Tornadoes form out of thunderstorms, where moist air rises, cools and condenses into clouds that release heat and force cooler air back down. If the updrafts are strong enough, the feedback loop forms an air vortex that continues to shunt more moist air upwards and eventually forms a tornado.
Should the feedback loop reach critical mass, the vortex descends out of the thundercloud and becomes a tornado, reaching towards the ground. The destructive force of a tornado comes from the kinetic energy of the swirling air, easily reaching speeds of 200 to 300 miles per hour. Should it touch the ground, it can rip apart structures and fling debris at lethal speeds. Small tornadoes may only last a few minutes, while large tornadoes can last for hours and cover stretches over 90 miles.
Tornadoes are self-sustaining as they continuously pull air upwards into the reaction to support themselves, so scientists are not sure how exactly they dissipate. The most supported theory is the tornado's need for instability and rotation. If the air intake or the moisture is removed, the feedback loop is broken and the tornado ceases to rotate, stopping the upward flow of additional air. Temperature changes may also break the cycle, as hot and cold air creates the instability that tornadoes need.