- Gales, Southern Australia, 1948 and 1994
Hot, humid, unsettled weather conditions. An approaching cold front or trough. The ideal conditions for severe thunderstorms, and their progeny of flash flooding, large hail, destructive wind gusts, and even tornadoes. Most Australians can vividly recall at least one major thunder-, wind- or hailstorm in their area, and occasionally these storms are of such note to be talked about for many years afterwards. In Melbourne, the February 1972 flash flood was such a storm; in Brisbane, the tornado of November 1973, in Sydney, the 1990 and 1999 hailstorms.
Hail-stones collected during the Sydney hail-storm of 14 April 1999 compared with a seven centimetre diameter cricket ball. (Photo courtesy of Milton Speer, Bureau of Meteorology)
According to Emergency Management Australia, severe thunderstorms cause more damage in Australia each year than any other natural hazard, and the damage bill in individual cases has gone into the hundreds of millions of dollars. Hail causes the greatest proportion of the damage, accounting for nearly half the total losses from severe storms. Insurance losses in the storm of 14 April 1999 exceeded $1.5 billion.
A storm over Sydney Harbour. (Photo courtesy of Mike Langford)
When are severe storms most likely?
Although all parts of Australia experience severe thunderstorms, their preferred time of occurrence varies. In most of northern Australia, autumn and spring are the most likely time, in eastern Australia it is the late spring/summer period. In southwestern Australia, they are most common in winter in areas nearer the coast, but in spring-early summer further inland. As a rule, severe storms are most common in New South Wales, Queensland and parts of Western Australia, and least common in Tasmania.
The Fujita scale of tornado intensity.
||Wind Speed (km/hr)
||Wind Speed (kts)
||Damage to chimneys and windows; branches torn off trees.
||Roof peels off; caravans bowled over; cars pushed off roads.
||Roofs torn off houses; large trees snapped or uprooted
||Roofs, some walls torn from well constructed houses; most trees in forest uprooted.
||Well-constructed buildings levelled; cars thrown, large missiles generated.
||Strong frame houses lifted, carried and disintegrated; steel-reinforced buildings severely damaged.