eli5: why are tornadoes common in north America but not other parts of the world?
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Tornados – well, the big and strong tornadoes, anyway – typically form in a very specific kind of storm that forms when powerful cold fronts and powerful warm fronts meet. Since this requires cold and warm air to both be present, it mostly happens in the mid-latitudes, with warm air flowing in from the tropics and cold air flowing down from the poles.
However, Earth’s winds mostly just blow in one direction by default (east to west near the equator and west to east elsewhere). So it’s relatively rare for a large polar air mass to dip towards the equator, or a large tropical air mass to head up to the midlatitudes, and especially rare for two such large masses to meet in a way that forms a powerful midlatitude cyclone.
But the shape of North American geography creates a unique opportunity. The Rocky Mountains form a massive barrier to the usual west-to-east air movement at that latitude, and so there’s less air flow from the Pacific to the central US than there would normally be. That creates a sort of “suction” effect that draws down polar air from the Canadian Arctic and dry air that has been moisture-squeezed by its passage over the Rockies. At the same time, the Gulf of Mexico provides a large and warm body of water to produce warm, moist tropical air that flows northward into the US.
These two dominant air masses meet over the Great Plains, and produce the most powerful midlatitude cyclones in the world as a result. And it’s those cyclones that produce most, and the most powerful, tornadoes.
The other areas in the world where this could happen lack the appropriate geography:
* In South America, the Andes block the west-to-east flow. But there’s no equivalent of the Gulf of Mexico, and the cold air is flowing up from the relatively narrow southern tip of South America, rather than a vast continental interior with lots of cold, dry air. South America – in the region of southern Brazil, Paraguay, Uruguay, and northern Argentina – does get some tornadoes, though; it’s the second most active area in the world for them.
* In southern Africa and Australia, the land doesn’t extend far enough into the midlatitudes to reach the area where tornadoes would form.
* In China, the flow of warm tropical air from the Indian Ocean is blocked by the massive Himalayan mountain range. In Eastern china, where some tropical air can flow in from the South China Sea, tornadoes do happen, but there’s less moisture than is available from the Gulf of Mexico. There’s also a small hotspot for tornadoes near Bangladesh, where cold dry air from the Tibetan Plateau meets the Indian monsoon air off the Bay of Bengal.
* Europe sits too far north, and lacks all three of a mountain range to its west (it has the Atlantic Ocean), a tropical moisture source (the Mediterranean provides some moisture, but the Sahara’s also right there), *and* a source of dry continental air (Europe has the relatively warm North Sea to its north).
TLDR: North America has the perfect storm of a tropical sea to its equatorward side, a big mountain range on its western side, and a continental interior on its poleward side.
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