Depressions are areas of low atmospheric pressure [atmospheric pressure: the weight of air resting on the Earth's surface. ] 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.
- At the warm front, lighter, warmer air from the south (tropical maritime air [tropical maritime air: A warm air mass that has formed over the sea, within 30° of latitude from the equator. ]) meets cooler air from the north (polar maritime air [polar maritime air: A cold air mass that has formed over the sea near the north (or south) pole. ]) and rises gradually over it.
- As the warm air slowly rises it cools, its water content condenses and clouds form (nimbostratus [nimbostratus: A type of thick dark grey cloud that produces precipitation in the form of rain or snow. ] then altostratus [altostratus: A type of cloud consisting of light grey sheets or layers. ]). The result is steady rain, later giving way to drizzle and finally clearer skies with high cirrus [cirrus: A type of high, thin, wispy, white cloud composed of ice crystals. ] 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 [cumulonimbus: A type of large, dense, tall, grey cloud which produces rain, hail, or snow. ], then cumulus [cumulus: A type of cloud made of dense mounds or heaps with dark bases and white rounded upper regions. ]). The result is heavy rain or thunderstorms, giving way to showers and finally to clear skies as the cold front moves away eastwards.