Tornadoes are a potential outgrowth of severe thunderstorms. A tornado is formed of rapidly rotating wind that blows around a small area of intense low pressure. The overall diameter typically ranges from 300 ft to 2,000 ft, but some are as small as a few yards or as wide as a mile. A tornado's circulation on the ground is marked by a funnel-shaped cloud or a swirling cloud of dust and debris. When viewed from above, most tornadoes are observed to rotate counterclockwise. Tornado wind speeds generally range from 40 to 280 mph or even higher.

The first stage of most tornadoes is the dust-whirl stage. As dust swirls up from the ground, it marks the tornado's circulation on the ground and displays a short funnel reaching down from the base of the thunderstorm's cumulonimbus cloud. Damage during this stage is usually light. During the organizing stage, the funnel extends downward and wind speeds increase. Damage is most severe in the mature stage; here, the tornado reaches its greatest width and stands almost vertically. The shrinking stage consists of a narrowing of the swath at the surface, an overall decrease in the funnel's width, and an increase in the funnel's tilt. Finally, the decay stage finds the tornado stretched thin and ropelike, contorting and then dissipating.

Because the lifetime of a tornado is generally brief and because, more often than not, tornadoes occur out of the range of weather stations or anemometers due to their relatively small size, the measurement of wind speeds by conventional methods is a difficult if not an impossible task. Measured wind speeds are rarely available and often instruments that are in the path of the storm are destroyed by the intensity of tornadic winds. The Fujita Scale, or F-Scale, was developed as an indirect method for classifying tornado intensity in the aftermath of the event based on observed damage. Because reliable estimates of wind speed are not generally available until long after a tornado occurs, AIR does not post loss estimates in real time.