What are they
... area of high pressure caused by sinking air - it has less heat so is more stable, giving dry conditions.
Winds tend to move outwards and clockwise in the Nothern hemisphere to the areas of low pressure. The more widely spaced the isobars, the shallower the pressure gradient. This means lower wind speeds are generated.
The highest pressure is at the centre of the anticyclone and decreases as you progress outwards
Anticyclones can be 3000km in diameter and give weeks of settled weather. Due to the air originating in the upper atmosphere where water vapour is limited
Associated weather conditions
- Absence of cloud means there is intense insolation, causing hot, sunny days and a lack of rain
- Clear skies and radiation overnight cause temperature inversions, dew mist and fogs
- After several days of increased temperature, there is an increased risk of thunderstorms and the 'spanish plume'
- Absence of cloud means low temperatures, the development of fog and frost - these then take time to disperse due to the weakness of the sunshine
- Cold polar air which is dry and stable reaches the north sea, its lower layers acquire warmth and moisture, causing heavy snowfalls on the east coast
these occur when cells of high pressure detach themselves from the major high pressure areas of the subtropics or poles.
Once created they last from severald ays and block eastward moving depressions to create anomalous conditions such as extremes of temperature, rainfall and sunshine