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Sunday, February 6, 2011

Severe cyclones - a rising tide or just more hot air?

Cyclone Yasi has been dubbed the worst cyclone to ever hit Queensland. Certainly it would appear to be the largest delivering gale force winds over 600km with effects being felt over 2,000km from Cape Melville to Shoalwater Bay.  When it made land-fall at Mission Beach it had a central pressure of 930hpa and was packing winds of up to 300km/h.  However, Yasi certainly was not the worst cyclone to hit Queensland.

Firstly, keep in mind that cyclones are low air pressure systems. The lower the air pressure, the more dangerous the system.  Normal air pressure at sea level is around 1013hPa (hectoPascals). Cyclones generally form when the air pressure drops below 990hPa.  Lower pressure results in worse weather and generally higher wind speeds.

One of the most severe cyclones to hit Australia was Cyclone Mahina which struck on 4 March 1899. Between 300 and 400 people died.  Mahina struck with a storm surge of 13m at Bathurst Bay on Cape York Peninsula, travelling 5km inland and with witnesses claiming dolphins were found on rocks 14m above normal sea height.  Ships in the area recorded a central pressure of 914hPa.  Mahina is considered one of the most severe cyclones ever to strike Australia.

The year 1918 was particularly notable for Queensland with two Category 5 cyclones within weeks of each other.  The first one struck Mackay in January 1918 with a central pressure of 933hPa.  It reportedly struck with a 2.7m storm surge hitting the centre of Mackay.  There were 30 deaths recorded, most of whom drowned in the surge.  As with all cyclones, there came rain.  In this case 1411mm in 3 days which resulted in severe flooding of the Pioneer River in Mackay and the Fitzroy River in Rockhampton which peaked over 10m.

On 10 March 1918, what can only be described as a super-typhoon hit the Innisfail, Mission Beach, Clump Point area.  This cyclone is one of the most powerful storms to have hit Australia in recorded history.  It had a central pressure of less than 910hPa.  It is unknown what the wind speed was but it would almost certainly have been far greater than 300km/h.  It has been reported that 12 houses remained standing in Innisfail, which was a town of 3,500 people at the time.

By contrast, Cyclone Tracey which hit Darwin in 1974 had a central pressure of 950hPa with recorded gusts of 217km/h (before the weather instruments failed) although it is estimated wind speed reached 250km/h.  Cyclone Larry, a category 4 system when it crossed the Queensland coast near Innisfail in 2006, had a central pressure of 960hPa with a highest recorded wind gust of 294km/h at Bellenden Ker and estimated gusts around Innisfail of 240km/h.

In 1996 Western Australia was struck by Cyclone Olivia which has the dubious honour of recording the world's highest wind speed of 408km/h.  Average wind speed for this system however was not quite so intense, being around 260km/h.

Australia has been struck by hundreds of cyclones, many very destructive.   Whilst Yasi is the largest system we have seen in a century, it certainly is not unique to this part of the world. Yasi devastated homes, crops and infrastructure but thankfully without taking any lives.

The devastation from cyclones is not dependant on their strength and size alone, important factors include population, building standards, industry such as crops and mining, infrastructure in the affected area.  For instance the most deadly cyclone in the world was the Bhola Cyclone, a Category 3 with 185km/h winds and air pressure of 966hPa which struck East Pakistan (now Bangladesh) in 1970 and killed 500,000 people.

For comprehensive details of any of the Australian cyclones mentioned or others that have not been delved into, refer to the website of the Bureau of Meteorology (

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