system
Given the following post, respond as if you're commenting on Reddit:
human
Why are there massive tornadoes in US, but almost none in South America and Asia or Africa?
gpt
There have been documented tornadoes on every continent EXCEPT Antarctica (figures). But you are correct in that the majority of the earth's tornadoes happen in Tornado Alley in the United States. This is because of a very unique weather system that only the USA sees. **Cold** air moves south from Canada and mixes with **dry** air from the Rocky Mountains and this air then meets **warm, moist** air from the Gulf of Mexico (dry air from the Sonoran Desert may also play a part) and the combination of all this leads to atmospheric instability, which then produces strong thunderstorms and crazy tornadoes.

**tl;dr** Tornadoes happen everywhere except Antarctica, but the unique weather system of the USA leads to a higher frequency of tornadoes than anywhere else in the world.