Anticyclones

ANTICYCLONES - BRITISH ISLES

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

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.

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’.

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

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)

Anticyclones give rise to different conditions, depending on the time of year.

Radiation Fog

Advection Fog

Summer

Winter