The Weather Map

by Ken Batt, Bureau of Meteorology on 5 Aug 2006 As navigator or skipper of a yacht, whether cruising or racing, how comfortable are you with your weather skills? A very good working knowledge of the weather is a very fundamental component to the successful navigation of any craft.


Amongst a number of tools available to the weather forecaster (which includes you) is the weather map. The best-known map would have to be the Mean Sea Level (MSL) analysis, commonly known as the surface weather map. This particular map or chart can be obtained from the internet as shown above(a weather map in this case of the South Pacific including New Zealand and Australia), and of course local newspapers and television.

How many times have you looked at the weather map and said what does this all mean? Don’t worry you are not alone! Hopefully this article should help to get you on your way to a better understanding.

The weather map is assembled at regular, normally 3 hourly, intervals during the day. It is constructed utilising a number of inputs, the most basic being surface observations which are taken at three hourly intervals around Australia (and around the world) and its off-lying islands. These observations are performed by Bureau staff, farmers, house-persons, ships at sea, just to name some, and comprise mean sea level atmospheric pressure, temperature, atmospheric moisture, wind, cloud, present and past weather. This information is then plotted onto the surface map.

The chart is then analysed, with smooth, curvy lines being drawn, known as isobars (lines joining places of equal mean sea level pressure) that show the position of major weather features (highs, lows, tropical cyclones, fronts, troughs and ridges). During the analysis stage the analyst will generally utilise weather satellite imagery to assist with the positioning of weather systems. The effect of topography is also taken into account.

Weather forecasters use a wide range of information and techniques to formulate weather forecasts. The surface weather map does not and cannot show all of these factors. Nevertheless it will help you very much in your forecasting endeavours.

Let us now test our knowledge of the weather map. Look at the map and name the main features. Can you associate its idealized weather with each system? What is the wind direction and speed at Sydney, Brisbane and Hobart? How did you go?

Now for the answers but in a systematic way:

We should first establish the time of the map. Look at the bottom left-hand corner of the map, you can see that the MSL Analysis was valid for 0000 UTC, Co-ordinated Universal Time or Greenwich Mean Time (GMT) on 14th April 2000. This translates to 10 am Eastern Standard Time (EST) on 14th April. (EST is 10 hours ahead of GMT, 11 hours during Daylight Savings Time). If instead of Analysis the word Prognosis appeared then the map becomes a forecast map, showing the position of the weather systems at the time quoted. The prognostic chart is one of the main tools of the weather forecaster.

Next we should be able to pick out the high and low pressure systems. Highs are denoted by the letter H, and lows by the letter L. The centre of these systems is marked by a cross and the central pressure is shown close by using a numerical value. Highs are located in the New Zealand area (central pressure 1028 hectopascals, hPa) and to the west and southwest of Perth (central pressures 1026 hPa and 1025 hPa respectively). The weather normally associated with a high can be described as settled - light winds, particularly close to the centre, less cloud when compared to a low pressure system and, mostly, rain-free. Highs move with an average speed of around 15 knots (30 km/h) in the Australian region.

A low pressure system usually presents unsettled conditions - stronger winds (though light or calm near the centre), cloudy skies and, very often, rain. But there are many variations!

There are four lows indicated on the map, one in the tropics and three south of Australia, the latter three being located near 52 south 132 east, 50 south 114 east and 47 south 130 east. Notice that in each case these three lows are associated with solid lines which are known as fronts (more later). This association is very typical in southern latitudes. Lows and their associated fronts move with speeds typically around 25 knots (50 km/h) over the southern states, but their speed generally increases as you move further south.

The low pressure system in the tropics, situated north west of Port Hedland, is a tropical cyclone (TC Paul). As shown on the map TC Paul has a central pressure of 985 hPa and is moving towards the south west at 15 knots (30 km/h).

Another feature of the weather chart is the ridge and the trough.

A ridge is an elongated area of high pressure. Ridges can be indicated symbolically on a weather chart using a saw-tooth shape. However this is not normally done in practice. On the chart provided there is a ridge emanating from the high pressure system to the east of New Zealand onto the northern NSW and Queensland coasts; and another is evident extending into southern Western Australia and western parts of South Australia from the high to the west of Perth. Ridges are normally associated with similar weather to that of the high itself.

A trough is an elongated area of low pressure. Troughs can be indicated symbolically by a dashed line. In our example, a trough is indicated well to the south of Perth at around 47 degrees south latitude. As with ridges, they will not always be directly indicated on a weather map. In the example, one can infer by the shape of the isobars (in this case cyclonic curvature) that a trough exists from the east coast of Tasmania to just east of Canberra. Troughs are often associated with inclement weather similar to that of a low.

Winds around low pressure systems in the southern hemisphere blow clockwise, and around high pressure systems they blow anti-clockwise. In both cases they blow near parallel to the isobars. Thus, from the weather chart it can be deduced that the wind direction in coastal waters off the NSW coast would be from the north-northwest (NNW), in coastal waters off the Queensland coast they would be from the east-southeast (ESE), and about the east coast of Tasmania they would blow from the northwest (NW). These winds would be known as the friction-free or Gradient Wind that operate generally at a height of about 900 metres above the Earth’s surface.

The friction-affected or surface wind (at a height of 10 metres above the earth’s surface) blows slightly across the isobars, slightly inwards towards low pressure and outwards from high pressure. On the chart for 14 April 2000, the surface wind would be more from the north (N) at Sydney and Hobart than is the gradient wind; and at Brisbane, more from the southeast (SE).

The rule of thumb for allowing for the effect of friction on surface wind direction is to veer the gradient wind by about 30 degrees over land and 10 degrees (less friction) over the sea. As an example from our map, the surface wind at Sydney, where the free wind is from the NNW or from 340 degrees true, would be a wind from 010 degrees true (340 plus 30 giving 010 degrees). As an exercise in applying this rule of thumb, estimate the surface wind direction at Brisbane and Hobart from the map.

Note: If your calculated wind direction does not tally to that being observed then firstly, check your math. If OK then local winds (sea/land breezes, katabatic or anabatic winds) or even local topography could be at work.

Next look at the isobars. They should be analysed with 4 hPa spacing. How close together or far apart are they? In areas where they are close together, for example, south of the Great Australian Bight, the pressure gradient (the difference in pressure per unit distance) is steeper and the winds are stronger in comparison to where the isobars are further apart, say to the east of Perth, whe