Hannah Ballew, at Westminster High School, wonders what conditions spawn tornadoes and keep them going. It requires a collision of warm, moist surface air with colder air aloft. The warm air becomes buoyant, rises into the colder air and forms a thunderstorm. If the rising air encounters "shear" -- winds aloft moving in a different direction -- it begins to spin. If it touches ground, it's a tornado. In 1917, perfect conditions kept a twister going for seven hours, across 293 miles.