A rare mother ship cloud formation hovers over Childress, Texas. Tornado chasers there spent seven hours and 150 miles (240 kilometers) tracking the supercell thunderstorm that produced this cloud formation. Supercell thunderstorms are known to spawn tornadoes with winds exceeding 200 miles an hour (322 kilometers an hour).PHOTOGRAPH BY CARSTEN PETER, NATIONAL GEOGRAPHIC

One tornado chaser with a camera poses while his partner gets into their truck as a tornado approaches in South Dakota. South Dakota had 35 tornadoes in 2009, and is part of "Tornado Alley," an area of the Midwest United States that sees more tornadoes than the rest of the country.PHOTOGRAPH BY CARSTEN PETER, NATIONAL GEOGRAPHIC

Storm-stained skies hover over the remains of a mobile home demolished by a passing tornado. Tornadoes kill about 60 people in the U.S. every year and cause billions of dollars of property damage.PHOTOGRAPH BY CARSTEN PETER, NATIONAL GEOGRAPHIC

A photographer caught this extreme close-up of a tornado funnel in Manchester, South Dakota. The combination of high winds, flying debris, and loud noise of the tornado would have made this photographer very uncomfortable.PHOTOGRAPH BY CARSTEN PETER, NATIONAL GEOGRAPHIC

Heavy clouds hang low over a dilapidated homestead in the Midwest, foretelling a possible tornado. Tornadoes form when the updrafts of air that supply storms with warm, humid air become a vortex, or high-speed whirlwind.PHOTOGRAPH BY CARSTEN PETER, NATIONAL GEOGRAPHIC

An EF5 tornado threw a van into a hotel in Greensburg, Kansas. An EF5 tornado produces winds greater than 200 miles per hour (322 kilometers per hour), and can pick up and throw vehicles and destroy buildings with ease.PHOTOGRAPH BY MIKE THEISS, NATIONAL GEOGRAPHIC

A waterspout touches down near a boat near Dubrovnik, Croatia, in August 2007. Waterspouts are weak tornadoes that happen over warm water. They can move onto land and become tornadoes.PHOTOGRAPH BY DANIEL PAVLINOVIC, MY SHOT

A thin funnel cloud touches down in a field in northwest Iowa. Funnel clouds become tornadoes once they touch the ground.PHOTOGRAPH BY MELANIE METZ, MY SHOT

A man throws a stuffed animal to his wife as they clean up the debris from their apartment, damaged by a February 2008 tornado over Lafayette, Tennessee.PHOTOGRAPH BY JIM WATSON, AFP/GETTY IMAGES

Several homes in a row were destroyed by a May 2008 tornado in Parkersburg, Iowa. Tornadoes can completely destroy some buildings while leaving others intact, depending on their construction. Mobile homes are notoriously flimsy in tornadoes.PHOTOGRAPH BY STEVE POPE, GETTY IMAGES

A category F3 tornado swirls across a South Dakota prairie. The F (Fujita) scale was used to measure wind speeds based on damage left behind after a tornado, and an F3 tornado had wind speeds between 158 and 206 miles an hour (254 to 332 kilometers an hour). The United States now uses the EF (Enhanced Fujita) scale, which takes more variables into account when assigning wind speeds to a tornado. An EF3 tornado now has wind speeds between 136-165 miles per hour (218-266 kilometers per hour), and the tornado pictured could be an EF3, EF4 or EF5.PHOTOGRAPH BY CARSTEN PETER, NATIONAL GEOGRAPHIC

A woman searches the wreckage of her mother's house in Arkansas after a tornado removed the roof. Forecasters in the United States can give an average warning time of 13 minutes before a tornado hits. The best place to be in a tornado is in a basement or interior room of the lowest floor of a sturdy building.PHOTOGRAPH BY RICK GERSHON, GETTY IMAGES

A Kansas vehicle license plate impaled on a tree after an EF5 tornado struck Greensburg, Kansas. Flying debris is the main cause of injuries and deaths in tornadoes.PHOTOGRAPH BY MIKE THEISS, NATIONAL GEOGRAPHIC

A woman looks through a broken window of a friend's house after a tornado over Atkins, Arkansas. Despite a myth that says otherwise, opening windows to equalize the pressure between the inside and outside of a house during a tornado will not prevent them from breaking. Flying debris is the main cause of broken windows in tornadoes.PHOTOGRAPH BY RICK GERSHON, GETTY IMAGES

A broken safety glass window frames a destroyed house after an EF5 tornado struck Greensburg, Kansas.PHOTOGRAPH BY MIKE THEISS, NATIONAL GEOGRAPHIC

Tornadoes are vertical funnels of rapidly spinning air. Their winds may top 250 miles (400 kilometers) an hour and can clear a pathway a mile (1.6 kilometers) wide and 50 miles (80 kilometers) long.

Twisters are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year.

Are tornadoes getting stronger? 01:44

Most tornadoes move at less than 35 miles per hour.

Most tornadoes last only a few minutes.

The most destructive and deadly tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.

A tornado over a body of water is called a "waterspout."

The United States has the highest number of tornado occurrences in the world with an average of more than 1,000 tornadoes reported each year.

According to the National Weather Service, in 2016 there were 17 tornado-related deaths in the United States.

Twisters are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year.

"Tornado Alley," a region that includes the area in the eastern state of South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.