Verisimilitude is hardly a quality I would associate with my dreams, particularly when it comes to settings. But in this case my brain got it just right. I once drove the breadth of Kansas, and the landscape in my dream looked just like that: low-lying and treeless in all directions, with nothing to obstruct my imagined view of eight or 10 jet-black twisters writhing all about me like giant leeches.

Receive emails about upcoming NOVA programs and related content, as well as featured reporting about current events through a science lens. Email Address Zip Code Subscribe

There's no better place to set a tornado nightmare than in the Great Plains. Here, a massive funnel near Seymour, Texas, in 1979. © NOAA

Tornado heaven

Support Provided By Learn More

My mind had good reason to set my nightmare there. The Great Plains of the U.S. suffers more tornadoes—and more violent tornadoes—than anywhere else on Earth. Movies from The Wizard of Oz to Twister have drummed this fact into our collective unconscious, and every year we hear of killer cyclones that, tragically, have ravaged some midwestern town or trailer park.

But have you ever stopped to wonder why the Great Plains, and by extension the country as a whole, gets the lion's share of our annual planetary quota of tornadoes? I hadn't—mostly, I suspect, because I'm an East Coaster, and for us tornadoes lie in the realm of the freak occurrence. As we'll see, most people elsewhere in the world appear to feel similarly about tornadoes.

All told, about 1,200 tornadoes occur annually in the United States.

The answer, I found, is two-fold. It has to do with what you might expect (climatological conditions in the Great Plains are unparalleled for spawning tornadoes), but also with what might come as a surprise (very few nations even bother to record tornadoes). One expert I spoke with believes that even countries that report their worst windstorms may be underreporting by a factor of seven. And "tornadic events" that get reported as a single tornado in a country with a nonexistent damage-assessment system might, with a better such system, be found to have been 10 separate tornadoes. This makes assigning twister numbers by country even trickier.

The truth is, the U.S. very well may

not get three out of every four tornadoes that occur on Earth; it may just look that way.

The Great Plains climate cooks up the right ingredients for twisters. Above, a graceful strand touches down near Union City, Oklahoma, in 1973. © NOAA

Accident of geography

The Great Plains has been likened to a funnel factory. It possesses all the ingredients needed to produce, as one expert put it to me, "some hellacious thunderstorms"—the parents of tornadoes. In spring and early summer, warm, moist air blowing in from the Gulf of Mexico can become trapped beneath a "lid" of hot, dry air gusting from the high desert region of the Southwest and, above that, cold, dry air sweeping over the Rockies. Like a lid on a pot of boiling water, this "convection cap" keeps the warm air from rising. The pressure builds, until a cold front or other boundary between air masses moves in and weakens the cap. Quite suddenly, the warm, humid air can burst forth, billowing upwards at up to 100 miles per hour and swelling into 50,000-foot-tall thunderstorms in minutes.

Some of these thunderstorms begin rotating through most of their depth. (This happens because of wind shear, a dramatic change in wind speed or direction over a very short distance.) Called "supercells," these storms serve as ideal generators of tornadoes, from those that scrape off a few shingles to those rare, mile-wide monsters that leave nothing in their wake but cleared concrete foundations. "No other place on the planet has the source of warm, moist air on the equatorward side and a wide, high range of mountains extending from north to south on the west side," says Harold Brooks, a tornado expert at the National Severe Storms Laboratory in Norman, Oklahoma. "The Andes aren't as wide as the Rockies, and the Himalayas don't extend very far from north to south."

Tornadoes regularly touch down in places far from Tornado Alley. In June 1998, a twister demolished this house near Pittsburgh, Pennsylvania. Liz Roll/FEMA News Photo

While the Great Plains gets the bulk of American tornadoes, other parts of the country witness them as well. Florida sees more twisters than Oklahoma, though they're far weaker. Cyclones also strike Colorado, and occasionally a ripsnorter will touch down in other states. In 1979, I was living in Hartford, Connecticut, when a tornado raked through nearby Windsor Locks. That tornado is fifth on a list of tornadoes that have caused at least $200 million in damage (in inflation-adjusted 1999 dollars). All 50 states, in fact, have experienced twisters.

