A tornado is a vortex or powerful whirling wind, often visible as a funnel-shaped cloud hanging from the base of a thunderstorm. It can be very violent and destructive as it moves across land in a fairly narrow path, usually a few hundred yards in width. Wind speeds are most often too strong to measure with instruments and are often estimated from the damages they cause. Winds have been estimated to exceed 350 mph (563 kph). Very steep pressure gradients are also associated with tornados and contribute to their destructiveness. Sudden changes in atmospheric pressure taking place as the storm passes sometimes cause walls and roofs of buildings to explode or collapse.

In some places such storms are referred to as cyclones. Tornados most frequently occur in the United States in the central plains where maritime polar and maritime tropical air masses often meet, producing highly unstable atmospheric conditions conducive to the development of severe thunderstorms. Most tornados occur between noon and sunset, when late afternoon heating contributes to atmospheric instability.