Pressure systems

The daily weather maps show regions of high and low pressure from which general predictions can be made. At ground level, air flows outwards from a high pressure system. So the column of air in the high-pressure system is generally sinking towards ground level. As air sinks, it is compressed (by the weight of air above) and warmed. Clouds evaporate and the weather is generally sunny.

On the other hand, at ground level air flows into areas of low pressure. So the column of air in a low-pressure system rises and expands. The air cools and any water content tends to condense into clouds. Clouds may form and, depending on the amount of water vapour in the air, rain and other forms of precipitation may occur.

The weather patterns over Australia show interspersed areas of high and low pressure, as shown in Figure 18.1. These generally move from west to east.

Wind direction, shown by arrows, is predominantly parallel to the isobars. It spirals outwards from highs (where air is sinking and warming) and inwards to lows (where air is rising and cooling). Wind travels clockwise around lows and anticlockwise around highs. Thus, if you face into the wind, the low is on your left. Usually the isobars are closer around lows, indicating stronger winds. Knowing that Australian weather systems generally move from west to east, you can infer from a strong northerly wind that a cold front may be on the way, and conversely that a cold southerly wind heralds improving weather. However, beware of the occasional low pressure system moving down the east coast of Australia. This can bring worsening weather with southerly and southeasterly winds, particularly in the northeastern regions of Victoria and southern New South Wales.

In the equatorial regions, particularly in summer, the ocean waters warm significantly. In areas of low pressure, moisture-laden air rises. At the condensation layer, cloud forms and the heat released by the condensing vapour causes upward moving air currents. More heat is released as more vapour condenses, large thunderstorms are created and the monsoon rains are experienced. This leads to periods of very high humidity and temperature, known as the ‘build up’, typically in November–Decem-ber. This is then followed by several weeks of very heavy, monsoon rains.

The incredible energy stored in weather events in northern Australia can lead to cyclones—very deep lows with destructive winds reaching 200 km/h at times. Cyclones form over the sea, and generally lose energy as they pass over land, making coastal areas and offshore islands at most risk. Cyclone weather alerts are issued at times of high, direct threat from a particular storm. These must never be ignored by outdoor enthusiasts—the energy of cyclones can devastate whole communities. If walking in cyclone prone areas during the cyclone season (typically November–March) always carry a transistor radio to monitor weather forecasts.

Within southern Australia, the patterns of highs and lows are established to the south and west of the continent. They flow over Australia from west to east. Within central and northern Australia, ‘heat’ lows are produced by contact between land that is heated to summer temperatures of above 40°C. Air in contact with the land warms and rises. Often there is no moisture associated with these lows.