Reasons why the largest number of and strongest tornadoes occur in the U.S.:
1. Cold dry air comes from the North, from Canada.
2. It collides with warm, moist air from the Gulf of Mexico.
3. Warm air rises through the cold air, water vapor condenses, forms clouds which
develop into thunderstorms.
4. The greater temperature difference between the cold air and dry air, the faster the warm air shoots up through the atmosphere and this leads to severe thunderstorms instead of ordinary air mass
thunderstorms.
5. These severe thunderstorms can then spawn tornadoes.