Tornadoes are completely natural -- humans have witnessed them on every continent except for Antarctica. However, the United States has far more observable tornadoes than any other country. This is because the 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. (There are other definitions of Tornado Alley's geographic locations; this one is based on the National Oceanic and Atmospheric Administration's reckoning of the incidence of tornadoes [source: NOAA].)
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 [source: University of Illinois].
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) [source: NOAA].
No matter where they happen, though, one thing's for sure: Tornadoes can be lethal. They kill, on average, about 60 Americans a year, with wind speeds that reach 320 miles per hour (515 kilometers per hour). The deaths are usually caused by flying debris landing on them [source: NOAA].
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