Anticyclones are high pressure weather systems that form over the British Isles frequently. They are characterized by calm winds with widely spaced isobars, sinking air that warms and decreases humidity, causing clear skies. In winter, anticyclones bring cold days and frosts with the potential for pollution inversions, while summer anticyclones result in hot days with hazy sunshine and the risk of thunderstorms from moist air masses. Anticyclones can influence weather conditions for up to 10 days as they move slowly across the region.

1. Anticyclones
ANTICYCLONES - BRITISH ISLES

2. Sketch a copy of the anticyclone below.
Remember anticyclones form
as a result of air piling up
aloft.
This encourages air to
subside / sink down.
As the air sinks, it warms, this
increases the amount of water
the air can hold, and thus
keeps the sky relatively cloud
free.
Leading edge

4. Anticyclones, Highs
• Anticyclones over the British Isles occur
frequently, but are less common than
depressions.
• Anticyclones give rise to different weather
conditions, depending upon the time of year.
• Anticyclones move slowly and are stable
features.
• They can be 3000km wide – much bigger than
depressions
• High pressure systems tend to be longer lasting
than depressions, once ‘blocked’ they can last
up 10 days. They deflect depressions away from
UK & Western Europe.

5. Anticyclones are characterised by:
• High pressure readings in the middle.
• Far apart isobars = calm wind.
• The air subsides (sinks). It warms adiabatically
which leads to a decrease in relative humidity =
lack of cloud & dry conditions.
• Air moving away from the centre of the high.
• Air spins clockwise around from the high.
• They normal travel to the British Isles from
mainland Europe.
• High pressure systems form commonly over
the Azores – ‘Azores High’.

6. Anticyclones give rise to different
conditions, depending on the time of year.
Winter
• Cold day time temps,
from below 0oc to 5oc
• Very cold night time
temps – frosts below
0oc
• Generally clear skies
by day & night
Summer
• Hot day time temp,
over 25oc
• Warm night time
temps
• May not fall below 15oc
• Generally clear skies
by day & night

7. Anticyclones give rise to different conditions,
depending on the time of year.
Winter
• Low level cloud may
linger and radiation
fog may remain in low
lying areas
• Causes high levels of
air pollution in urban
areas as pollutants
are trapped by
temperature inversion
Summer
• Hazy sunshine in some
areas
• Early morning mists
which disperse quickly
• Heavy dew in the
mornings
• East coast has sea frets
or haars (onshore winds)
• Thunder storms can
occur when RH is high
(Spanish Plume)

8. Radiation Fog

11. Advection Fog

14. Summer

21. Winter