A Change Is On The Way – How To Read Weather Maps

The weather map is one of the most familiar images in the community. The best known map is the mean sea level analysis, compiled from hundreds of weather observations (synoptic data) taken simultaneously around the Australian region. It is seen daily on television and in the newspapers.

Television and newspapers also often carry forecast weather maps which indicate how the weather patterns are expected to develop.

Meteorologists use a wide range of information and techniques to formulate weather forecasts. The weather map does not and cannot show all of these factors. It is a fairly simple representation of past and probable future locations of surface weather systems (highs, lows, fronts, etc.).

Without postgraduate studies in atmospheric physics, access to supercomputers, and years of experience, it's difficult to predict weather. And there is a great difference between weather and climate.

There is a variety of ways to explain the difference between climate and weather.

Climate is what you expect; weather is what you get.

Climate is about long-term records, trends and averages; weather is the day-to-day experience.

If weather is the watch, then climate is the calendar.

Reading A Weather Map

The lines on a weather map join places of equal mean sea level pressure and are called isobars ('iso' means equal).

Atmospheric pressure varies with both height above mean sea level, and from place to place at mean sea level (that is, horizontally).

We measure mean sea level pressure (MSLP) in units called hectopascals (hPa). The lowest MSLP ever recorded (870 hPa) was in “Typhoon Tip” in 1979; the highest ever recorded (1084 hPa) was in Siberia in 1968.

At the broadest scale, the equatorial region tends to be an area of relatively low mean sea level pressure. The mid-latitudes tend to be areas of relatively high pressure (typically around 30°N, 30°S) with another belt of low pressure further poleward (40°- 60° latitude), and relatively high pressure over the poles.

The main features of a typical weather map are:

  • Regions of high and low pressure, delineated by a pattern of isobars. The isobars are generally drawn at intervals of 2 or 4 hectopascals, depending on the scale of the map.
  • Cold and warm fronts that delineate major boundaries between air masses. These are shown as a bold line with either triangles for cold fronts or semi-circles for warm fronts along it.
  • Troughs of low pressure, shown as dashed lines.
  • Areas where rain has fallen in the past 12 or 24 hours, shown by hatching.
  • Surface wind direction and speed at spot locations. The direction from which the wind is blowing is designated by an arrow shaft and the speed by barbs along the shaft. (A long barb designates a speed of approximately 20 km/h. A combination of two long and one short barbs would designate 50 km/h.)

In some cases weather maps are overlaid on satellite images showing cloud bands associated with fronts and low pressure systems.

On a daily weather map (synoptic chart), such as the one shown here, there will be meandering patterns of relatively high and low pressure systems over Australia. It is these systems that generate variations in wind, temperature and rainfall. In particular the direction and speed of the wind is closely related to the distribution of mean sea level pressure.

In the southern hemisphere, winds flow clockwise around low pressure systems (and slightly inward) and anticlockwise (and slightly outward) around high pressure systems.

The strength or speed of the wind is closely related to the spacing of the isobars: closer spacing means stronger winds.

Inflowing air associated with a low pressure system results in convergence and ascent of air often resulting in deep cloud formations and rainfall. Conversely, outflowing air associated with a high pressure system results in divergence and descent of air and more stable weather.

Text and images courtesy of the Australian Bureau of Meteorology.


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