Isobars

Isobars are lines on a weather map joining together places of equal atmospheric pressure.

On the map the isobar marked 1004 represents an area of high pressure, while the isobar marked 976 represents an area of low pressure.

Europe weather map with isobar symbolsIsobars on a weather map
  • The numbers measure the atmospheric pressure in millibars.
  • Usually isobars are drawn at intervals of two or four millibars (one thousandth of a bar).
  • The closer the isobars are together, the windier it is.
  • If the lowest number is in the middle circle, this is a low pressure or depression. Often low pressures mean wet and windy weather.
  • If the highest number is in the middle circle, this is a high pressure or anticyclone. Often high pressures mean dry, sunny weather.
  • Air pressure tends to range from 890 mb (a hurricane) to 1060 mb (an anticyclone).

The weather map below shows an area of low pressure centred to the north of Scotland.

As we move away from the centre, the isobars increase in atmospheric pressure and become wider apart.

There is a zone of high pressure over Italy in the south-east corner of the map.

UK weather map showing pressureWeather map of the UK

Note the three different kinds of front shown on the map - a cold front, a warm front and an occluded front.

Fronts occur where two different air masses meet.