What exactly is a tornado, one of the deadliest storms on Earth?
Learn how tornadoes form, where they happen most often—and how to stay safe.
Tornadoes are vertical funnels of rapidly spinning air. Their winds may top 250 miles an hour and can clear a pathway a mile wide and 50 miles long.
Twisters are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive.
These violent storms occur around the world, but the United States is a major hotspot, with the number of tornadoes clocking in at about 1,200 per 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 often home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year, according to the National Weather Service.
(Where is ‘Tornado Alley’? How the deadliest storm zone in the U.S. is shifting east.)
What is a tornado?
A tornado is a violently rotating column of air that extends from a thunderstorm to the ground. It’s often portended by a dark, greenish sky. Black storm clouds gather. Baseball-size hail may fall. A funnel suddenly appears, as though descending from a cloud. The funnel hits the ground and roars forward with a sound like that of a freight train approaching. It tears up everything in its path.
(Sustainable Earth: Disasters)
Every U.S. state has experienced twisters, but Texas holds the record. They’ve been reported in Great Britain, India, Argentina, and other countries, but they are most often seen in the U.S.
Waterspouts are weak twisters that form over warm water. They sometimes move inland and become tornadoes.
(Can We Really Make Tornadoes for Energy? This Man Wants to Try)
Dust devils are small, rapidly rotating columns of air that are made visible by the dust and dirt they pick up. Dust devils are not associated with thunderstorms, neither are fire tornadoes.
“Firenados,” like the one that swept through Utah, have become much more frequent due to climate change. But these vortexes tend to spawn from wildfires, not thunderstorms.
What causes tornadoes?
The most violent come from supercells, large thunderstorms that have winds already in rotation. About one in a thousand storms becomes a supercell, and about 30 percent of supercells produce a tornado.
Tornadoes can occur at any time of year, but they are more common during a distinct season that begins in early spring for the states along the Gulf of Mexico.
The season follows the jet stream—as it swings farther north, so does activity. May generally has more than any other month, but April’s twisters are sometimes more violent. Farther north, they tend to be more common later in summer.
(Summer storms can strike suddenly. Here’s what causes them.)
Although they can occur at any time of the day or night, most form in the late afternoon. By this time the sun has heated the ground and the atmosphere enough to produce thunderstorms. They form when warm, humid air collides with cold, dry air.
The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. The updraft will begin to rotate if winds vary sharply in speed or direction.
As the rotating updraft, called a mesocyclone, draws in more warm air from the moving thunderstorm, its rotation speed increases. Cool air fed by the jet stream, a strong band of wind in the atmosphere, provides even more energy.
Water droplets from the mesocyclone’s moist air form a funnel cloud. The funnel continues to grow and eventually it descends from the cloud. When it touches the ground, it becomes a tornado.
Characteristics of tornadoes
Twisters are usually accompanied or preceded by severe thunderstorms and high winds. Hail is also common.
Once one hits the ground, it may move for as little as a few seconds or as long as three hours.
The average twister is about 660 feet wide and moves about 30 miles an hour. Most don’t travel more than six miles before dying out. Massive ones, however—those capable of widespread destruction and many deaths—can roar along as fast as 300 miles an hour.
These measurements are scientists’ best estimations. Anemometers, which measure wind speed, cannot withstand the enormous force of tornadoes to record them.
Using units F0 to F5, the Fujita scale measures a tornado's intensity by analyzing the damage the twister has done and then matching that to the wind speeds estimated to produce comparable damage. The United States now uses the EF (Enhanced Fujita) scale, which takes more variables into account when assigning wind speeds to a tornado.
(Night tornadoes are twice as deadly. Here’s why they’re becoming more common.)
Tornado destruction
Tornadoes can cause billions of dollars in damage. Extremely high winds tear homes and businesses apart. Winds can also destroy bridges, flip trains, send cars and trucks flying, tear the bark off trees, and suck all the water from a riverbed.
High winds sometimes kill or injure people by rolling them along the ground or dropping them from dangerous heights. But most victims are struck by flying debris—roofing shingles, broken glass, doors, metal rods.
The number of average deaths per year in the U.S. used to be higher before improved forecasting and warning systems were put into place.
(Survivors recall the terror of the deadliest F5 tornado to hit the U.S.)
Tornado watch vs. tornado warning
Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.
Doppler radars record wind speeds and identify areas of rotation within thunderstorms. Since Doppler radar has been in use, the warning time for tornadoes has grown from fewer than five minutes in the 1980s to an average of 13 minutes by the late 2000s.
When weather conditions are conducive for tornado formation, the National Weather Service issues a tornado watch. When one has been sighted or indicated on radar, a tornado warning is issued.
Some scientists, meteorology buffs, and adrenaline junkies hit the road to chase storms. Researchers race to place sensors in tornadoes' paths. The sensors measure data such as wind speed, barometric pressure, humidity, and temperature.
(‘Twisters’ put storm chasing on the map. But here’s what it’s really like.)
The challenge for researchers is being in the right place at the right time. Every morning they study weather conditions and head for the area that seems most likely to spawn a twister. They drive through severe storms, dodge lightning, face flash floods, and get pounded by hail—sometimes for years—before ever spotting one.
All at considerable risk. In 2013, National Geographic Explorer Tim Samaras and his team were killed while trying to study one in Oklahoma. (See photos of Samaras’s work.)
How to survive a tornado
If a tornado is imminent, immediately seek safe shelter inside a sturdy building on the lowest level. While certain parts of the U.S. are more vulnerable to tornadoes than others, they can happen anywhere, at any time of year. The federal government’s Ready.gov is a good place to start preparing.
Before tornado season, understand your area’s risk. In the U.S., tornadoes are more common in the Midwest and Southeast. Tune into the emergency alert systems and the weather radio operated by the National Oceanic and Atmospheric Administration.
Many areas employ volunteer “storm spotters,” typically firefighters and police personnel trained to look for telltale signs and provide on-the-ground conditions to local, state, and national networks.
Stock up on emergency supplies, including first aid kits, dust masks, and extra batteries. Make sure you have enough supplies to last through the aftermath of a storm.
And don’t forget your pets—prepare an emergency kit for them that includes travel items, in case a tornado causes you and your loved ones to evacuate.
