How do tornadoes form?

There are thousands of thunderstorms in the United States each year, but only a few become strong enough to spin up a tornado.

Few weather phenomena capture the imagination—or instill fear—like tornadoes. These swirling columns of wind can level entire communities in minutes, yet much about their formation remains shrouded in mystery. Still, scientists have uncovered key clues about how these powerful forces emerge from severe thunderstorms.

Tornadoes don’t just appear out of nowhere. They need the perfect storm to form. It starts when warm, humid air clashes with cold air, which can generate a powerful thunderstorm known as a supercell.

Although this can happen in many different parts of the world, it happens most often in the Plains, Midwest and Southeast regions of the United States, where the flat landscape across the center of North America allows cold air from the Arctic to dive south into the moisture-rich air from the Gulf. Thousands of thunderstorms form this way during the spring and summer, but only a few produce tornadoes.

That's because one other ingredient is needed: wind shear. Wind shear, which is when winds move at different speeds and directions in different parts of the atmosphere, is present in only the most severe thunderstorms. This often creates a horizontal spin in the air.

The leading research on tornado formation says that the horizontal vortex of air turns vertical inside a severe thunderstorm with updrafts and downdrafts helping to create a mesocyclone, the rotating parent cloud of a tornado. A study published in 2018 found that tornadoes form from the ground up to the mesocyclone, contrary to popular thought.

While most tornadoes form within powerful thunderstorms and follow the classic mesocyclone model, they’re not the only swirling threats out there. From landspouts to waterspouts and gustnadoes, more than a dozen other types of atmospheric vortexes exist—each with their own unique origins and behaviors.

Scientists continue to study these phenomena, pushing the boundaries of what we understand about some of nature’s most intense and unpredictable forces.

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