A dark greenish tinge covers the sky, and black storm clouds threaten above. The wind roars loudly while cats hide under couches and dogs start howling. A tornado is forming.
Also known as twisters, tornadoes are violently spinning, funnel-shaped columns of air that stretch from the dark thunderclouds they form in all the way to the ground. The wind from a tornado can top 250 miles an hour—that’s faster than a race car! These extreme gusts can rip apart buildings, destroy bridges, flip trains, and send cars flying. They can even tear the bark off trees and suck all the water from a riverbed.
Tornadoes occur all over the planet, but the United States leads the world in the strength and number of storms: About a thousand twisters touch down every year. (Argentina and Bangladesh are next.) As part of larger U.S. storm systems, they cause about 80 deaths a year and close to $1 billion in damage.
How tornadoes develop
Thunderstorms form when cold, dry air is pushed over warm, humid air. When that warm air rises through the colder air, it causes an updraft, or a change in wind direction.
If winds from the thunderstorm vary greatly in speed or direction, the updraft will begin to rotate. As the rotating updraft draws in more warm air from the moving thunderstorm, its rotation speed increases, and a funnel cloud begins to form. As the twister gains strength, the funnel becomes longer. (The funnel becomes more visible as more dirt and debris is caught in its rotation.) It’s most dangerous when it touches the ground.
The most violent tornadoes come from supercells—the name scientists give to large thunderstorms with winds already in rotation. About one in a thousand thunderstorms becomes a supercell, and one in five or six supercells creates a tornado.
For all their power, the average tornado lasts only about 10 minutes, though some can rage for over an hour. Twisters usually die when they move over colder ground or when the clouds above it start to break up.
The Tri-State Tornado of 1925 holds the record for time and distance of a tornado. Named for the three states it hit—Missouri, Illinois, and Indiana—the tornado took three and a half hours to tear through 219 miles.
Where and when tornadoes form
Although tornadoes have been spotted in every U.S. state, many form in a region called Tornado Alley. This zone in the Midwest extends from Texas to Ohio and includes Iowa, Kansas, South Dakota, Oklahoma, and Nebraska.
These states are in the path of warm, moist air traveling from the Gulf of Mexico as well as cool air blowing from the Rocky Mountains. When the warm and cool airstreams meet, tornadoes are likely to form.
Though the storms can happen at any time of the year, tornado season in Texas, Oklahoma, and Kansas occurs in May through early June. In North and South Dakota, Nebraska, Iowa, and Minnesota, tornadoes are more likely to form in June or July.
During thunderstorms, meteorologists use weather satellites, weather balloons, and buoys to gather data like wind speed and temperature. Then they analyze that data with supercomputers. This helps scientists pinpoint where and when a twister might form, as well as how strong or long-lasting it might be.
If weather conditions are right for a tornado to form, experts issue a tornado watch for a region, like a county or large part of a state. This doesn't mean that a tornado is on the way. But it could be—meteorologists issue the watch so people can be prepared.
When a tornado is spotted or picked up on weather radar, scientists issue a tornado warning for a smaller area, like a town or part of a city. People there are urged to take cover.
Some experts actually drive into areas where storms are forming! Vehicles tricked out with special science equipment measure things like temperature, humidity, and air pressure that’s submitted to meteorologists at weather service headquarters. But the information that these “tornado chasers” gather also helps scientists better understand the science of tornados.
Thanks to these tools, meteorologists are now able to quickly predict when and where a tornado will form, giving people in a twister’s path more time to seek shelter. For instance, in the 1980s, people only had about a five-minute warning before a tornado hit; by the late 2000s, the warning time grew to 13 minutes.
How to survive a tornado
Before a tornado
• Listen to weather reports for tornado warnings.
• Close all the windows.
• Prepare a safe room. This can be your basement or the lowest room in the center of your house or apartment building, away from outer walls and windows. Window-free closets and bathrooms work well, too.
• Fill the safe room with blankets, pillows, sleeping bags, and your family emergency kit (water, food, flashlight, and radio).
• Have an emergency safety plan. If you live in a trailer or mobile home, make sure you know where to go to evacuate.
During a tornado
• Stay inside—never attempt to watch or chase a tornado.
• Do not go near windows. It’s not the tornado itself that causes most injuries; it’s the flying debris—or the windows they crash through—that cause most injuries.
• Cover yourself with the pillows, blankets, and sleeping bags. If there’s a sturdy piece of furniture, like a table, crawl under it.
• Cover your head and neck with your arms.
• If you’re caught outside and can’t find shelter, look for a ditch, gulley, or just a flat piece of ground away from trees and cars. Lie down as flat as you can and cover your head and neck with your arms. (Do not try this under a bridge or overpass.)
After a tornado
• Do not leave your shelter until local authorities give the OK. Follow their instructions.
• If you go outside, watch where you step—dangerous debris can be anywhere.
• Continue to monitor tornado reports. Sometimes more than one can whip up in the same place.
Adapted by Laura Goertzel from the National Geographic article “Tornadoes, explained” by Nat Geo Staff