Learn about tornadoes, one of the most mesmerizing weather phenomena and tips for staying safe when it strikes.
When a wall cloud begins rotating, tornadoes might form.
For many of us, our first image of a tornado is that of the one chasing Dorothy and Toto in the classic film “The Wizard of Oz.” The movie made an indelible impression on many of us. Tornadoes could lift up a house and carry it off to faraway places, like Munchkinland.
Call it a twister, a cyclone, or by its actual name – a tornado – this weather phenomenon is one of, if not the most, powerful storms nature can produce. Wind speeds as high as 300 mph can destroy everything in its path. In a typical year, more than a thousand tornadoes are recorded in the United States. They have occurred in every state, including Alaska and Hawaii, and in every month of the year, at any time of day.
Now that I have your attention, allow me to put the tornado risk into perspective. The majority of these tornadoes are fairly weak, and they do little if any damage at all. Over a lifetime, chances are these tornadoes will miss you, but you can help ensure your safety by following some of the safety tips I’ll be giving you – and by never setting foot in Oklahoma or Kansas again. (Just kidding.)
What exactly is a tornado? It’s a spinning column of air, and it can spin very fast. The strongest winds ever measured in a tornado were about 300 mph. (The strongest hurricane winds are just over 200 mph.) Fortunately, tornadoes anywhere near that strength are very rare – less than 1 percent. Most tornadoes have winds under 100 mph.
If a tornado is made of air, how come you can see it? What you’re seeing is called the condensation funnel. It’s made up of water droplets. Besides spinning around, the air is also being lifted, and the lower part of the funnel picks up stuff from the ground – dirt, debris, cars, cows and more (yes, cows have been sighted in tornadoes, but not sharks).
Tornadoes are a type of cyclone. Cyclone in weather-talk just means a low-pressure area with winds rotating around it. Hurricanes are another type of cyclone, as are winter storms. Tornadoes are much smaller though. Most are only a few hundred feet across. The biggest and strongest can be more than a mile wide. From the few measurements we have, as few instruments survive a tornado, the pressure created in a strong tornado can measure about 27 inches on your home barometer. It was once believed that houses exploded due to this great drop in pressure, and it was recommended that you open your windows when a tornado approached. This has since been proven wrong. Before the pressure drops enough to inflict that type of damage, your house is already sustaining winds over 100 mph, which will take care of your windows for you. You should just be seeking shelter.
Today, tornadoes are ranked by strength. The man behind the classification of tornadoes is famed meteorologist Dr. Ted Fujita. He created a six-category scale ranking tornadoes 0 through 5 in terms of wind speeds based on resulting damage. It was named the Fujita scale, or F-Scale, which was later modified and is now officially called the Enhanced Fujita or EF-Scale. The EF-Scale has been used to measure tornadoes in the U.S. since 2007 and in Canada since 2013.
Unlike hurricanes, the small and relatively short-lived tornadoes (most only last for a few minutes; the strongest can go on for more than an hour) are only classified by the destruction they have caused after they’re gone. An EF0 tornado will cause damage but not destruction. An EF2 can severely damage a typical house. An EF3 and up bring major destruction. What’s the difference between an EF4 and an EF5? The aftermath of an EF4 is complete destruction, while an EF5 will likely whisk that destruction away. A structure will be gone, reduced to small pieces with only a bare slab or basement remaining. If a tornado never hits a man-made structure, and most don’t, its true strength will never be known.
How do tornadoes form? All significant tornadoes are produced by strong thunderstorms, the strongest of which are called supercells. If conditions are just right, the warm, moist surface air being sucked up into the base of a thunderstorm will start to spin. This is typically toward the rear of the storm, often to the southwest. Storm spotters will look for a low, rain-free cloud base around that area, which is called a wall cloud. If this cloud is rotating, it’s possible a tornado may be produced.
Although tornadoes have occurred in every state, they’re more likely to occur in the southern and central Great Plains. It’s here in “Tornado Alley” where atmospheric conditions favoring tornado development come together most often, particularly in the late spring. May and June are the peak months. Tornadoes do occur in other parts of the world, but nothing like the activity we see here.
It’s impossible to forecast exactly where a tornado will hit. It’s only possible to generally forecast where potential tornado-producing thunderstorms will develop. If conditions look particularly ripe for tornado development, meteorologists at the Storm Prediction Center (SPC) in Norman, Oklahoma, will
issue a tornado “watch,” which typically covers a large area. Only a small portion of this area will actually see tornado activity. The watch is issued hours before the storm is forecasted to occur, and for the public, it means to be alert for possible severe weather.
