Tornadoes are a worldwide phenomenon, touching down in every continent save Antarctica. They can occur wherever dry polar air sits atop moist, warm air in the troposphere, and the unstable warm air rises rapidly, forming a funnel. In fact, the United Kingdom reports the most tornadoes by land area.
But for total tornado count, there's no place like home: the United States reports more than 1,000 annually, and it's certain that Tornado Alley, the area extending from west Texas up through Kansas to North Dakota, bears the brunt.
The flat, wind-friendly Great Plains make a perfect meeting place for dry Canadian air to encounter tropical breezes from the Gulf of Mexico, both buffered by the Rocky Mountains. The result is a unique breeding ground for the world's most violent twisters.