Atlantic Depressions and the British Isles
Frontal depressions have a significant impact on weather across the British Isles. They bring the wet and windy conditions we are used to in the UK.
Depressions are one of the most common features on weather maps across the UK and Ireland. The weather map below shows an example of a depression over the British Isles. If you are not familiar with weather charts the BBC have produced a really handy guide to decoding a weather forecast.
A depression over the UK
A depression is an area of low pressure, where the air is rising. As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. Consequently, the weather in a depression is often cloudy, wet and windy (with winds blowing in an anticlockwise direction around the depression). When the black lines (isobars) on a weather map are close together it means air pressure is changing quickly and therefore windy.
There are usually frontal systems associated with depressions. The red, blue and purple lines represent weather fronts. A red line with semi-circles is a warm front. A warm front has warm air behind it and forms the boundary between a large mass of warm air and cold air. A blue line with triangles shows a cold front. A cold front has cold air behind it and forms the boundary with warmer air. A purple line with semi-circles and triangles is an occluded front. This is an area where the warm front is no longer in contact with the surface of the Earth and sits above cold air.
The video below explores fronts in more detail.
The UK sits between a warm air masses to the south and cold air masses to the north. When a wedge of warm air moves north (or a wedge of cold air moves south) weather fronts form.
Where warm and cold air meets warm humid air is cooled by contact with the cold air. The moisture is condensed and forms cloud and rain. This forms two bands of rain, which can take 1-3 days to pass the UK.
Features of a depression
Depressions typically move over the UK from west to east. The UK is affected by around 100 depressions every year.