Knowing how to read a weather map will help you prepare for the unexpected on your next bushwalk.
One of the most important things you need to do when planning a bushwalk is check the weather. It’s easy to find out predicted temperature and if there’s a chance of rain with the help of the television or internet, but this doesn’t tell you which direction the rain will be coming from, whether there’s a chance of a thunderstorm coming through or what the wind will be like on the day. Once you unlock the mystery of reading a weather map, you’ll be able to predict if there’s any chance of rain or storms and what gear you’ll need to take. Just don’t forget that chaos theory (and the butterfly effect) will always play a role in the weather.
Actual and possible rainfall is indicated as blue-filled shapes on colour weather maps. It can also be predicted by looking at the pressure cells on a map and understanding the direction the weather system is moving in – weather systems move west to east in southern Australia, while in northern Australia they tend to move east to west with the exception of cyclones, which are highly unpredictable.
Wind speed can be indicated by small lines on the map, with an increasing number of ticks indicating strengthening wind speeds. It can also be predicted using the isobars on a weather map.
Isobars and pressure cells
Isobars are the lines on a map that link areas of equal air pressure – when they form a closed loop, this is known as a pressure cell. These play an important role in wind direction, as air flows anticlockwise in high pressure cells (marked H), which are generally associated with stable weather. Air flows in a clockwise direction in low pressure cells (L), which often bring unsettled weather. The winds are caused by the flow of air from high pressure cells to low pressure cells – the closer the isobars, the greater the variation in pressure and the stronger the winds as they circulate around the pressure cells.
These relatively small but intense low pressure cells are marked with TC and their name on weather maps. They usually bring heavy rain and strong winds to coastal areas and further inland.
When a new air mass arrives in a region it forces the old one out; the barrier between the two is called a front. Cold fronts are marked as a line with solid triangles, which indicate the direction that the front is moving. They form when a cooler body of air moves underneath a warmer one, forcing the warm moist air to rise, and are usually associated with a decrease in temperature, clouds and possible thunderstorms. Warm fronts occur when warm air mass moves towards and over a cooler air mass, creating clouds and even rain as it rises and cools. These are marked as lines with semi-circles. In both cases, developing fronts may be marked with a small circle between the semi-circles or triangles, while a decaying front is marked with a plus symbol.
Ridges and troughs
When air from a high pressure cell enters a region of low pressure, a ridge forms (usually shown as a protrusion from the isobars of the cells) – these are usually associated with fine weather. Troughs, on the other hand, appear when air from a low pressure cells enters a region of high pressure (shown as U-shaped fluctuations in the isobars) and are associated with unsettled weather and precipitation. A dashed line is usually drawn along the centre of a trough.
The butterfly effect
According to chaos theory creator Edward Lorenz, a tiny shift in the initial conditions, such as a butterfly beating its wings, can cause large changes in the long-term. As a result, forecasters can only accurately predict the weather for about a week ahead – and even then it’s probably going to change. So don’t forget to look at a weather map the night before and on the day of your trip and be prepared for any sudden changes.
If doing a coastal walk check when low and high tides occur so you can cross an area in safety. Unlike the weather, this can be accurately predicted (thanks to the moon) and can be found on state maritime services websites, which also give alerts for dangerous conditions.
Need to know: Bureau of Meteorology bom.gov.au