Breaking News: Cyclones

Given the approach of Tropical Cyclone Yasi towards the Queensland coast, I have delayed publishing Part 2 of the UV and skin series in order to present this rapid article on cyclones.

Information specific to TC Yasi is accurate at time of publishing but should not be used to plan evacuation from affected areas.

Cyclones mainly form along a belt near the equator where the temperature of the water is warmer. The minimum water temperature needed for a cyclone to form is 26.5 degrees Celsius, however already formed cyclones can travel over areas of cooler water temperatures, but will lose intensity.

Map of cyclone areas worldwide. Orange areas are regions where conditions are suitable for cyclone formation. Image courtesy of NASA

The warm moist air above this warm water rises, resulting in a local low air-pressure area near the ocean surface. Surrounding air is literally sucked into this low-pressure area, which then too become warm and moist and rises. As this process repeats, fed by the warmth from the ocean and water evaporation, the air begins to swirl around the low-pressure area (the same effect as water swirling as it drains into a plug hole), and as the warm moist air cools away from the ocean surface it forms clouds. The combination of the swirling winds caused by the low-pressure area and cloud formation forms the characteristic spinning cloud.

Cyclone formation. Air swirls the area of low-pressure before rising upwards from the ocean surface. Image courtesy of Geoscience Australia

The swirling of air around a low-pressure area is caused by the Coriolis Effect, a result of the earth’s rotation. The direction of the Coriolis Effect depends on the hemisphere, in the southern hemisphere a cyclone will always spin clockwise.

The category of tropical cyclone is determined by wind strength. In Australia the minimum category of a tropical cyclone is reached when sustained wind speeds exceed 63km/h.

Australian Region – Tropical Cyclone Intensity Scale
Category Sustained winds Gusts Expected damage
Tropical Low <34 kts
<63 km/h
<49 kts
<91 km/h


One 34-47 kts
63-88 km/h
49-67 kts
91-125 km/h
Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings 


Two 48-63 kts
89-117 km/h
68-89 kts
125-164 km/h
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings. A Category 2 cyclone’s strongest winds are DESTRUCTIVE winds
Three 64-85 kts
118-159 km/h
90-121 kts
165-224 km/h
Some roof and structural damage. Some caravans destroyed. Power failures likely. A Category 3 cyclone’s strongest winds are VERY DESTRUCTIVE winds 


Four 86-107 kts
160-200 km/h
122-151 kts
225-279 km/h
Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures. A Category 4 cyclone’s strongest winds are VERY DESTRUCTIVE winds
Five >107 kts
>151 kts
>279 km/h
Extremely dangerous with widespread destruction. A Category 5 cyclone’s strongest winds are VERY DESTRUCTIVE winds 


Table adapted from BoM

Cyclone Intensity

There are two main factors which affect cyclone intensity – the warmth of the water and wind conditions. Warm water is required for cyclone formation, and cooler waters will degrade the intensity. In the case of Yasi it is encountering warm north Queensland waters caused by the La Nina climate.

High winds surrounding the cyclone can also reduce the cyclone’s intensity. The cyclone requires the swirling winds caused by the Coriolis Effect, high incidental winds can disrupt this swirling pattern, literally tearing the cyclone apart. Yasi is encountering only mild winds which have very little affect on the cyclone formation.

The shape of Yasi is also described as being very efficient, with good ventilation at both the top and bottom, allowing a very efficient formation of the low-pressure system which cyclones form around, and high wind velocities.

Storm Surge

As the air surrounding the low-pressure area swirls it pushes water towards the centre of the storm, literally forming a mound of water. This mound is the “storm surge”, which when reaching landfall effectively acts like a tsunami and causes significant flooding of the coastal areas. A storm surge will be greater in areas where the ocean floor slopes gradually towards the coastline, such as the areas around north Queensland.

Storm surge. The top imageis normal conditions. The lower image is the rise in sea levels caused by a storm surge at high-tide. Image courtesy of BoM

Storm surges typically measure around 60-80 kilometres across, meaning areas more than 30 km from the crossing point of the cyclone can become flooded. A category 4 cyclone will produce a storm surge approximately 4-6 metres higher than normal tidal levels.

The actual height of the storm surge depends on the tides. If the storm surge hits the coast during low tide, destruction will be minimal. However if it hits during high tide, the effective height and reach of the surge will be greater.

Cyclones and climate effects

Cyclones are more common in Australia during La Nina climate periods than during El Nino periods. La Nina is characterised by a large scale cooling of water temperatures across the Pacific, except near the north Australian coasts where there is a gathering of warm ocean waters. This area of warm water and changes in wind patterns during La Nina periods together contribute to the increased numbers of cyclones during these periods.

The La Nina period being experienced by Australia at the moment is one of the strongest on record and was also a cause of the Queensland floods.

Cyclone paths

Not all cyclones are unpredictable; their path is determined by the environment around it. Yasi is situated in a well characterised belt of easterly winds which are carrying it towards the Queensland coast. While it can be said with some certainty Yasi will strike the Queensland coast because of these winds, there is still some variability in exactly where on the coast it will hit. Latest BoM predictions are that it will reach landfall at Cairns.

The spread of Yasi has been estimated by the Bureau of Meteorology to be around 800km, stretching along the coast from Cooktown to Sarina.

Predicted path of Tropical Cyclone Yasi. Orange areas are regions of high impact. Image courtesy of BoM

Inland reach

Cyclones require warm water to maintain intensity, so once the cyclone makes landfall it will begin to lose intensity. However given its size and high intensity, Yasi may take a long time to degrade. This means it may reach as far inland as Mt Isa, however it will have likely degraded into a tropical storm by this time and not a categorised cyclone.


The wetness of the catchment areas affects the amount of runoff from rains and storm surges. Simply, the wetter the ground, the less water that can soak in, increasing runoff and the potential for flooding. Townsville and Cairns have had slightly above normal rains this season, but it is hard to tell if it will cause abnormal flooding.

Yasi no longer appears to be headed for areas affected by the January Queensland floods, so it is less likely serious weather events will affect those areas.

Cyclones and Climate Change

At the moment the BoM and World Meteorological Organisation cannot draw certain conclusions about the effect of climate change on cyclones. There is no clear evidence of long-term trends in the numbers of cyclones, and limitations in knowledge and technical limitations limit the ability to predict changes in cyclone frequency. However, they expect little or no change in global frequency of cyclones, nor significant changes in the regions affected by cyclones.

Increases in the economic damage and disruption by tropical cyclones have been caused mainly by increased population centres in coastal areas.

However, rising sea levels may increase the effects of storm surges. This is particularly of concern as historically the primary cause of death during cyclones has been due to flooding.

Satellite Images of Yasi:

NASA Earth Observatory

University of Wisconsin Cooperative Institute for Meteorological Satellite Studies

(links at the top change image type)


Thanks to Jim Davidson, Regional Director Queensland at the Bureau of Meteorology and Andrew Western from the University of Melbourne for providing their expertise.

References and further information

Geosciences Australia,

Magdalena Roze, Weather Channel,

Dick Whitaker, Weather Channel,

Bureau of Meteorology,

Statement on tropical cyclones and climate change, Bureau of Meteorology,


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