Depressions are areas of low atmospheric pressure which 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.
The diagram shows a depression with a leading warm front and a trailing cold front moving from west to east across Britain.
Diagram 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 (nimbostratus then altostratus). The result is steady rain, later giving way to drizzle and finally clearer skies with high cirrus clouds.
- Behind the warm front is an area of warm, rising air and low pressure – 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.
Anticyclones are the opposite of depressions – they are an area of highatmospheric pressure where the air is sinking.
- As the air is sinking, not rising, no clouds or rain are formed. This is because as the air sinks it warms, meaning it can hold more water.
- The absence of fronts means winds may be very light.
- Consequently, high-pressure areas are often associated with settled, dry and bright conditions.
- In summer, anticyclones bring dry, hot weather. In winter, clear skies may bring cold nights and frost.
- In cold conditions, anticyclones may also bring fog and mist. This is because the cold forces moisture in the air to condense at low altitudes.