Australia Weather News

When most Australians think about the term "cyclone" they think of tropical cyclones – ones that impact the northern half of the country.

A few famous tropical cyclones include Tracy in 1974, Larry in 2006 and Yasi in 2011 – all had wind gusts well in excess of 200km/h and caused significant damage.

However the term "cyclone" actually refers to any intense low pressure system, this means that cyclones can occur anywhere around the country and they often do!

NSW experiences cyclones off the NSW coastline several times a year – these are normally called "East Coast Lows" but they're still a type of cyclone, just not a tropical cyclone.

Then there's the big polar lows that track south of the continent each winter – these can have pressures lower than 960mb which would ordinarily be a category 3 tropical cyclone! However the structures of these systems are all different. Tropical cyclones occur due to the instability created by very warm water (typically above 27-28C).

Here even in a warm atmosphere, conditions are unstable thanks to the heat released by the water as it condenses in cloud. Tropical cyclones exhibit their strongest winds close to the surface – but actually have only light winds high in the atmosphere.

Compare this to a polar low where its strongest winds are often high in the atmosphere (ie sometimes in excess of 200km/h around 8-10km above the surface). Close to the surface wind gusts can still reach in excess of 150km/h in the most intense systems. Polar lows occur due to very cold air in the upper atmosphere helping to produce instability which is quite different to how a tropical cyclone develops.

East Coast Lows develop in a similar manner but they develop very suddenly when intense cold pools move over the Tasman Sea and help lift the moist air above the surface of the ocean.

There are also "hybrids" which are more complicated again and tend to be a cross between East Coast Lows and tropical cyclones. These most commonly occur around Southeast Queensland and Northeast NSW. Tropical Cyclone Wanda that produced the 1974 floods in Brisbane is thought to most likely have actually been a hybrid system.