People like to talk about the weather every day. It impacts what we wear and our mood. In this section we are going to look at some of the things that create the weather across the British Isles and consider some of the weather systems that affect us.
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.
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.
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.