Areas of sinking air which result in high pressure are called anticyclones. The opposite to an anticyclone is the cyclone or depression. High pressure systems have low pressure gradients (ie the air pressure doesn’t change rapidly). This means that the winds are gentle. As the air sinks, it warms up, leading to warm and dry weather.
Anticyclones are much larger than depressions and can lead to many days or weeks of settled and calm weather. Anticyclones often block the path of depressions, either slowing down the bad weather, or forcing it round the outside of the high pressure system. They are then called ‘Blocking Highs’.
As air descends, air pressure increases. When air hits the ground, it has to go somewhere. The earth’s rotation makes the air change direction. In the Northern Hemisphere the air is pushed clockwise. In the Southern Hemisphere the air is pushed anticlockwise.
Characteristics of Summer Anticyclones
– Hot days with few or no clouds.
– Light winds.
– Cooling of ground leading to morning mist.
– Warm moist air rising from the ground forming thunderstorms.
– Cloud cover over Eastern England caused by light winds blowing over the cooler North Sea.
Characteristics of Winter Anticyclones
– Cloudless skies but less radiation due to the low angle of the sun.
– Temperature drop, making the days cold and the nights even colder due to lack of cloud cover.
– Fog and frost forming at night.
– Cold air from Asia bringing snow to the East.
The phenomena known as a ‘blocking anticyclone’ is a persistant area of high pressure which sits over UK/Northern France and ‘blocks’ the passage of frontal systems moving in from the North Atlantic. The frontal systems have to route to the North or to the South of the blocking anticyclone. The blocking anticyclone starts as a ridge of high pressure extending from the sub-tropical Azores High which breaks off and establishes itself as a seperate warm anticyclone; the UK rarely sees cold anticyclones.