storm.

Tornado outbreaks are a cluster of several tornadoes caused by the same storm system. They happen only a few times a year, but when they do they can be enormously destructive.

The worst tornado outbreak in U.S. history, called the "super outbreak," happened in 1974. It produced 148 twisters in 13 states, from Mississippi to Ohio.

Sunday's storms will also go down in history books. After more than 70 tornadoes swept through five states, 16 deaths were reported in Tennessee, 12 in Alabama, five in Ohio and one each in Mississippi and Pennsylvania. More than 200 people have been injured.

Outbreaks

For a tornado outbreak to happen, you need a "perfect" storm system, explained Dan McCarthy of the Environmental Prediction Storm Prediction Center in Circle Norman, Okla. That is exactly what happened on Sunday.

"Tornadoes are a spontaneous thing when related to storms," McCarthy said. "We have to have the exact conditions of amount of moisture and heat combined with vertical wind sheer — that's the winds that change — they usually turn in a clockwise manner to really get a tornado outbreak like this," he said.

Tornado outbreaks are caused by the same forces that form individual tornadoes — the meeting of hot and cold. Typically, cold air coming in from Canada hits warm, moist air from the Gulf. When the cold air meets the warm air, they tumble over one another and create the vortexes called tornadoes.

On Sunday, the hot and cold air generated hugely powerful, swirling currents and the tornadoes were formed nearly simultaneously, amid a line of thunderstorms moving from the southwest in a northeasterly direction.

After a tornado whipped through Carbon Hill, Ala., at 8:15 p.m. Sunday night, Leslie Johnson said she was shocked by how it simply tore her town apart.

"There was a subdivision behind me that is gone," Johnson said. "It's just gone."