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Super cell thunderstorm (Image courtesy of BoM) - More>>

What causes severe weather?

Intense low pressure systems

On the largest scale severe storms are associated with the low pressure systems which are depicted on weather maps. Intense low pressure systems generate strong winds because of tight pressure gradients surrounding the centre of the system. Intense low pressure systems and their associated cold fronts can generate strong winds and heavy rain over large areas, causing local flash flooding and riverine flooding. The main wind damage from these low pressure systems is often in coastal areas and adjacent mountain ranges. The systems can cause coastal erosion also through the combined effect of large waves and an increase in sea level known as storm surge. Severe winds can also contribute to extreme bushfire activity.

A notable example of such a severe storm is the deep low which seriously disrupted the Sydney to Hobart yacht race in December 1998, resulting in the deaths of six sailors and the forced withdrawal of a number of damaged yachts.

Thunderstorms

This is a general term for relatively small-scale convective processes which develop when warm, humid air near the ground receives an initial upward push from converging surface winds and rises quickly in an unstable atmosphere. Under these conditions cumulonimbus clouds develop rapidly to potentially reach heights of up to 20 kilometres with associated lightning, thunder, severe wind gusts from downdraughts, heavy rain and hail. Many thunderstorms are typically short-lived, lasting less than one hour and are limited to a maximum of around 10 kilometres in diameter. However, they can traverse significant distances and are capable of inflicting severe damage. They frequently are embedded in low pressure systems or generated along a cold front. The strong cold fronts which affect Australia’s southern coast during winter and spring also may spawn severe localised winds, including tornadoes capable of unroofing houses.

• There are three recognised types of thunderstorms:
• Ordinary cells which are of limited size and lifespan and can produce short bursts of severe weather;
• ‘Multicells’ which are more persistent and have a greater impact; and
• ‘Supercells’ which are even more persistent with a constant rotating updraft known as a ‘mesocyclone’ The Sydney hailstorm of April 1999 was a supercell thunderstorm.

Lightning

Lightning is an atmospheric discharge of electricity. All thunderstorms will exhibit some lightning and the resulting thunder. Lightning also can occur within the ash clouds from volcanic eruptions, as a result of violent bush fires or during dust storms which generate sufficient dust to create a static charge. The process in the generation of lightning is still a matter of debate, but it is believed that large quantities of ice in the clouds enhance lightning development. The charge build-up will neutralize itself through any available path that assists in the forcible separation of positive and negative charge carriers within a cloud.

Hail

Hail forms in strong thunderstorm clouds, particularly those with intense updraughts, great vertical extent, high liquid content with large water droplets and where a good portion of the cloud layer has a temperature below freezing (0°C). Hail forms on condensation nuclei such as dust, insects or ice crystals when supercooled water freezes on contact. If the hailstones grow large enough, they become too heavy to be supported by the thunderstorm's updraught and fall out of the cloud.

While hailstones are most commonly only a few millimetres in diameter they can grow to 15 centimetres and weigh more than half a kilogram. Pea or golfball sized hailstones are common in severe thunderstorms. Hail can cause serious damage to cars, skylights, glass-roofed structures, crops and vegetation. On rare occasions, massive hailstones have been known to cause concussions and even death.

Storm wind gusts

In a mature thunderstorm, the falling rain and hail drags the surrounding air downwards while evaporation from raindrops cools the air and accelerates its descent. On reaching the ground, this strong downdraught spreads out to produce a gusty wind which has the potential to cause significant damage. Severe thunderstorms produce wind gusts of 90 km/h or more, with peak winds exceeding 160 km/h in the most damaging storms. In Australia, the strongest measured wind gust during a thunderstorm was 196 km/h at Double Island Point in Queensland on 16 December 2006 while a gust of 174 km/h was recorded at Richmond in New South Wales on 3 December 2001.

Tornadoes

The most intense of all atmospheric circulations on a local scale, tornadoes are the rarest and most violent of thunderstorm phenomena. According to the Glossary of Meteorology (2000) a tornado is a violently rotating column of air suspended from large storm clouds in contact with the ground. It has a funnel shape and its vortex can range in width from a few meters to hundreds of meters. The majority of strong and violent tornadoes occur in association with supercell thunderstorms. These often occur at the connecting zone between updraught and downdraught regions of a thunderstorm. Weaker tornadoes may spawn from either multicell or single cell thunderstorms.

Tornadoes come in many sizes but are typically in the form of a visible funnel with the narrow end touching the ground. A cloud of debris commonly encircles the lower portion of the funnel. They usually rotate with wind speeds of 175 km/h or less, but can have wind speeds up to 350 km/h. They usually are around 75 metres or less in diameter and travel several kilometres before dissipating, although some are in excess of a kilometre across and stay on the ground for more than 100 kilometres. Severe tornadoes can have winds exceeding 400 km/h but typically these are restricted to widths of less than 100 metres.

Tornadoes have a short life span of only a few minutes, but are powerful events which can cause extensive damage. The six-point Fujita scale (F0 to F5) was established to quantify the intensity of a tornado and its danger. This scale relates tornado intensity indirectly to structural and/or vegetation damage and corresponds that information to wind speed intervals from 65 to 510 km/h.