A severe thunderstorm is one that produces tornadoes, winds of at least 50 knots (58 mph), and/or hail at least 0.75 inch in diameter. Severe thunderstorms are more powerful than regular thunderstorms because of the extra long duration of the updraft—warm moist air that rises and condenses into a visible cumulonimbus cloud. (Within this cloud, hail forms.) In the United States, only about 10% of annual thunderstorms are classified as severe. The Great Plains is the region most prone to such storms.
Hail from severe thunderstorms is produced within the cumulonimbus cloud when frozen raindrops accumulate liquid droplets. Violent air currents carry the droplets high above freezing level, where they are covered by ice. When the particles become too heavy to be supported by the rising air, they begin to fall as hail, which can cause significant property damage.
Tornadoes are another 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. 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.
Straight-line windstorms—an often overlooked peril that can cause a significant percentage of total losses from severe thunderstorms—are localized wind events. Maximum wind speeds can exceed 100 mph. Unlike tornadoes, a straight-line windstorm has no central vortex, so there is no circular motion of the wind. An example of a straight-line wind is the downburst, which is a small area of rapidly descending air beneath a thunderstorm.
Because reliable estimates of wind speed, hail size, length, width, and duration are not generally available for the hundreds of tornadoes, hailstorms, and windstorms originated from a single severe thunderstorm outbreak until long after it occurs, AIR does not typically post loss estimates for these events in real time.