Depressions pass over Britain frequently. They form in the Atlantic and move east across the country, bringing changeable weather.
The British Isles are affected by a number of different air masses. When warm and cold air meet, a depression forms:
In the UK, depressions often follow a similar pattern. First, a warm front passes over, bringing rain and then warmer air. Then a cold front follows, bringing more rain and cooler air.
On synoptic maps, warm fronts are shown by a red line with red semi-circles. Cold fronts are shown by a blue line with blue triangles.
When the land warms up, it heats the air above it. This causes the air to expand and rise. As the air rises it cools and condenses. If this process continues then rain will fall. This type of rainfall is very common in tropical areas but also in areas such as South East England during warm sunny spells.