Tornadoes originate in thunderstorms. The May tornadoes emerged from a network of hundreds of thunderstorms over Oklahoma and southern Kansas. While tens of thousands of thunderstorms occur every year, swarms of thunderstorms are rare. And not every thunderstorm forms a tornado. To find out what causes a tornado, we must first learn what causes thunderstorms. A storm requires two things: moist air and a force to make that air rise and become unstable. In May, warm, humid air came from the Gulf of Mexico. When it reached Oklahoma, it collided with the colder air moving south from Canada. What made that air rise?
Warm air is lighter, and more buoyant, than cold air. In other words, warm air rises above cold air. The humid air inside a thunderstorm can rise as high as 21,000 meters. As the air from the Gulf of Mexico ascended, it encountered wind shear -- a gradual change in the speed and direction of the wind at different altitudes. The wind turns the rising air, forcing it to rotate. This rotating column of air is called a mesocyclone.
What causes a tornado to develop out of this humid column of rotating air? No one knows for sure. In fact, most mesocyclones don't spawn tornadoes. But something happens to shape all that rotating and rising air into a more compact column -- a tornado -- which then spins down to the ground.