What is a depression?

A depression is a weather system of low pressure which moves across the British Isles from west to east. It involves two air masses which meet at a front. The air masses are tropical maritime which brings warm rainy from the tropics and polar maritime air which brings cold rainy weather from the north. As the depression approachs the warm front hits the UK first, the warm lighter air meets the colder air and rises over it. As the air rises it cools, condenses and forms nimbostratus clouds, bringing rainy weather. As the warm front passes the UK moves into the warm sector, which is an area of steady pressure, calm winds and clear skies. As the cold front approaches next, the colder air undercuts the warm air forcing it to rise and resulting in a decrease in pressure. This causes cumulonimbus clouds to form and brings heavy rain.