The anatomy of a storm Tornadoes and water spouts Lightning Thunder Wind categories

The storm warning comes over the radio. Outdoor furniture and loose materials are put away, windows closed and all are prepared for the onset of the winds and rain. A familiar occurrence in coastal Australia, storms form a very important component of our weather systems. Their violence is often destructive, their rain welcome and power awe inspiring.

Severe storms are more common than any other natural hazard and they occur everywhere in Australia. On average, unclassified storms are responsible for more damage each year than tropical cyclones, earthquakes, floods and bushfires. Each year between 5 and 10 people are killed by lightning. Tornadoes have caused at least 41 deaths in Australia's short recorded history.

The anatomy of a storm
A storm is produced when hot air and cold air mix, releasing huge quantities of energy and creating strong winds, rain, hail, lightning and thunder. Sometimes very large storms are driven by massive quantities of hot moist tropical air, these types of storms are known as cyclones in Australia or as hurricanes and typhoons overseas.

Tropical cyclones occur in northern Australia during summer when the overhead Sun evaporates immense quantities of water creating huge reserves of stored thermal energy. The warm moist rising causes wind to blow in towards the centre of the storm while the rising air soon cools forming storm clouds and releasing some of the stored energy from the moist air. As more and more energy is released and more hot moist air is fed into the storm, it becomes larger and stronger eventually reaching proportions large enough to sustain itself and become a cyclone.

Tropical cyclones soon die out when they move over land because the source of energy, moisture from the sea, is no longer available to drive the cyclone.

Tornadoes and water spouts
Tornadoes occur when hot moist tropical air meets cold dry air. Tornadoes are especially well known over the southern states of the U.S.A. but also occur in Australia, mainly in south-eastern Australia.

A tornado is a very violent windstorm in which the air whirls rapidly upwards in a vortex, forming a funnel shaped cloud. Tornadoes are associated with larger thunderstorm activity and form in the edges of the storm clouds and descend until they reach the ground. They can range in width from a few metres to hundreds of metres, their winds have been measured at more than 450 km/h.

There is no generally agreed theory for the formation and maintenance of tornadoes, waterspouts, and other vortices. Eyewitness accounts associate particularly bright blue lightning in the eye of tornadoes and theories involve both electrical activity and the formation of circulating winds formed by the enormous energy released when large quantities of warm moist air are suddenly cooled when two air masses meet.

The largest and most dangerous tornadoes definitely occur in the southern U.S.A. with over 1000 tornadoes recorded in some years. In Australia we quite often see much smaller vortices such as 'Willy-Willys', 'Dust Devils' or 'Whirl Winds'.

Waterspouts are tornadoes that occur over water. The whirling air sucks up water creating a very well defined column of rising water. Fish caught in waterspouts have been known to fall to the ground many kilometres from the sea having been carried by the waterspout and associated storm.

Lightning
The lightning flashes that accompany thunderstorms are enormous electrical sparks caused when electrical charges build up in the storm clouds.

Most lightning occurs within or between adjacent storm clouds. Friction within clouds created by updraughts and the movement of air creates the build up of negative and positive charge within a storm cloud. Lightning within clouds occurs when enough charge difference is built up and an electrical discharge jumps from a negative region to a positive region within the cloud.

Lightning strikes from cloud to Earth occur slightly differently. Here negative charge build up within a cloud repels negatively charged electrons within the ground and induces (or creates) a positive charge on the surface of the Earth. Lightning will tend to strike the nearest place of accumulated charge on the ground, hence church spires, trees and even golf clubs can act as lightning conductors as they allow the accumulation of charge to occur.

Lightning itself involves a 'leader' stream of electrons descending from the cloud towards the ground in a series of jagged steps and branches as it tries to find the path of least resistance. Once the lightning strikes the ground, it tries to dissipate its charge again along the route of least electrical resistance. This may be back up to the cloud along the original lightning strike path, so what at first appears to be a single movement of the lightning can actually be a series of rebounds back and forth between the cloud and the ground.

The light from lightning is created as the electrons steaming along the lightning path, smash into and ionise molecules in the air making them emit light that we see as the flash.

Thunder
Thunder occurs when lightning dramatically heats nearby air, expanding the air very quickly. This creates an area of low pressure around the lightning flash. Upon cooling, surrounding air rushes back into the low-pressure zone. The sound we call thunder is created as part of the initial explosion of air and as the inrushing air meets again.

Wind categories
The Bureau of Meteorology uses the Beaufort Wind Scale to measure wind strength. Using this scale, it is possible to estimate wind strength and understand the effects that may be expected from a wind warning in a forecast.

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