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Why does the U.S. have so many more tornadoes than other countries?

Why does the U.S. have so many more tornadoes than other countries?

Oklahoma’s devastating tornado, which killed at least 24 people and injured more than 200 others, is drawing comparisons to past U.S. twisters today, including the massive tornado that hit the same region in 1999. And the United States has plenty of examples to draw from. According to the National Oceanic and Atmospheric Administration (NOAA), the United States, in averaging more than 1,000 tornadoes each year, is by far the global leader when it comes to number of twisters recorded. Canada finishes a distant second with roughly 100 per year.

Here’s NOAA’s map of the regions of the world that are most likely to experience tornadoes. In addition to the United States and Canada, the organization highlights many European countries and parts of other nations including Argentina, South Africa, Bangladesh, and Japan (click on the image below to expand):

So why is the United States so disproportionately prone to tornadoes? According to a Discovery Channel explainer on the subject, the distinction is a result of climatology, geography, and topography (the NOAA image at the top of this post shows this week’s storm system over Moore, Oklahoma):

[T]he United States has an abundance of flat, low-lying geographic regions, and it also has a climate that is conducive to intense thunderstorms, and tornadoes tend to form during thunderstorms.

Turning for a moment from topography to geography, the United States has a few places that might be called tornado hotspots. Most prominent among them, of course, is “Tornado Alley,” a slice of America’s mid-section running horizontally from Texas up to North Dakota — taking in portions of Oklahoma, New Mexico, Colorado, Kansas and Nebraska….

Tornado alley’s tornadoes usually happen later in the spring time and sometimes into the fall. The region is considered a prime breeding ground for supercell thunderstorms, which tend to produce the strongest tornadoes. Supercell thunderstorms contain something called a mesocyclone, which has a rotating updraft — they’re very dangerous but also, when identified as supercells, can provide a good heads-up that the extreme weather they can produce, like tornadoes, is possible….

Florida, too, has lots of tornadoes. That’s because the state has many thunderstorms on a daily basis, and it’s also a pit stop for many tropical storms or hurricanes (the tropical storms and hurricanes don’t tend to produce the kind of killer tornadoes that come about during non-tropical storms).

While the United States leads the world when it comes to sheer volume of tornadoes, the ranking changes when you apply other filters. The United Kingdom, for example, has more tornadoes relative to its land area than any other country (a fact one expert attributed to the country’s position on the Atlantic seaboard, at the nexus of polar air from the North Pole and tropical air from the Equator). And factors such as high population density, ineffective warning systems, and shoddy infrastructure mean tornadoes can be particularly deadly in countries like Bangladesh, which experienced a tornado that killed 1,300 people in 1989.

Writing for PBS, Peter Tyson points out that America’s tornado tally may be so high relative to the rest of the world in part because other countries aren’t as diligent about recording twisters. And he adds that all nations that experience tornadoes have something 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 twisters have yet to strike? Antarctica.