Weather & Climate- British Isles Climate

Anticylones

Characteristics:

• Anticyclones are characterized by high atmospheric pressure
• Represented by closed isobars with pressure increasing towards the centre
• move slowly and may remain stationary over an area for several days or weeks
• isobars are far apart therefore there is little pressure difference between the centre and edges of the anticyclone
• winds are weak and flow gently outwards in a clockwise direction in the northern hemisphere, anticlockwise in the southern hemisphere

Formation:

• The air that descends from the upper atmosphere is cooler, drier and denser than air at ground level. 
• As it descends it warms and can hold a lot of moisture leading to limited condensation and reduced rain. 
• This stable air can cover several thousand KM and presents a gentle pressure gradient with weak winds blowing clockwise out from the centre of the high pressure area. 
• The passage of anticyclones tends to be much slower than depressions. Weather can be days or weeks depending on the season.

Winter Anticyclones:

• Cold daytime temperatures and freezing night-time temperatures as less isolation is available to warm the British Isles and the clear skies of a high pressure zone mean that the moderate gain of warmth during the day can be more than offset by rapid loss of heat at night.
• Surface cooling frequently gives rise to radiation fog and frost which may persist because of the weak sunshine during the day. 
• High levels of atmospheric pollution in urban areas caused by subsiding and no wind.
• When dry cold polar continental air from central Asia and Europe moves across the North Sea it can bring heavy snowfall to eastern regions. 

Summer Anticyclones:

• Hot daytime temperatures and warm night-time temperatures as clear skies allow maximum isolation producing temperatures of over 25 during the day time.
• Generally clear skies, hazy sunshine in some areas.
• Formation of dew & mist which clear quickly in the morning.
• The east coast of Britain may have sea frets caused by onshore winds.
• Thunderstorms when the air has high humidity.

Where blocking anticyclonic cells exist for days over Europe the deflection of warmer depressions away from Britain can bring dry freezing conditions in winter and heatwaves in summer.

Fog

Radiation fog:

• Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. 
• The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. 
• Radiation fogs usually dissipate soon after sunrise as the ground warms. An exception to this can be in high elevation areas where the sun has little influence in heating the surface.
• Radiation fog is common under temperature inversion which often occur in valleys:

Valley fog:

• Valley fog forms where cold dense air settles into the lower parts of a valley condensing and forming fog. 
• It is often the result of a temperature inversion with warmer air passing above the valley. 
• Valley fog is confined by local topography and can last for several days in calm conditions during the winter.

Advection fog:

• Advection fog occurs when moist air passes over a cool surface and is cooled. 
• A common example of this is when a warm front passes over an area with snow cover. 
• It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction then sea fog can become transported over coastal land areas; a sea fret or haar.