Frontal depressions across the British Isles

An area of low pressure is called a depression.

Air rises in a depression so clouds and rainfall are formed.

Depressions therefore bring unsettled weather and rain. Winds are normally stronger.

They usually form over the Atlantic Ocean and are carried across Britain by westerly winds.

They produce cloudy, rainy and windy weather.

These low-pressure systems often begin in the Atlantic, moving eastwards towards the UK. They are responsible for the UK's changeable weather.

Depression formation

In the Atlantic Ocean, the cold polar maritime and warm tropical maritime air masses will meet.

The lighter warm air will start to rise up over the denser, colder air and this creates an unstable area that will develop a front.

Sequence of a depression

The diagram shows a depression with a leading warm front and a trailing cold front moving from west to east across Britain.

Characteristics of a depression
  • At the warm front, lighter, warmer air from the South (tropical maritime air) meets cooler air from the North (polar maritime air) and rises gradually over it.
  • As the warm air slowly rises it cools, its water content condenses and clouds form (cirrus, altostratus and then nimbostratus). The result is steady rain, later giving way to drizzle and finally clearer skies.
  • Behind the warm front is an area of warm, rising air and low pressure (the warm sector) - the centre of the low-pressure system. As this part of the depression passes over, there may be a short period of clear, dry weather. However, at the trailing cold front, heavier, cooler air meets the warm air at the centre of the depression, undercutting it and forcing it steeply upwards. Quickly moving air masses produce high winds and cooler temperatures.
  • As the rapidly rising warm air cools, its water condenses and clouds form (cumulonimbus, then cumulus). The result is heavy rain or thunderstorms, giving way to showers and finally to clear skies as the cold front moves away eastwards.
Illustration showing depressions on a UK weather chartA depression on a weather chart

Key features of a depression

  • Pressure – Low and falling (below 1000 mb).
  • Temperature – Varies depending on the air mass passing overhead (and the time of year).
  • Cloud cover – An ever-changing selection of clouds will change as the depression passes. The maximum amount of cloud will be when the cold front passes overhead.
  • Wind speed and direction – Strong winds will usually blow in an anticlockwise direction. The depression will usually move in an NE direction across the British Isles.
  • Precipitation – There are variable amounts of rainfall as a depression passes. A depression usually takes about 5 days to pass and this can bring a wide variety of rain, sleet and snow during that time.