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Typhoons, tropical cyclones, tornadoes and hurricanes; which is the odd one out? Tornadoes. All the rest are different names for what we in Australia call cyclones. However our cyclones rotate in a clockwise direction, while hurricanes (USA) and typhoons (NW Pacific) go anti-clockwise. Tornadoes are also rotating storms but are much smaller (usually less than 1km across) and more intense than tropical cyclones.

Tropical cyclones are low pressure systems that form in the tropics which, in the southern hemisphere, have well defined clockwise wind circulations with average surface winds exceeding gale force (34 knots). Short period wind gusts are often 40 per cent or more higher than the average wind speed.

The circular eye of a tropical cyclone is an area characterised by light winds and often by clear skies. The eye is typically 20 nautical miles or so across but the eye of a cyclone can range from under 5 to over 50 nautical miles wide. The eye is surrounded by a dense ring of cloud known as the eye wall which is the area of heaviest winds and seas.

Following the passage of the eye the winds shift to the opposite direction with equal force. Tropical cyclones vary in both size and intensity. Small cyclones may be only 60 nautical miles across whereas large storms may be up to 300 nautical miles across. Both large and small cyclones can have equally devastating wind speeds near the centre.

Category 1 Strongest gust to 125 km/h Beaufort Scale 8-9
Typical Effects    Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.
Sea Conditions High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility
Category 2 Strongest gust 125 - 170 km/h Beaufort Scale 10-11
Typical Effects    Minor house damage. Significat damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.
Sea Conditions Very high waves with long overhanging crests; the resulting foam, in great patches, is blown in dense white streaks along the direction of the wind; on the whole, the surface of the sea takes a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected.
Category 3 Strongest gust 170 - 225 km/h Beaufort Scale 11-12
Typical Effects    Some roof and structural damage. Some caravans destroyed. Power failure likely.
Sea Conditions Exceptionally high waves (small and medium sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected.
Category 4 Strongest gust 225 - 280 km/h Beaufort Scale 12
Typical Effects    Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failure.
Sea Conditions The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected.
Category 5 Strongest gust of more than 280 km/h Beaufort Scale 12
Typical Effects    Extremely dangerous with widespread destruction.
Sea Conditions The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected.

Travelling at speeds of up to 20km/h, a large cyclone system can exert cyclonic conditions on a particular area for 6-24 hours as they pass over it. Most cyclones in North Queensland are Category 3 or less.

The Queensland region averages four to five tropical cyclones per year. The greatest number of cyclones to be recorded in one season in the Queensland region was 12 (1962/63). Tropical Cyclones can occur in North Queensland from November to April, with most during January, February and March. Some have occurred as late as June, but these are very rare.

How Are They Named?

There are three alphabetical lists totalling 60 names (six letters, such as q and u, are not included). Male and female names alternate on the list. At an average of five cyclones per season it takes about 12 years to work through the list before the first name is used again. Names of particularly bad cyclones such as Tracey (category 4) which completely destroyed Darwin in 1974, and Orson (category 5) which produced waves to 20 metres off the Western Australian coast in 1989, are removed from the list.

Each Cyclone Warning Centre (Brisbane, Perth, and Darwin) has its own name list and is responsible for naming a cyclone which originates from that area. Even if a cyclone moves into another region, it retains its original name.

Well now that you know what they are, when they come, and what they do, you're probably assessing whether your vacation plans should alter. The answer is no.

Although they are more common at certain times of the year, most of them stay well out to sea, tracking down the coast which takes them into cooler water where they generally just fizzle out. Even when they seem to be "just off the coast" they don't have much effect on the weather on land or on the reefs near the coast. When they do come towards the land they need to pass directly over it to have any serious adverse effect.

The cities in cyclone-prone areas are also built to cyclone safety standards. Reef vessels keep going out to sea and dive trips are only called off when the port is closed. It usually re-opens after the threat is gone and everything gets back to normal.

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