Home > Disasters > Severe Storms

Statistically, lightning poses a greater threat to individuals than most other natural hazards.
On average lightning causes 5 to 10 deaths and over 100 injuries in Australia each year.
Severe storms can occur anywhere in Australia and cause more damage than any other natural hazard.

In 1996, of the 23 natural disasters in Australia, each with total estimated costs of $5 million or more, 15 were severe storms, accounting for $772 million of the total $1,258 million.

Above: Storm battered street in North Queensland (photo courtesy of Cairns Post).

Severe storms normally occur in localised areas and don't usually affect areas as large as cyclones or floods.

Severe storms are the most frequently occurring hazard in Australia, particularly Queensland.
Most severe storms in Queensland occur between the months of September and March, and can be divided into two categories - severe thunderstorms and land gales.

Severe Thunderstorms

Severe thunderstorms develop when dense, cold air overlies less dense, warm, moist air. They are triggered by solar heat, a front or a trough.

Strong rising currents of air develop and heat energy stored in the air and water vapour is converted into electrical energy.

When the atmosphere is particularly unstable and the wind flow provides an efficient input of energy to a growing cloud, a severe thunderstorm develops with accompanying up-draughts and down draughts.

The Bureau of Meteorology defines severe thunderstorms as those which produce:

Most thunderstorms are not severe enough to produce these phenomena, but they all produce lightning.

Heavy Rain
Intense up-draughts produce raindrops through condensation of moist air. As raindrops become too large to be supported they fall, producing heavy rain which can exceed 200 mm per hour, causing flash floods.

Hail
Hailstones form in a thunderstorm when raindrops freeze at high levels and then recycled through up and down draughts, growing all the time. Hailstones larger than cricket balls have been observed in Australia, and such large, jagged ice hazards can inflict serious damage or even fatal injury.

Lightning and Thunder
Lightning is the discharge produced when differences in ground and atmospheric electrical charges are large enough (several hundred-million volts) to overcome the insulating effect of air. An average thunderstorm can release several hundred megawatts of electrical power.

Right: Severe lightning in Sydney. Photo courtesy of Australian Severe Weather.

Lightning strikes may occur within a cloud, between clouds, or between the cloud and the ground.
Thunder is the sound produced by the explosive expansion of air heated by the lightning stroke to temperatures as high as 20,000 degrees Celsius.

Land Gales

Land gales are gale force winds over land with a speed of 62 km/hr or more.

Land gales are caused when large differences in atmospheric pressure are concentrated over a small distance. This can occur between a low pressure system and strong high pressure systems, or near intense cold fronts.

In northern Queensland the strongest winds usually occur in summer and autumn, often due to tropical cyclones.

Severe Thunderstorm Warnings

Severe thunderstorm warnings are issued by the Bureau of Meteorology www.bom.gov.au/weather/qld.

A severe thunderstorm warning is issued when:

• A severe thunderstorm is reported, or there is strong evidence of a severe thunderstorm, and it is expected to persist;
• Existing thunderstorms are likely to develop into a severe thunderstorm.

How often are warnings issued

While the threat remains, a severe thunderstorm warning will be issued every three hours, however more detailed city warnings may be issued every 30-60 minutes.

Severe Weather Warnings

These warnings are issued by the Bureau of Meteorology (www.bom.gov.au/weather/qld/) when severe weather is expected that is not directly related to severe thunderstorms, tropical cyclones or bushfires.

Examples of severe weather include damaging wind gusts, land gales, squalls, storm tide, flash-flooding, dangerous surf, east coast low, monsoon low.

A severe weather warning is issued when

• severe weather is expected to affect land-based communities within 6-24 hours; and
• it is not directly the result of severe thunderstorms; and
• it is not covered by tropical cyclone or fire weather warnings.

How often are warnings issued

While the threat remains, a severe weather warning will usually be issued every six hours, however the more frequent warnings may be issued in some serious circumstances.