2.2 Depressions

Depressions tutorial

If you read the section on Fronts you will have learned that when areas of warm and cold air meet, the warm air rises up and over the colder air. When a fast moving area of cold air moves into a region of warmer air it forces its way under the warm air, which is pushed upwards. As it rises, air pressure falls.

This rising air can lead to a low pressure system or depression. Here is a reminder of the diagram showing warm rising over colder air. At the front, air pressure falls. 

cold front formation diagram

On weather charts and forecasts you will see fronts associated with a depression:

 

Low and fronts

 

Viewed from above, the air within a depression circulates in an anti-clockwise direction in the Northern hemisphere (they circulate clockwise in the Southern hemisphere). This is the opposite of anticyclones. Here's a diagram of a low pressure system over the UK:

 

Direcion of winds in a depression over the UK

 

Low pressure systems or depressions bring the cloud, rain and wind that we usually associate with British weather.