Depressions are areas of low pressure that bring unsettled, cloudy weather. These low pressure systems develop to the west of the British Isles in the Atlantic and are blown in a north-eastwards direction. They are responsible for the very changeable weather which is common across Britain.
- Depressions start where cold polar air from the north and warm tropical air from the south collide along an imaginary line called the “polar front”.
- Warm tropical air gradually rises up over cold polar air to form a warm front.
- This rising air results in less air at the surface, creating an area of low pressure.
- The cold, polar air then rushes in behind to replace the rising warm, tropical air and forms a cold front.
- Soon winds begin to spiral around the low pressure centre as cold chases warm.
- The two gradually merge to form an occluded front.
- Winds blow out from the centre of the low-pressure cell anti-clockwise and give very changeable weather and frontal rainfall.
- As the warm front approaches, it rains steady, later giving way to drizzle.
- After the warm front has passed, the weather becomes clear and milder but not for long...
- The occluded front brings continuous rain.
- As the cold front approaches, it becomes windier and colder.
- As the cold front passes over there is heavy rain or thunderstorms, giving way to showers.
- As the front moves away, the rain stops but the air becomes colder.