What comes to mind when you hear the Bureau of Meteorology (BOM) reports mention severe wind?
Well, severe wind is a term used to describe high wind conditions mostly experienced during days of heavy thunderstorms, tornadoes, cyclones (within tropical areas) and the extensively deep low-pressure systems. These are often described in weather reports as gale-force winds or storm-force winds.
How a severe wind hazard happens
Severe storms are often associated with the low pressure system displayed on most weather maps. Intense low-pressure systems coupled with cold fronts can produce strong winds and heavy rainfall in a wide area leading to local flash floods and riverine flooding.
The most serious wind damage caused by these low-pressure systems occurs in the coastal areas and the neighbouring mountain ranges. The low pressure systems can also lead to coastal erosion through combined effects of huge waves and a rise in the sea level referred to as a storm surge. Severe winds can also play a role in fueling dangerous bushfire activities.
Severe thunderstorms produce highly damaging wind gusts usually brought about by downbursts such as heavy rainfall and hailstorm push the surrounding air down to the ground. Severe thunderstorms generate wind gusts of speeds of at least 90km/h and wind gusts that exceed 160km/h for most damaging storms. Australia recorded its strongest wind gusts moving at 196km/h on 16 December 2006 in Double Island Point, Queensland.
A tropical cyclone refers to low pressure systems that usually develop in the tropics and are adequately intense to generate well-sustained gale force winds of at least 63km/h. If sustained winds attain a hurricane force of at least 118km/h, the system evolves into a severe tropical cyclone. These are also referred to as typhoons or hurricanes.
Wind damage is majorly caused by peak gusts in a cyclone. As such, the renowned tropical cyclone severity categories employed by the BOM to give warnings are based on peak wind gust speeds.
A storm surge is considered the most catastrophic weather hazard related to tropical cyclones which causes a landfall. Storm surge has resulted in more deaths than any other element of tropical cyclones. Generally, a storm surge is a dome of water raised at about 60 to 80 kilometres wide and about two to five metres above the ordinary tide level.
A storm surge features a combination of strong winds that drive water onshore and the lowest atmospheric pressure present within a tropical cyclone. Within the southern hemisphere, the onshore winds emerge on the left of the track used by the tropical cyclone. In Australia, this falls on the eastern side on the northwest and the northern coasts and the southern side of the east coast.
While tornadoes rarely happen, they are considered the most violent of all thunderstorm events. Basically, a tornado is a highly violent rotating air column hanging from huge thunderstorm clouds onto the ground. Tornadoes feature a funnel shape which ranges from just a few to hundreds of metres. Most of the strong and very violent tornadoes are associated with supercell thunderstorms. Naturally, tornadoes occur at the junction of updraught and downdraught zones of a thunderstorm. Weaker tornadoes transpire from either a single-cell or multi-cell thunderstorm.
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