As cyclones go, Oswald was a small one

ISOLATED: This aerial photo was taken of the Gresham family farm in the Gympie district during the recent floods.
ISOLATED: This aerial photo was taken of the Gresham family farm in the Gympie district during the recent floods. Craig Warhurst

GIVEN the impact of ex-tropical cyclone Oswald, I thought I'd review the cyclone activity expected for this season.

Typically, the Australian tropical cyclone season runs between November and April.

According to the Bureau of Meteorology's 2012/13 tropical cyclone outlook for Queensland and the Coral Sea, there is a close to average chance of more than four cyclones developing this year.

Tropical cyclones are low pressure systems that form over warm tropical waters and have gale force winds.

Gale force winds are sustained winds of 63kmh or greater, with gusts of more than 90kmh near the centre.

For a tropical cyclone to develop it needs a low pressure system, a trough or surface level low, and warm ocean temperatures greater than 26.5 degrees celcius.

There also needs to be a moist unstable atmosphere and little wind to allow for the central column and eye, to develop.

A category one rated cyclone will have wind gusts less than 125kmh, an average maximum wind speed between 66 to 88kmh and an approximate central pressure of not lower than 985hPa.

Tropical cyclone Oswald was category one.

A category five rated cyclone, an extremely dangerous event, will have wind gusts in excess of 280kmh, an average maximum wind speed in excess of 200km/hr and a central pressure of less than 930hPa.

TC Yasi in February 2011 was a marginal category 5.

For more information on cyclones, current warnings and updates, visit au/cyclone.

A key trigger for cyclones is an MJO, which is a band of low air pressure which originates off the east coast of central Africa.

A strong MJO last crossed Australian longitudes in early to mid January. Based on standard patterns, the next MJO could be expected in early March.

An MJO travels east across the Indian Ocean and northern Australia roughly every 30-60 days.

It can be an indicator of potential rainfall events.

For more information and updates try

Topics:  dave mcrae el nino oswald weather

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