A booklet for Students and the Community
Chapter 5
Severe
Storm Hazards and Disasters
SEVERE STORM
CATEGORIES
Severe storms can be divided into two types, severe thunderstorms and land gales:
Severe Thunderstorms
By definition, these produce:
u flash flooding; or
u damaging hailstones (2 cm diameter+); or
u destructive wind gusts (90 km/h+); or
u tornadoes (or a combination of any of the above).
Thunderstorms that do not produce any of these dangerous phenomena are not regarded as severe but may still cause death, injury, or property damage due to lightning strikes.
What Causes Them? - Thunderstorms develop when dense cold air overlies less dense, warm, moist air, resulting in strong upward currents and conversion of heat energy into wind and electrical potential. When the atmosphere is especially unstable and windflow can provide the most efficient input of energy to the cloud, a severe thunderstorm develops, with well-organised, complementary up- and down-draughts, capable of producing the following:
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 intensities of 200 mm per hour, causing
flash floods.
Hail - Hailstones form in a thunderstorm when raindrops freeze at high levels and then are recycled through up- and down-draughts, growing all the time. Hailstones larger than cricket balls have been observed in Australia. Such large, usually 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. Lightning
strokes may occur within the cloud, between clouds, or between the cloud and ground. Thunder is the sound produced by the explosive
expansion of air heated by the lightning stroke to temperatures as high as 20,000°C.
Tornadoes - Tornadoes (or ‘twisters’) are rapidly-rotating columns of air that descend in a funnel shape from thunderstorm clouds and should not be confused with ‘willie willies’ (or ‘dust devils’) which are much smaller, and not usually associated with storms. A tornado vortex, which can range in diameter from a few metres to several hundred metres, usually whirls clockwise (southern hemisphere) and contains extreme winds that may exceed 450 km/h (ie the top end of the F5 range on the Fujita Scale) with a forward speed of 30 to 100 km/h. The combined action of powerful rotary winds and low air pressure in the centre can collapse buildings and hurl lethal debris, or even large vehicles, through the air. In June 1997, an F5 twister in Texas, USA which killed 27 people, actually ripped bitumen from roads, sucked fence posts out, left bare slabs where houses once stood and sent railway wagons literally flying! In the USA even tornadoes with multiple vortices, rotating in a cluster have been observed. Tornadoes are not confined to the USA as many people believe, in fact estimates suggest that up to 10% of the world’s twisters occur in Australia. Due to their usually smaller size and our very low average population density however, relatively few affect people and property or are actually observed and reported. Nevertheless, tornadoes have caused over 40 deaths and 500 injuries in Australia and hundreds of millions of dollars damage (see examples following).
Extreme Wind Gusts - In a mature thunderstorm, falling rain and hail drag the surrounding air down, causing a strong down-draught which accelerates as the air cools due to evaporation of rain drops. It spreads out upon reaching the ground producing a cool gusty wind which can be strong enough to cause significant damage. At their most violent, these dangerous, damaging gusts are known as ‘micro-bursts’ and ‘down-bursts’ which can snap large trees in two and demolish buildings.
Land
Gales
These are simply ‘gale force’ (62 km/h or greater) winds over the land, and usually affect a very much larger area than thunderstorms. They can also last for up to several days (much longer than thunderstorms) and often cause significant damage (see Example 7). Land gales occur when large differences in atmospheric pressure are concentrated over a relatively small distance. This happens between a deep low pressure system and a large high pressure system, or near an intense cold front.
WHERE AND WHEN DO SEVERE STORMS OCCUR?
At any time of the year severe thunderstorms can occur throughout Australia. Most strike from September to March when solar energy is greatest, but severe winter storms are common in Western Australia. The frequency of land gales is difficult to specify. In southern Australia, extreme winds tend to occur in winter and spring. In the tropical north, extreme winds usually hit in summer and autumn, often due to cyclones.
Sparse population in many areas and lack of observations over much of the continent make severe thunderstorm distribution studies difficult. Records suggest that most occur in a crescent from Brisbane, through coastal NSW and Victoria to Adelaide, but it is likely that many severe thunderstorms strike less-populated areas without being recorded. The most-damaging individual storms, including tornadoes, have hit south-eastern Queensland and the central NSW coast, south to Sydney.
EFFECTS AND COST
Dangers to People - Statistically, lightning poses a greater threat to individuals than most other natural hazards. On average, it causes 5 to 10 deaths and over 100 injuries in Australia each year. Deaths and injuries also occur when boats are hit by thunderstorm squalls, trees or limbs fall, buildings are damaged, or debris is hurled about in high winds. As mentioned, tornadoes also cause deaths.
Huge Damage Bills - Severe storms can occur anywhere in Australia, doing so more frequently, and causing more physical damage (including vehicles, buildings and crops) than any other natural hazard. In 1996, for example, of the 23 natural disasters, each with total estimated costs of $5 million or more, 15 were severe storms, accounting for $772 million of the total of $1,258 million. (Only droughts cost more). On average, about 35% of storm damage is insured.
AUSTRALIAN SEVERE STORMS - Examples
covering all associated hazards.
1. 26 January 1971
Severe thunderstorms dumped torrential rain on Canberra, ACT causing flash floods in the Woden Valley where 7 deaths resulted, mainly as cars were swept from the road into an overflowing creek.