Stay up-to-date with developments through the host of media outlets now available, including radio, television and Internet. A tornado “warning” will only be issued if a tornado is reported or is indicated by radar. Doppler radar, which detects rotation, is excellent for pinpointing a tornado’s location. Warnings are often issued for much smaller areas (a county or part of a county) than watches, and they mean for folks to take cover immediately.
Despite what you might see in movies or videos, you can’t always see a tornado coming. Some are obscured by rain, dust, etc. Even though they are very loud close up, the sound of a tornado doesn’t travel far. It’s important to heed all tornado warnings issued, and take cover.
Suppose a tornado warning is issued for your area. What do you do? If you’re in your home, go to your basement or storm shelter. Some homes even have above-ground tornado-proof safe rooms. If none of these are available, go to the middle of your house away from any windows. An interior closet or bathroom is best.
Don’t hunker down in a mobile home to wait out a tornado. Many fatalities occur in mobile home parks, because such homes don’t have the structure to withstand tornados. Even an EF0 can roll a mobile home, so have a backup shelter you can get to quickly.
The same is true for cars. It’s possible to outrun a tornado if there’s a road going in the right direction and there’s no traffic, but tornadoes can have a forward speed of up to 70 mph. If you see a tornado, you’ll have to judge whether or not you can get to a safe distance in your car. If you can’t, get out of the car. Bridges or underpasses are not safe hiding places. If you’re in the open without shelter, seek low ground and cover your head, but make sure that low ground isn’t a rapidly filling stream.
Rather than hiding from tornadoes, there are some who chase after them. The first storm chasers were meteorologists who wanted to observe tornadoes firsthand to learn more about them. With the advent of portable video cameras, though, a new group of chasers evolved – those wanting to film the storms and try to sell the videos. The 1996 movie “Twister” added to the allure of chasing while minimizing the actual danger of it.
Today, small convoys of storm chasers can often be seen on the highways of Tornado Alley. This can be a very dangerous undertaking, as chasers are taking bigger and bigger risks to get even closer to the storms, if not even into one. Chasers have even been killed in their pursuit.
The largest tornado ever recorded was a result of the El Reno, Oklahoma, storm, which occurred May 31, 2013. At its widest point, this EF3 storm measured 2.6 miles across.
Although most tornadoes only last for a few minutes and remain on the ground for a few hundred yards, the “Tri-State Tornado” on March 18, 1925, traversed 219 miles through Missouri, Illinois and Indiana. While on the ground for several hours, it had a record number of fatalities – 695 people.
The strongest tornado winds were measured by Doppler radar on May 3, 1999, during a storm near Bridge Creek, Oklahoma. Wind speeds of 318 mph were recorded.
The greatest tornado outbreak ever recorded occurred April 3 and 4, 1974. In the “Super Outbreak,” 148 tornadoes covered an area from the Mississippi River to the Appalachians and from Mississippi to New York.
Tornado risk varies geographically with the seasons. In winter, the greatest threat is in the South and Southeast, even Florida. By spring, it moves into the Plains and Midwest, peaking in May and June. In summer, the tornado threat decreases greatly and is mainly along the northern tier states. During fall, risk increases again to the south but is much smaller than in spring.
Tornadoes have a few weaker cousins. Water spouts form over water, often from benign-looking clouds. Dust devils are common in areas where the ground gets very hot in the summer, but they have no parent cloud. Fire whirls, twisting columns of air filled with burning debris or flames, are caused by air superheated by a major fire, either wild or man-made.
A great website for all severe weather data and future forecasts is the National Weather Service Storm Prediction Center.
The original Fujita scale was created to categorize tornadoes from estimated wind speeds based on resulting damage.
The Enhanced Fujita scale was later implemented in the U.S. in 2007 and in Canada in 2013, and serves as a more accurate and consistent way to estimate the strength of a tornado. To see the full scale, visit 1.usa.gov/1QbXKmd.
EF Number and corresponding wind speed (3-second gust in mph)
0: 65 to 85
1: 86 to 110
2: 111 to 135
3: 136 to 165
4: 166 to 200
5: Over 200
For 30-plus years, Ed Brotak taught thousands of college students about the weather and helped hundreds of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife (also a meteorologist) and his two daughters (who vow never to be “weather weenies”). He still goes outside when he hears thunder.
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