Severe weather

With the atmosphere always in motion, air speeds sometimes become high enough to cause considerable damage.

At the smallest scale, local 'hot spots' can develop on the ground, drawing in cooler air from around them and causing spiralling of the air. Such 'dust devils' or 'willy willies' move randomly over the ground but are too small to cause much damage.

Severe thunderstorms are very localised, so their devastating effect is often underestimated. They are more frequent than any other natural hazard. Deaths are caused by lightning and wind-borne debris. Large hailstones can also cause severe damage. Thunderstorms are associated with a very tall cloud mass reaching up to 15km. They are constantly evolving with organised regions of upward moving air (updraughts) and downward moving air (downdraughts). While a typical thunderstorm lasts about an hour, severe thunderstorms are much longer. Here wind shear - a marked change in wind speed with height - tilts the updraughts, causing precipitation falling from them to move into the downdraughts, allowing the updraughts to persist instead of being destroyed by the cooling effects of precipitation falling within them.

Tornadoes have violent winds which may exceed 400km/hr. These rotating funnel clouds, extending down from the cumulonimbus clouds of severe thunderstorms, may be only several hundred metres across. In some regions they cause massive destruction on a track rarely more than 20 or 30 kilometres long.