What is severe wind?
Severe wind is a general description of high-wind conditions that occur during severe thunderstorms, cyclones (in tropical areas), extensive deep low pressure systems and tornadoes. These are often described as gale-force or storm-force winds in weather reports.
How does a severe wind hazard come about?
Severe storms are associated with low pressure systems depicted on weather maps. 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 also cause coastal erosion 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.
Severe thunderstorms generate damaging wind gusts, often caused by downbursts as heavy rain and hail drag surrounding air towards the ground. Severe thunderstorms produce wind gusts of 90 km/h or more, with peak wind gusts 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, Queensland, on 16 December 2006.
A tropical cyclone is a low-pressure system which develops in the tropics and is sufficiently intense to produce sustained gale force winds (at least 63 km/h). If the sustained wind reaches hurricane force of at least 118 km/h, the system is defined as a severe tropical cyclone. In other parts of the world they are called hurricanes or typhoons.
In tropical cyclones, wind gusts in excess of 90 km/h can be expected around their centre, or eye, while in the most severe events, gusts can exceed 360 km/h. Although the strongest winds are near the eye, damaging winds can extend hundreds of kilometres from the centre. The eye can have quite calm winds and cloud-free skies, but this lull is temporary and is followed by destructive winds from another direction. This is because the winds spiral around the eye in a clockwise direction (in the southern hemisphere). The effect of this on the ground is that winds on opposite sides of the eye blow in different directions.
Wind damage is mostly caused by the maximum gusts in the cyclone. For this reason, the well-known tropical cyclone severity categories used by the Bureau of Meteorology to communicate warnings are based on maximum gust wind speeds.
Potentially the most dangerous hazard associated with tropical cyclones which make landfall is storm surge. Storm surge has been responsible for more deaths than any other feature of tropical cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere, the onshore winds occur to the left of the tropical cyclone's path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast.
The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye. Its magnitude also depends on the local bathymetry of the seafloor and the angle and speed at which the cyclone crosses the coast. If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.
Tornadoes are the rarest and most violent of thunderstorm phenomena. A tornado is a violently rotating column of air suspended from large thunderstorm clouds in contact with the ground. It has a funnel shape and can range in width from a few metres to hundreds of metres. The majority of strong and violent tornadoes occur in association with supercell thunderstorms. Tornadoes often occur at the connecting zone between updraught and downdraught regions of a thunderstorm. Weaker tornadoes may spawn from either multi-cell 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 average 75 metres or less in diameter and travel several kilometres before dissipating, although some are in excess of a kilometre across and can stay on the ground for more than 100 kilometres. Severe tornadoes can have winds exceeding 400 km/h but are mostly restricted to widths of less than 100 metres.
Tornadoes typically have a short life span of only a few minutes, but are powerful events which can cause extensive damage. The six-point enhanced Fujita scale (EF0 to EF5) 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 65-510 km/h.
Where does severe wind hazard occur?
Severe storms can occur anywhere in Australia and do so more frequently than any other major natural hazard. Insurance statistics reveal that severe thunderstorms account for the majority of damaging events. The most common areas for severe thunderstorms are along the east coast from central Queensland to central NSW.
The main source of energy for tropical cyclones is the warm oceans in the tropical regions. To initiate a tropical cyclone the sea-surface temperature generally needs to be above 26.5 °C. However, existing cyclones often persist as they move over cooler waters.
The development of a tropical cyclone also relies on favourable broad-scale wind regimes and can persist for several days with many following quite erratic paths. They lose their source of energy when they move over land or colder oceans causing them to dissipate. Weakening may also occur if the cyclone moves into an unfavourable wind regime which disrupts the structure of the system. Sometimes a decaying tropical cyclone may interact with a weather system in higher latitudes to cause impacts far from the tropics.
In an average season, tropical cyclones are mostly experienced in northwest Australia between Exmouth and Broome in Western Australia and in northeast Queensland between Port Douglas and Maryborough. In the Australian region, the official tropical cyclone season runs from 1 November to 30 April, with most occurring between December and April. On average, about 10 cyclones develop over Australian waters each year and around six of these cross the coast.
There is some correlation between cyclone frequency and the El-Niño Southern Oscillation status, with activity subdued in El Niño events and enhanced during La Niña events, especially in the Coral Sea.
Although tornadoes have been observed throughout Australia, they occur most commonly in the south western and eastern parts of the continent and are not as rare as once thought. The perceived rarity was because Australia's sparse population and lack of detection equipment in the past resulted in many tornadoes not being reported. There have been more than 40 tornado-related deaths in Australia in the past 100 years.
- If Tropical Cyclone Tracy (1974) was to occur today, Geoscience Australia has estimated (Arthur, Schofield and Cechet 2008) that the average damage (based on full reconstruction costs) would reduce by 90 per cent. This reduction is due to the implementation of very high building standards in the years after Tracy, which greatly reduces the damage from extreme winds.
- Tropical Cyclone Tracy (1974) is the smallest observed cyclone in the world, with a radius of gales extending less than 50 kilometres. The largest tropical cyclone on record is Typhoon Tip (1979) which had gales over a radius 1100 kilometres in the north-west Pacific Ocean.
- The greatest number of tropical cyclones in the Australian region in any one year was 20 in 1983. Since 1950, the lowest number of tropical cyclones in any season was just three in 2015.
- The most intense cyclone observed in the Australian region was Tropical Cyclone Monica (2006). When it made landfall near Maningrida, NT, the estimated maximum wind gusts were over 350 km/h.
- The highest (non-tornadic) wind speed ever recorded was observed in Tropical Cyclone Olivia (1996). As it passed over Barrow Island, an anemometer recorded a wind gust of 408 km/h.
What is Geoscience Australia's role in reducing risk to Australians from severe wind?
Geoscience Australia is committed to support Australia's capability to managing the impact of natural hazards, including severe wind. Geoscience Australia:
- develops an understanding of natural hazards and community exposure to support risk mitigation and community resilience
- provides authoritative, independent information and advice to the Australian Government and other stakeholders to supportrisk mitigation and community resilience
- maintains and improves systems for effective natural disaster preparedness, response and recovery
- contributes to Australia's overseas development program.
In particular, Geoscience Australia:
- develops national-scale tropical cyclone wind hazard assessments
- assesses the potential impact of severe wind on the community and infrastructure
- supports Emergency Management Australia to understand what is exposed to tropical cyclones before, during and after events
- collaborates with the Bureau of Meteorology to develop a pilot impact-based forecasting capability
- collaborates nationally and internationally to enhance the wind impact and risk modelling capability
- assesses strategies to increase the resilience of buildings to severe wind
- conducts post-disaster surveys to understand how severe wind impacts the built environment
- Tropical Cyclone Hazard Assessment (TCHA) data access
- Tropical Cyclone Hazard Assessment (TCHA) interactive map
- Wind multipliers data access
- National Exposure Information System (NEXIS)