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COMMUNITY SAFETY

SEVERE STORMS

 

 Storm debris

CHARACTERISTICS OF SEVERE STORMS

Severe storms are very localised events, not usually affecting wide areas as tropical cyclones and floods do, so their devastating impact is often under-estimated. These storms can occur anywhere in Australia and do so much more frequently than any other major natural hazard. On average, each year severe storms are responsible for more damage (as measured by insurance costs) than tropical cyclones, earthquakes, floods or bushfires. Unfortunately, storms also kill people; between five and ten deaths are caused by lightning strikes each year. Deaths also occur when strong winds cause tree limbs to fall, debris to become projectiles and small boats in open water to capsize. In fact, although many people believe that tornadoes do not occur in Australia, 41 tornado-related deaths have been recorded here.

The Bureau of Meteorology is responsible for provision of warnings of dangerous weather to the Australian community, with the aim of minimising injury and damage. The service is provided from Regional Forecasting Centres of the Bureau in State and Territory capitals, and information is transmitted to authorities such as Police, State and Territory Emergency Services and to radio and television stations. 

Further information can be obtained from the Bureau of Meteorology web site

WHICH STORMS ARE SEVERE STORMS? 

Severe storms can be divided into two types: severe thunderstorms; and land gales. Severe thunderstorms are more common, and generally more dangerous than land gales, which are described later.

SEVERE THUNDERSTORMS

They are defined by the Bureau of Meteorology as those thunderstorms which produce either:

Most thunderstorms do not reach the level of intensity needed to produce these dangerous phenomena, but they all produce lightning which can cause death and injury.

Lightning and Thunder
Lightning is the discharge produced when differences between ground and atmospheric electrical charge are large enough (several hundred million volts) to overcome the insulating effect of the air. Lightning strokes can occur within the cloud, between clouds or between clouds and the ground. An average thunderstorm can release several hundred megawatts of electrical power. Thunder is the sound produced by the explosive expansion of air heated by the lightning stroke to temperatures as high as 20,000º C.

Hail
Hail stones form in a thunderstorm when raindrops freeze at high levels and then grow steadily in size as they are recycled through powerful up- and down-draughts. Hailstones larger than cricket balls have been recorded in Australia. (Injury to people and severe damage to buildings or vehicles can be caused by hailstones much smaller than these!)

Wind Gusts
In a mature thunderstorm, falling rain and hail drag surrounding air downwards. Also, evaporation from raindrops cools nearby air, accelerating the downward rush. This strong downdraught spreads out upon reaching the ground, producing a cool, gusty wind which can cause damage.

Flash-Floods
The updraught of a mature thunderstorm produces rain drops through the condensation of moist air which cools as it rises. When rain drops become too large to be supported they fall, but the intense updraught of a severe storm can suspend huge amounts of rain before releasing a deluge onto the ground. Such rain can reach intensities of more than 200 millimetres per hour, provided the environment is humid enough to feed sufficient moisture to the storm. Flash-floods often result when the storm moves slowly, so that a small area receives most of the rain, but drainage and run-off characteristics on the ground can also control where the greatest impact occurs.

Tornadoes
These rarest and most violent of thunderstorm by-products are rapidly rotating columns of air that descend in the well-known funnel shape from the base of the storm cloud. A tornado vortex can range in width from a few metres to several hundred metres, usually whirls clockwise and contains winds that may reach more than 450 km/h.

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WHAT CAUSES SEVERE THUNDERSTORMS?

Thunderstorms develop when dense cold air overlies warm, moist air, which is less dense. A 'trigger' such as solar heating, a front, or a trough can then begin the development of a thunderstorm. Strong upward currents of air develop and heat energy stored in the air and water vapour is converted into wind and electrical energy. When the atmosphere is especially unstable and the surrounding wind flow happens to be arranged to provide the most efficient input of energy to a growing cloud, a severe thunderstorm develops with well-organised, complementary up- and down-draughts.

WHERE AND WHEN DO SEVERE THUNDERSTORMS STRIKE?

Severe thunderstorms can occur throughout the year, although they are very rare during the dry winter months in the north. Most strike from September to March when the supply of solar energy is greatest, but severe winter storms linked to cold fronts are common in the south west of WA.

The geographical spread of severe thunderstorms in Australia is difficult to determine because of our low population density and lack of observations over most of the continent. While records of storm impacts show that the most damaging storms have occurred in the populous south east quarter of the continent, analysis of wind, hail and tornado data suggests that severe thunderstorms are a significant threat throughout the Country. The most damaging individual storms have hit south eastern Queensland and t he central NSW coast.

LAND GALES

Land gales are simply gale force winds (62 km/h) or stronger over the land, and usually affect a much wider area and last much longer than thunderstorms. Gales blow when large differences in atmospheric pressure are concentrated over a small distance. This can happen between 'deep' low pressure systems and strong highs or near intense cold fronts.

In the southern half of Australia extreme winds generally occur in winter and spring and are usually due to land gales. In the tropical north, the strongest winds usually hit in summer and autumn, and are often due to tropical cyclones.

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PROTECTION AGAINST LIGHTNING STRIKES

In statistical terms, lightning poses a greater threat to individuals than almost any other natural hazard in Australia, accounting for five to ten lives and well over 100 injuries annually. 

During thunderstorms the following precautions should be taken:

OUTDOORS PROTECTION

INDOOR PROTECTION

FIRST-AID

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SEVERE STORMS ACTION GUIDE

Severe storms, including tornadoes, are our most frequently damaging natural hazards, and can occur Australia-wide.

