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Reading weather maps

All weather maps use a standard set of symbols to portray features of the weather. Figure 2 below shows some of the more commonly used symbols.

Figure 2: Symbols used on weather maps

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Isobars

Isobars are lines on a weather map that join locations of equal air pressure. The number written on an isobaric line is the atmospheric pressure for that isobar measured in hectopascals (hPa). Air pressure readings at ground level typically range from 980 hPa to 140 hPa. The lower the air pressure is, the more likely the chance of an extreme weather event such as a cyclone.

On weather maps, concentric isobars are usually drawn increasing or decreasing by an increment of 4 hPa. The weather map in Figure 3 below shows the isobars increasing in pressure from north to south across Australia.

Figure 3: Weather map featuring a tropical cyclone over north-western Australia

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Pressure cells

Where isobars are drawn as closed curves they surround a pressure cell. High pressure cells are indicated by “H” on a weather map. They indicate the location of the highest air pressure measured in a region at a particular point in time. Similarly, low pressure cells indicate the point of lowest air pressure and are indicated by an “L”.

On the weather map in Figure 3 above a high pressure system is located to the south of Western Australia. A low is developing to the east of Tasmania.

Fronts

Cold and warm air masses are moved around the earth by winds. When an air mass arrives in a region it pushes the existing air mass out. The boundary between two air masses is called a front.

A cold front usually forms out of a low pressure cell where a cold mass of air is moving towards warmer air. In Australia, cold fronts bring southerly winds and sudden decreases in temperature. They are often associated with thunderstorms. Cold fronts are drawn on weather maps as a line with solid triangles. The triangles point in the direction that the front is moving.

The cold front shown on the weather map in Figure 4 below developed out of the low off the east coast of Tasmania. As it moves north through New South Wales it will bring cold south-west winds with the chance of thunderstorms.

Warm fronts occur when a mass of warmer air moves towards cold air. The warm air slides up and over the cold air forming clouds with the possibility of precipitation. Warm fronts are drawn on weather maps as a line with solid semicircles.

Figure 4: Weather map featuring a cold front over south-eastern Australia

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Ridges and troughs

When air from a high pressure cell pushes into a region of lower pressure, a ridge forms. Ridges are shown as protrusions in the isobars out from the high pressure cell. A ridge has formed from the high pressure cell in Figure 4 over central Australia and northern Queensland. Fine weather is usually indicated by ridges.

A trough appears when a region of lower pressure appears in the air circulation around a high pressure cell. Troughs also occur at the junction between two highs. Troughs are drawn as “U”-shaped fluctuations in the isobars. Meteorologists and weather reporters describe troughs as ‘a dip in the isobars’.

A dashed line is often drawn along the central line of a trough. The weather map in Figure 3 shows a trough in eastern Australia. The trough line runs north from central New South Wales to northern Queensland. Troughs are usually associated with unsettled weather and precipitation.

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Cyclones

A cyclone is a relatively small but intense low pressure cell. Most cyclones that occur in Australia develop over warm oceans to the north and are called tropical cyclones. They usually occur from November to April. In the North Pacific Ocean cyclones are called typhoons and in the North Atlantic Ocean they are referred to as hurricanes.

Cyclones are drawn on weather maps as a series of tight circles. They have the letters “TC” with the name of the cyclone in the centre. Both male and female names are selected alphabetically. Cyclones are rated from category one to five, with five being the most severe. The weather map in Figure 3 shows a tropical cyclone approaching the coast of north-west Australia.

A cyclone is an intense low pressure storm, with gale force winds rotating clockwise around the centre or ‘eye’. Winds of over 100 km/hr are common at the leading edge of a cyclone, with wind gusts of up to 300 km/hr. The gales can produce high seas with waves over 30 metres in height. Torrential rains always accompany cyclones.

It is hard to predict where and when cyclones will cross the coast due to their erratic movement. When they do reach land they can cause severe damage, flooding and loss of life. Once they have moved inland cyclones weaken to become rain depressions. Despite the damage they cause cyclones contribute substantially to the much needed rainfall in northern Australia.

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Winds

Meteorologists have a series of symbols that they use to indicate wind speed and direction. They use the data gathered from the hundreds of weather stations located around Australia. Most weather maps available on internet sites, in newspapers and on television do not show these symbols.

However, wind direction and relative speed can be determined from the isobars. Remember that air circulates clockwise around a low pressure cell and anticlockwise around a high pressure cell. The closer the isobars are together, the stronger the winds. The wind direction is given as the compass point from which the wind is blowing.

Table 1 below gives a summary of the wind direction and relative speed from points around Australia on the weather maps in Figures 3 and 4.

Table 1 - Wind speed and direction

Weather map Location Wind direction Relative wind speed
Figure 3 A Northerly Gale
Figure 3 B Easterly Moderate
Figure 3 C South-easterly Light
Figure 4 D Easterly Calm to light
Figure 4 E South-westerly Strong
Figure 4 F Southerly Strong to gale
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Rainfall

Rainfall is indicated on weather maps as a series of diagonal lines (Figure 2). This symbol is used to show both potential and current rainfall.

Rainfall can also be predicted from the isobars and pressure cells on a weather map. Intense low pressure cells coming off the ocean, such as cyclones, always bring heavy rain to coastal areas. The rain may also extend inland. Troughs are often associated with unstable weather and precipitation.

Winds blowing from the ocean onto the land are called onshore winds. These winds can bring rain if they are associated with clouds containing moisture picked up as the wind blows across water. Onshore winds also bring cooler weather.

Offshore winds blow from the land out to sea. They are usually associated with fine weather. In summer, high pressure cells over inland Australia bring hot, dry winds and the risk of bushfires.

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