Pressure is the weight of air pressing down on the earth's surface. Pressure varies from place to place and and results in pressure systems. At times of low pressure the air is usually rising. As the air rises, it cools, condenses and forms clouds. Areas of low pressure are known as depressions.
Depressions occur when warm air meets cold air.
The point where warm air meets cold air is called a front. Along the front there is usually cloud and rainfall. Depressions, or 'lows', bring rain, strong winds and changeable conditions. Changeable weather is a feature of British weather, depressions are responsible for much of this. Strong winds blow in an anti clockwise direction.