Internet Geography - GeoTopics - Weather and Climate

What is pressure?

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.

What is low pressure?

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. This occurs because the warm air cools and condenses when it meets the cold air (you can find out more about frontal rainfall here).

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.

What is high pressure?

Areas with above average pressure are called anticyclones. Anticyclones occur when air is sinking. As a result there are usually few clouds in the air. In the UK anticyclones bring cold clear days in winter and hot and sunny days in summer.

Light winds blow in a clock wise direction.

Weather experienced during a winter anticyclone	Weather experienced during a summer anticyclone
In winter the skies are cloudless so heat is allowed to escape. Therefore temperatures are usually very cold. The ground cools rapidly at night so frost often forms. Fog can also form as the cold air makes water vapour condense into tiny droplets. Fog can last long into the day as there is insufficient heat from the sun to evaporate the water droplets away.	Summer anticyclones bring very different weather. As the air descends it is heated causing water in the air to evaporate. Therefore there are few clouds in the air. The skies are clear allowing the suns rays to reach the surface of the earth. This causes temperatures to rise. Heat waves can occur if anticyclones remain over Britain for a number of weeks.