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Reading
a Weather Map
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What do Weather Maps show?
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| The most obvious features of the
media's weather maps are patterns of high and low pressure, and
the barbed lines identifying cold fronts. In the southern hemisphere,
the earths rotation causes air to flow clockwise around low pressure
systems and anticlockwise around high pressure systems.
Friction over the earth's surface causes the winds to be deflected slightly inwards towards low pressure centres, and slightly outwards from high pressure systems. Wind strength is directly proportional to the distance between isobars - the closer the lines, the stronger the winds. This rule does not apply in the tropics where the effect of the earth's rotation is weak. For this reason, tropical meteorologists usually replace isobars with streamline arrows which indicate wind and direction without directly relating to the pressure gradient. Shaded areas on weather maps show where there has been rain in the previous 24 hours, and wind direction is shown with arrows that have a series of barbs on their tails to indicate speed. The coverage on media weather charts is usually limited to the
continent and surrounding oceans. The Bureau of Meteorology also
produces global charts to take account of weather systems interacting
with each other over great distances. Global charts are necessary
when preparing forecasts up to four days ahead, and framing the
monthly climate monitoring bulletins. |
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Typical Weather Map Patterns
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- A high pressure belt in the mid-latitudes (usually 30-50 degrees latitude) contains centres of varying strengths which generally move from west to east. Fluctuations in the intensity of these highs (anticyclones) strongly influence the behavior of the trade winds and the development and decay of tropical lows. - The belt of westerly winds south of the high pressure region
contains disturbances which usually travel west to east. Barbed
lines indicate the leading edge of traveling cold (and occasionally
warm) fronts, the boundaries between different types of air. The
term "front" was applied during World War I by European
meteorologists who saw similarities between atmospheric structures
and the large-scale conflict along a battle front. - A high pressure area over Antarctica - associated with extremely cold and dense air - ringed by easterly winds which form the boundary with the subpolar low pressure belt. These typical features vary in intensity and location according to the season. For instance, in summer the high pressure belt is usually found just south of Australia, while the subtropical easterlies cover most of the continent. Monsoon flows and associated lows over the tropics bring significant summer rain; tropical cyclones may develop. In winter the high pressure belt is usually located over the continent, allowing westerlies and strong cold fronts to affect southern Australia. It is important to be alert to significant exceptions to this 'normal' situation when, for example, strong high pressure systems move slowly across the oceans well south of Australia. Closed or 'cut off' lows may then move across southern Australia or intensify over the Tasman Sea, possibly causing prolonged heavy rain. It is also important to remember that all weather systems have a life cycle of development, maturity and decay/ They occasionally show unusual behavior. They may become stationary or even briefly reverse their usual direction of travel.
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Hor or Cold?
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| Remembering that air flows clockwise
around low pressure systems and anticlockwise around high pressure
systems, a fairly typical summer weather map (Figure 2) shows:
- Northerly winds over eastern Australia on the western flank of a Tasman Sea high. They carry hot, dry air from inland Australia southward over Victoria and Tasmania. With winds strengthening ahead of an approaching front, this represents a classic weather situation with extreme bush fire risk. - Moist, easterly flow from the Coral Sea onto the Queensland coast causes very warm, humid and sultry weather east of the Great Dividing Range. This air, often susceptible to the development of showers and thunderstorms, is described as 'unstable'. - The cold front passing South Australia replaces the hot, dry north westerlies with southerlies carrying cooler, often relatively humid air from waters south of the continent. Such summer fronts are often quite shallow and may not penetrate far inland, particularly if they are distorted and slowed over the Victorian mountains. |
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| In Figure 3, a relatively common
winter weather map shows:
- Very cold, unstable air from well south of Tasmania flows northward over Tasmania, Victoria and southeast New South Wales, reducing normal day temperatures typically by five degrees or more, Note the cold front, the deep low pressure (pressures below 976 hectopascals) south of Tasmania and the high (1020 hectopascals) south of the Bight. Occasionally, rapid interaction with other weather systems around the southern hemisphere can almost halt the pattern's eastward movement, causing successive cold fronts to bring a prolonged spell of cold, showery weather to southern Australia. - Easterly winds over inland Australia. Although southern cold fronts become shallow and diffuse as they move into northern Australia they often trigger a surge in the strength if the easterlies and this, combined with their extreme dryness, creates a very high fire danger in the tropical savanna region. - An active low pressure system near Perth is 'cut off' from
the southern westerlies. Situations of this type may cause rain
and rather cold weather over southern parts of Western Australia. |
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Rain or Fine?
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Features on the surface weather chart indicate likely rainfall patterns as well as temperature distribution and wind strength. In general, highs tend to be associated with subsiding (sinking) air and generally fine weather, while lows are associated with ascending (rising) air and usually produce rain or showers. While cloud can exist without rain, the opposite is not the case. Clouds form by the condensation of water vapor through cooling, Causes of cooling include: - Convection, which may be caused through air mass instability. It may be initiated by warning of low-level air, forced ascent over mountainous country, or dynamic causes associated with severe weather systems. Cumulus clouds often form as a result of convection. The most exceptional forms are often associated with severe thunderstorms and occasionally tornadoes. Cumulonimbus, for instance, may reach altitudes above 15,000 metres. - Systematic ascent of moist air over large areas linked with large-scale weather systems such as low pressure systems, including tropical cyclones. In mid-latitudes, this systematic ascent often occurs ahead of active fronts, or with 'cut off' lows. This type of rain may be persistent and heavy and cause floods, especially if enhanced by forced (orographic) ascent over mountains. - Orographic ascent which occurs when air is forced upwards by
a barrier of mountains or hills. Cloud formation and rainfall
is often the result. Australia's heaviest rainfall occurs on the
Queensland coast and in western Tasmania, where prevailing maritime
air streams are forced to lift over mountain ranges. As a typical cold front approaches, winds freshen from the north or northwest, and pressure falls. After the front passes, winds shift direction anticlockwise ('backing' to the west or southwest) and pressure rises. Cold fronts are much more frequent and vigorous over southern Australia then elsewhere. Warm fronts, relatively infrequent over Australia, are usually found in high latitudes where they can occasionally cause significant weather. They are often shown on weather charts over the Southern Ocean. Warm fronts progressively displace cool air by warmer air. - Convergence lifting which occurs when more air flows into an area at low levels then flows out, leading to forced rising of large air masses. Convergence is often associated with wave-like disturbances in tropical easterlies and may also occur with broad tropical air masses flowing to the south. Given sufficient atmospheric moisture and instability, it may cause large cloud clusters and rain.
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How Strong will the Winds
be?
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| 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 air streams 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 then those
in the tropics with similarly spaced 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 then these tropical storms.) |
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All content and images on this page have come
from the "Bureau of Meteorology - The Weather Map" brochure
For more information please contact the Bureau of Meteorology |
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