The National Oceanographic and Atmospheric Administration (NOAA) website notes that a tornado is a narrow and violently rotating column of air that extends from the base of a thunderstorm to the ground.
A storm brews when warm, moist air rises and mixes with cold air. Then if winds change their speed and start to rotate, causing a high-level rotating storm (called a mesocyclone), a tornado may eventually touch down.
But as to which specific combination of these these weather factors causes a tornado, no one really knows. In fact, predicting when a tornado may form is so complicated, the U.S. Weather Bureau discouraged issuing tornado forecasts until 1950.
In the U.S., about 1,200 tornadoes hit each year -- and some researchers have pointed to the nation's midsection, known as "Tornado Alley," as most likely to harbor tornado-producing weather patterns. In addition to Oklahoma, this area includes eastern South Dakota, Nebraska, Kansas, northern Texas, and eastern Colorado.
For more tornado science, check out these seven dangerous misconceptions.