2. 13 November 1976
Near Sandon, in central Victoria, two people were killed when a tornado with winds estimated at 300 km/h (F3) lifted their car up 9 metres in the air and hurled it 100 metres into a ditch. It left a trail of destruction 400 metres wide and 6 km long, which included wrecked vehicles, homes, farms and crops.
3.
18 January 1985
A severe hailstorm and tornado passed over Brisbane, breaking thousands of windows, damaging many cars and tearing roofs off. In just 30 minutes, it caused insured losses of $299 million and a total estimated cost of $385 million (1997 values).
4. 18 March 1990
Australia’s costliest hailstorm pelted Sydney’s south-western suburbs with giant hailstones as large as oranges, resulting in severe damage to homes, businesses, thousands of vehicles, trees and powerlines. An insured loss of $384 million and total estimated costs of $550 million (1997 values) resulted.
5. 21 January 1991
Our most-damaging severe thunderstorm with winds estimated at 230 km/h, large hail and torrential rain, tore through northern areas of Sydney, causing one death and injuring over 100 people. It damaged over 7,000 homes, hundreds of businesses, and downed 140 km of powerlines including three steel towers. The extreme winds which were thought to have included a tornado also felled or damaged at least 50,000 significant trees! Insurance losses of $226 million and total estimated costs of $670 million (1997 values) occurred.
6. May 1994
Perth, Mandurah and south-western WA suffered violent winds up to 140 km/h during a storm which seriously damaged 600 houses, caused two deaths and 20 injuries, blacked-out 60,000 homes and caused heavy losses to commerce and industry.
7. November 1994
The worst land gales for 10 years lashed Victoria, ACT and NSW for several days at up to 145 km/h. 500,000 Melbourne region homes were blacked-out, many being damaged. Huge duststorms spread for hundreds of kilometres in dry conditions removing millions of tonnes of topsoil.
8.
29 September 1996
A very powerful severe thunderstorm struck Armidale, NSW with 160 km/h winds and 70 mm hail causing damage, ranging up to severe, to 90% of the town’s 6,500 houses. Many large buildings and 5,000 cars were also damaged. Three tornadoes were reported from surrounding areas adding to the losses which were $150 million (insured) and a total estimate of $340 million (1997 values).
9. 27 January 1997
At Geelong, Victoria, during a thunderstorm, lightning struck a group of people sheltering under a park gazebo, killing two and injuring another four (one critically).
SEVERE STORM SURVIVAL
Most communities have damage prevention and reduction measures at local level. These may include enforcement of building standards, anchoring of light structures and caravans, clearing of tree limbs above roofs and powerlines, provision of timely weather reports and safety warnings, and the support of emergency help with temporary repairs to damage (provided by local State/Territory Emergency Services). You can minimise personal storm and lightning injury or property damage as follows:
Before the Storm Season
Trim tree branches well clear of your house. Check/clean roof, guttering and downpipes.
Have a portable radio, torch, spare batteries, and a first aid kit (and basic knowledge).
Clear backyard of loose objects that could cause damage during high winds.
Purchase masking tape (for windows), plastic sheeting and large garbage bags (for emergency rain protection).
List emergency contact numbers. Familiarise your family with this storm action guide.
As the Storm Approaches
Listen to your portable radio and disconnect all electrical appliances.
Shelter and secure pets/animals. Shelter vehicles or cover with tarpaulin/blankets.
Tape (cross fashion ‘x’ plus strips) or cover large windows.
When the Storm Strikes
Stay inside in the strongest part of the house (bathroom, cellar). Don’t use the phone.
Keep clear of windows and glass doors, electrical items, pipes and metal fixtures.
If necessary, cover yourself with a mattress, doona, blankets, tarpaulin, under a table etc.
Listen to your portable radio for storm updates.
If outdoors find solid, enclosed shelter or a ‘hard top’ vehicle (not under a tree).
If far from shelter, crouch (alone, feet together) preferably in a hollow. Don’t lie down.
If driving, stop clear of trees, powerlines and streams. Stay in car, clear of metal parts.
After the Storm Passes
Check your house for damage, listen to your radio and heed official warnings/advice.
If you need emergency assistance contact your State/Territory Emergency Service.
If unable to contact emergency services by phone, form a self-help group with family and neighbours. Watch for emergency crews who will be checking your area.
If you don’t need help, check neighbours. Don’t go sight-seeing, stay and help others.
Beware of damaged powerlines, buildings and trees and flooded streams.
STUDENTS - FIND FURTHER INFORMATION:
As a project on severe storms, find out more about:
tornadoes - frequency, deaths and damage in Australia;
common factors in the most-damaging severe storms; and
‘micro-bursts’ and ‘down-bursts’ during severe thunderstorms.
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EMERGENCY MANAGEMENT AUSTRALIA, PO
Box 1020 Dickson, Australian Capital Territory 2602, AUSTRALIA Telephone: + 61 (0) 2 6266 5402 Facsimile: +61 (0) 2 6266 5029 Email: ema@ema.gov.au AUSTRALIAN EMERGENCY MANAGEMENT INSTITUTE,
Mount Macedon Road, Mount Macedon, Victoria 3441, AUSTRALIA |
| This page was last updated May 31, 2001 |
