Storms and a major storm in Australia

Storms are weather patterns characterised by high winds and sometimes high precipitation. In Australia, there are two major types of storms, dust storms and thunderstorms. Both of these types of storm are usually fairly harmless, but in some cases, they can be highly destructive.

Thunderstorms

Thunderstorms are the most common type of storm in Australia. They are a familiar phenomenon in New South Wales, Queensland and parts of Western Australia. Thunderstorms are common in eastern Australia in late spring and early summer, in northern Australia in autumn and spring, in coastal south-western Australia in winter and in inland south-western Australia in spring and early summer.

Thunderstorms form when there is instability in the atmosphere, a source of moist air and a trigger to start their development. The most common trigger is a front, or a boundary between two air masses, a low pressure system, or an area with low air pressure in the high part of the atmosphere, or a trough, which is an elongated low pressure system located between two high pressure systems. Topographic features such as hills and mountains also affect the formation of thunderstorms.

The clouds in thunderstorm systems are called cumulonimbus clouds. These clouds are very tall, extending between about 3050 and 12 200 metres into the air. Their tops are often anvil-shaped and they appear very dark near the ground. When a thunderstorm hits an area, it produces thunder and lightning, high winds and usually heavy precipitation. See image 1

Since cumulonimbus clouds are so tall, hail forms. Hail is a solid form of precipitation made up of alternating layers of clear ice and frozen mist. These balls of ice, also called hailstones, form from ice crystals found in the cumulonimbus cloud. The ice crystals exist at a layer of the atmosphere where the temperature is below freezing. An updraft, or upwards-blowing air current, pushes these ice crystals higher into the atmosphere. This causes more water to freeze around the ice crystals. The hailstones circulate in the cumulonimbus cloud, gaining more and more ice until they become so heavy that the updraft cannot push them upwards any longer. At this point, the hailstones fall to the ground. Hailstones are usually about the size of a pea, but the largest hailstone ever recorded was the size of a melon. See image 2

Another phenomenon that happens in cumulonimbus clouds is lightning, or electrical discharges between two oppositely charged bodies. Lightning occurs because of electrical charges within clouds and on the ground. Within a cumulonimbus cloud, the top half is positively charged while the bottom half is negatively charged. During a thunderstorm, the ground is positively charged.

There are three major types of lightning, in-cloud lightning, cloud-to-cloud lightning and cloud-to-ground lightning. In-cloud lightning happens when positively charged particles at the top of the cloud attract negatively charged particles at the bottom of the cloud. Cloud-to-cloud lightning occurs when oppositely charged particles in two clouds attract one another. Cloud-to-ground lightning, which is the type of lightning most people are familiar with, is when lightning strikes the ground. Cloud-to-cloud and in-cloud lightning are more common than cloud-to-ground lightning. Thunder, which is the rumbling sound heard after lightning, is the sound of the electrical discharge. Thunder is usually heard after lightning because sound takes longer to travel than light. See image 3

A severe thunderstorm: 14 April 1999

On the evening of 14 April 1999, a severe thunderstorm hit the eastern and city suburbs of Sydney. In this extremely destructive thunderstorm, very large pieces of hail fell, damaging 60 000 cars. The largest piece of hail that fell that was measured had dimensions of 9x8x8 centimetres. Evidence shows, however, that larger hailstones may have fallen in severely damaged areas. In addition, heavy rain flooded 25 000 homes. Overall, the damage is believed to have cost at least $1.5 billion to repair.

This storm puzzled meteorologists (people who study weather) because thunderstorms are not common in Sydney during that time of year and at that time of day, and because the storm was so violent. After an investigation, it was found that this thunderstorm was the result of a supercell, or a very large and strong cumulonimbus system. Since the storm was so large and strong, it was able to generate very large hailstones and cause significant damage.

Dust storms

A dust storm occurs during times of drought. Drought causes soil to become very dry, allowing it to be easily picked up by wind. If winds grow strong enough, large amounts of dusty soil can be thrown into the air, filling the air with dust. This can reduce visibility, cause breathing problems and fill the streets with dust. These storms can become enormous, with dust rising as high as 305 metres into the air and spreading for hundreds, even thousands, of kilometres. Winds in dust storms often reach speeds of 40 kilometres per hour, causing additional problems due to high winds. See image 4

A severe dust storm: The Great Dust-Up of 1902

In 1902, eastern Australia was experiencing a difficult drought. Dust storms were commonplace during this time, but things became particularly difficult on 12 November. Starting in South Australia, the dust storm quickly spread to Victoria and south-western New South Wales. From early morning in Adelaide, dark clouds of dust coloured the sky. In Victoria, telegraph poles were blown down by high winds, and some towns experienced total darkness as the dust engulfed everything. Even rail lines had to be cleared so that trains could pass. Temperatures soared as high as 38 degrees Celsius. By the next day, the dust storm reached Sydney, but by then it had dissipated a little and was not as severe.

An interesting phenomenon from this dust storm was reports of 'balls of fire' that fell to the ground, showering sparks everywhere, and even setting fire to buildings. It is believed that these balls of fire formed because of static electricity that was generated as dust particles moved past each other quickly in the storm. The static electricity then lit organic material in the dust, causing balls of fire to seem to rain down from the sky.