A series of strong twisters brought death and destruction to parts of the South and Midwest last week as violent weather pulverized homes in half a dozen states from Iowa to Tennessee. At least three dozen deaths were reported from as many as 90 tornadoes. Damaged buildings include a hospital in Louisville, Miss., where winds knocked down two walls and tore holes in the roof. Wind speeds in Tennessee reportedly exceeded 190 mph.

"All homes gone" said a National Weather Service bulletin for a community in Lincoln County, Tenn. President Obama declared a major disaster in Arkansas and ordered federal aid to supplement state and local recovery efforts in a county hit by tornadoes, other severe storms and flooding. Later in the week, heavy rain flooded parts of Florida's Panhandle and Alabama. Nearly 2 feet of rain fell in about 24 hours, knocking out power and causing damage in Escambia and Santa Rosa counties in this state. Some people were rescued from rooftops.

A tornado is a violently rotating column of air that emerges from what;s called a supercell thunderstorm. When cold, dry air pockets in the upper atmosphere meet humid, warm air below, that raises the risk of tornadoes – which typically occur in spring. "We're on a part of the planet where these ingredients can come together sometimes very quickly," says weather expert Ernest Agee, a professor at Purdue University in Indiana. "Other places have tornadoes, but not as many." Twisters extend between the ground and a cloud, usually a cumulonimbus cloud. Most last for less than 10 minutes, though large ones can spin a half-hour. They can be more than two miles wide and race across land for dozens of miles.

Presidential message: "Your country will be there to help you recover and rebuild as long as it takes."

Survivor says: "It makes you just take a breath. It makes you pay attention to life." – Kenneth Billingsley of Louisville, Miss.

Tenn. official says: "This is the worst this area has been hit in many years." – Chelsea Bobo of Lincoln County Sheriff’s Office