What causes a tornado?

Answer

According to the Weather Channel, tornadoes can occur when powerful thunderstorms move through areas of atmospheric instability and excess low-level moisture. Under these conditions, a cold front or wind shear zone can create the rotating and rising air currents necessary to create a mesocyclone, the precursor to a tornado. Not all such storms spawn tornadoes, but any time thunderstorms occur under dangerous conditions, meteorologists take note.

The precursor to any tornado is the mesocyclone, an area of rotation that forms high in a thunderstorm cell. So-called "supercell" storms are more prone to develop into tornadoes, but even relatively weak storms can create funnel clouds if the conditions are suitable. Tornadoes commonly occur when a cold front overlaps a warm front, preventing warm air from rising directly up into the atmosphere and creating rotating air currents. This draws in air from the surrounding area, and the moisture in this air condenses into a heavy cloud layer called a wall cloud. Inside the cloud, the rotation intensifies, extending high into the atmosphere and creating a funnel-shaped vortex. Eventually, the vortex may twist and work its way back down to ground level, wrapping itself in the wall cloud and creating the distinctive silhouette of a tornado.

1 Additional Answer
Ask.com Answer for: what causes tornado
Clouds form when water vapor condenses in the air. This releases heat, which increases the updraft temperature and the kinetic energy of air movement. In supercell thunderstorms, updrafts are strong and may create a vortex of air that may form a tornado.
Most scientists consider this to be a simplified and incomplete explanation. The finer details of tornado formation and dissipation are still unknown.
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