A severe thunderstorm is defined by the Bureau of Meteorology as one which produces hail with a diameter of 2 cm or more; or wind gusts of 90 km per hour or greater; or flash floods; or tornadoes; or any combination of these. Severe storms can range from an isolated thunderstorm to a widespread intense low pressure system.
Thunderstorms occur when humid air near the ground is lifted by converging surface winds and rises quickly in an unstable atmosphere and typically do not last long. In Australia there are three recognised types of thunderstorms: single-cell thunderstorms which are of limited size and lifespan and can produce short bursts of severe weather; multi-cell thunderstorms which are more persistent and have a greater impact; and super-cell thunderstorms which are even more persistent with a constant rotating updraft known as a mesocyclone.
All thunderstorms will exhibit some lightning, atmospheric discharge of electricity, and result in thunder. Mature storms will produce storm wind gusts which are of speeds of 90 km per hour or more, with peak winds exceeding 160 km per hour in some storms.
Image: Storm front, Sydney 2008, Jonny Ross (Flickr)