All told, about 1,200 tornadoes occur annually in the United States. The entire rest of the world collectively reports just 200 to 300 every year. Yet only in this country is the number of reported tornadoes roughly equal to the number of actual tornadoes in any given year. The U.S. began officially collecting tornado reports back in 1953 and rating tornadoes using the Fujita Scale 20 years later. No other nation has such a robust or long-standing system.

As a sign of how lackluster tornado reporting is elsewhere, can you name a single country outside the U.S. where tornadoes regularly occur? I couldn't before starting this article. In fact, I couldn't remember hearing of a single tornado that ever struck anywhere else in the world. I'm sure I've heard of some, but they didn't stick in my mind.

Though outdated and far from comprehensive, Theodore Fujita's 1973 map of tornadoes expected to occur over a four-year period gives a rough idea of where they are known to occur around the globe. © WGBH/NOVA, adapted from "Tornadoes Around the World," by Fujita, T. T. 1973 Weatherwise, Volume 26, pp 55-83.

Tornado countries

Not surprisingly, the planet does have other tornado seedbeds, and some occasionally germinate twisters to rival the nastiest the U.S. has to dole out. In raw numbers, Canada probably comes in second to the U.S. The same climatological regime that brings tornadoes to the southern Great Plains in early spring moves north through the year to unleash more of the same on western Canada in July.

After the U.S. and Canada, Bangladesh and eastern India probably get the most violent tornadoes; they certainly suffer the deadliest. On April 26, 1989, the most lethal tornado on record swept Bangladesh, killing about 1,300 people, injuring 12,000, and leaving 80,000 homeless. High population density, flimsy housing, and a nonexistent tornado warning system mean killer tornadoes are all too common there, says Jonathan Finch, a meteorologist at the National Weather Service in Dodge City, Kansas, who is an expert on that region's tornado climatology.

The only continent where tornadoes have not been reported is Antarctica.

Europe may come next in sheer number and intensity. Tornadoes have been reported across the continent, from Scandinavia south to Italy and from Spain east to European Russia. The United Kingdom gets more than 30 tornadoes a year, which per unit area compares with the Great Plains. But as in Florida, they're sired by nonsupercell storms and thus pack a much softer punch.

The Southern Hemisphere has several hotspots, including southeastern and southwestern Australia; the region encompassing northern Argentina, Uruguay, and southern Brazil; and southeastern South Africa. Records show that more than 200 tornadoes hit South Africa in the 20th century, including one in 1999 that knocked down part of a pharmacy in which President Nelson Mandela was shopping at the time (he was uninjured).

Virtually all areas that regularly see tornadoes share one thing in common—they lie 20° to 50° on either side of the equator, in the mid-latitudes. "You could probably get a tornado anywhere on the planet, but there are places where they are far less frequent," says John Snow, a tornado expert at the University of Oklahoma. "For good meteorological reasons, these tend to be in the tropics and the very high latitudes." The only continent where tornadoes have not been reported is Antarctica.

Tornadoes strike not only many countries other than the U.S. but their cities as well. Perhaps the earliest tornado on record for London descended on October 17, 1091, killing two. The one that damaged this house in West London appeared in 1954. © Corbis Images

Telling it like it is

In 1973, the year his Fujita Scale went into effect, tornado expert Theodore Fujita called for the creation of an international tornado reporting system. Such a system would help in better understanding and preparing for potentially devastating tornadoes. Yet 30 years later, no such thing exists. International reports remain so incomplete that rather than rely on them, Brooks has taken to studying environmental conditions worldwide to make estimates of the frequency of the severe thunderstorms that give rise to tornadoes.

The situation is improving. This November representatives from across Europe will meet about creating a Europe-wide tornado database. A similar database for Bangladesh tornadoes is in the works. Perhaps even Russia and China—two nations notorious for keeping mum about their tornadoes—will begin to collect and share reliable data. Both countries are thought to get their share of deadly tornadoes. One tornado north of Moscow in 1984 may have been an F5—the most destructive type of funnel—and southeastern China is regularly pounded by typhoons that likely trigger tornadoes. Perhaps it's just a matter of time before we have a truly accurate notion of how many tornadoes are not American-made.

That won't change where the most and worst tornadoes occur, however. The Great Plains, as my dreaming mind knows, will always hold that distinction. Incidentally, a mea culpa: I did describe my dream. For the very moment I realized I was surrounded, I got so spooked I woke up.