AT THE BEGINNING OF THE STORM SEASON

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PROTECTING CARAVANS FROM HIGH WINDS

From November to April each year, the Australian coast north from PERTH through DARWIN and around to northern NEW SOUTH WALES faces the threat of cyclone, with consequent casualties and destruction of property. Other types of storms, even tornadoes, can strike any part of Australia at almost any time of the year. No area can be singled out as being safe from destructive winds. These notes are for the guidance of caravan users in making safe vans and equipment during times of high winds. They apply especially to caravan users in established caravan parks, but should be followed as closely as possible by those in other situations. If it is impossible or impracticable to provide the recommended ground anchors, lesser protection may be obtained by making anchors of buried logs, steel or concrete.

THE DANGER TO CARAVANS

Caravans are necessarily of lightweight construction with generally flat sides, and will be endangered in high winds unless special precautions are taken to reinforce their structure, to protect them from damage from flying debris, and to prevent them from being tossed off their supports.

POSITIONING

Placement with a narrow end facing the prevailing wind will provide some degree of protection for your van. Caution: Proper positioning alone cannot assure safety. Your van should be tied down securely, the parking brake applied and the wheels chocked. Natural barriers such as trees provide worthwhile windbreaks. Even if your caravan is securely tied down you should seek other shelter when warned of approaching high winds. In some areas flooding is associated with storms, and you should consider this w hen positioning your van.

MAKE SAFE IN STAGES

The precautions suggested in these guidelines should be taken in three stages. However, it is important to realise that even if the suggested precautions are taken, it is not possible to cyclone-proof a caravan to make it a cyclone shelter. Consequently broadcast instructions for the public to occupy shelters should be heeded immediately.

The first stage is chassis tie-down, which should be done as a matter of normal course even when temporarily located in high wind areas (principally cyclone areas). This is to save time later. Stage 1 and Stage 2 must both be carried out to make the van safe. One stage without the other is not sufficient.

The second and third stages, which are roof tie-down and equipment make-safe, should be done when a high wind warning or tropical cyclone warning is issued by the Bureau of Meteorology.

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STAGE 1 - CHASSIS TIE-DOWN

The chassis tie-down is designed to prevent the van from being blown off its supports.

Established caravan parks in high wind areas usually provide ground anchor points for chassis tie-down. In these cases use either short lengths of strong chain and turnbuckles or sound rope of at least 9.5 mm (3/8 in) diameter to secure the chassis to the ground anchor points. When these points are not provided the suggested tie-down system is shown in the drawings on the reverse side of this folder. It comprises a nylon rope anchored at ground level and attached to the van chassis. Before starting work on the tie-down, ensure that the van supports are sound and stable. The effectiveness of the tie-down will be lost if the van supports fail during high winds.

The steel pin type anchor shown on the drawings is suitable for most installations. The pin should be angled to the vertical as shown and driven into position, preferably by a jack-hammer with an adaptor on the driving ram. After driving the pin, weld to top of pin a bar or loop made from 8 mm m.s rod. Where rock is encountered, pre-drill a 25 mm (1 in) diam. hole 600 mm (24 in) (plus any earth cover) into the rock and drive a star picket to full depth of hole. Inquiries should be made and care taken to avoid damaging underground pipes or cables or other facilities when driving pickets into the ground.

When completed, the chassis tie-down will not interfere with use of the annex, but in most cases the annex will need to be removed during the tie-down installation.

STAGE 2 - ROOF TIE-DOWN

The roof tie-down keeps the van from overturning or swaying off its chassis. Because of the interference caused to the use of an annex, installation of the roof tie-down can be left until a high wind or tropical cyclone warning is issued.

The first step in roof tie-down is to remove and roll up the annex for stowage. This should be done immediately a high wind or a tropical cyclone warning is broadcast.

The most effective roof tie-down is a net firmly fixed at ground level and passing over the van roof for the full length of the van. This also affords protection against flying debris. The roof tie-down shown on the drawings is an approximation of this concept, and is simply a continuous nylon rope anchored at ground level using the same anchors installed during Stage 1, and passed over the van roof as indicated. The nylon rope should be tensioned as much as possible by hand. The corners of the caravan and the ropes should be protected from damage from each other by the use of metal angle pieces or some other form of padding.

STAGE 3 - EQUIPMENT MAKE-SAFE

After a high wind or tropical cyclone warning has been broadcast and the Stage 1 and Stage 2 tie-down of the van has been completed, as much equipment as possible should be stowed inside the van. This includes the annex, awnings and other items such as lawnmowers, bicycles, toys, chairs, garbage bins and the countless small household articles which seem to accumulate.

Heavy or bulky items that cannot be stowed in the van should be laid on their side, tied together with strong rope and lashed to suitable anchor points.

Before leaving the site to seek shelter in a place protected from wind-borne objects:

Folding caravans with canvas sides would need to be closed down before the roof tie-down is tightened. If alternative shelter is not available, go to one of the designated high wind or cyclone shelters in your area.

ROPE USE IN A CARAVAN TIE-DOWN

STAGE 1: Chassis tie-down.

This system is recommended by the Darwin Reconstruction Commission and the Australian Department of Housing & Construction. No guarantee can be given for the structural sufficiency of individual caravans.

 

STAGE 2: Roof tie-down.

* A useful conversion factor when converting from Imperial to metric Breaking Force is 0.1 Ton F = 1 kN (Kilo Newton) and for converting Breaking Force expressed in kN into Breaking Force expressed in kg is 1 kN = 100 kg.

 

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