Although science is very advanced, there is not enough data to fully understand all the reasons behind tornado formation, but they are believed to occur when rotating thunderstorms form mesocyclones. Once the mesocyclone forms, temperature differentials cause air to wrap around it.
Mesocyclones are vertical columns of air rotating and rising around an axis. In addition to tornadoes, they form severe thunderstorms and hail. Scientists believe they form due to strong changes in the direction or speed of the wind. These changes cause the lower regions of the atmosphere to begin spinning in a horizontal tube-like role, invisible to the human eye. The updraft of the thunderstorm causes the spinning air to transform from horizontal rotation to a vertical rotation.
Typically, tornadoes form as the intensity of the thunderstorm begins to decrease. The momentum and vacuum in the upper atmosphere siphon the air away from the ground. Small pockets of warm air create a back draft to draw the base of the storm to the ground. Tornadoes are generally less than one mile in diameter at the ground. The length of time the tornado remains on the ground depends on the amount of warm air remaining. Once the warm air is exhausted, the tornado dies.