How strong will the winds be?

A mean sea level pressure chart shows the direct relationship between isobar spacing (pressure gradient) and orientation, and the strength and direction of surface winds. The general rule is that winds are strongest where the isobars are closest together. Thus the strongest winds are usually experienced near cold fronts, low pressure systems and in westerly airstreams south of the continent. Winds are normally light near high pressure systems where the isobars are widely spaced.

However, because of a latitude effect winds in middle latitudes are lighter than those in the tropics with similarlyspaced isobars.

In Australia, the most destructive winds over broad areas are generated by tropical cyclones. (Tornadoes, associated with some severe thunderstorms, have the potential to generate higher wind speeds, but areas affected are much smaller than these tropical storms.)

Tropical cyclones are low pressure systems in the tropics which, in the southern hemisphere, have well defined clockwise circulations with mean surface winds (averaged over ten minutes) exceeding gale force (63 kilometres per hour) surrounding the centre. Tropical cyclones exhibit a relatively clear eye, surrounded by dense wall clouds and a series of spiral rain-bands. The Bureau tracks cyclones with weather watch radar, special service reports and frequent satellite images. Figure 4(a) and Figure 4(b) show a tropical cyclone approaching, and crossing the Queensland coast near Rockhampton.

Figure 4a.	Figure 4b.

Figures 4 (a) and (b).    Charts of a cyclone moving from the Coral Sea to the Queensland coast demonstrate how isobars indicate wind speed and direction.

The pressure gradient is very steep towards the cyclone's centre and wind speeds on the nearby coast in this case would have been about 110 kilometres per hour with gusts 50 per cent or more above this mean wind speed. In Figure 4(b), 12 hours later, the cyclone has moved inland. Cut off from its heat energy source, the ocean (it requires sea surface temperatures above 26.5 C), its intensity has decreased and wind speeds have dropped to 85-90 kilometres per hour. Figure 5 graphs the relationship between wind speed and pressure as the eye of tropical cyclone Winifred crossed Cowley Beach, in Queensland, in February 1986. The relationship is characteristic of tropical cyclones.

Figure 5.    Typical wind speed and pressure relationships for a tropical cyclone.
(Cyclone Winifred, Cowley Beach, Queensland, February 1986.)