Weather & Climate- British Isles Climate

Climate of the British Isles

TEMPERATURE:

• Temperature is reduced towards the north due to the reduction in amount of isolation at higher latitudes. Places further from the sea experience higher summer temperatures, on average as the cooler temps from the sea have less influence inland. 
• In larger land masses this is referred to as ‘continentality’ but is evident in the UK.
• Areas at higher altitudes are cooler as temperatures drop by 6.4 degrees for every 1000m increase.
• The effect of prevailing winds and oceanic currents are evident in the higher winter temperatures in the land bordering the Irish Sea as warmer air associated with the North Atlantic Drift brings warmer Gulf Stream waters to Western Britain.

Average summer and winter temperatures

WIND:

• Most common direction of wind is the South West but this is variable from day to day & winds from other directions are quite frequent. With long spells of easterly or north easterly not unusual in winter. Strongest winds are found in West and North of the country as these areas face the prevailing SW winds. Wind speed increases with height so mountains experience higher wind speeds.  
• A gale is defined as: wind speed of over 63 km/hr for a duration of at least 10 minutes.

PRECIPITATION:

North & West of the British Isles shows greater rain. The key factors affecting this pattern are the direction of the prevailing winds and altitude.

• Relief rainfall occurs when moist air that has been travelling over the sea is forced to rise over upland areas. As it rises and cools, the air reaches dew point (saturated) and condensation occurs leading to rainfall.

In areas such as Keswick in the Lake District rainfall averages around 1500mm per year whereas Tynemouth at similar latitude on the east only receives 660mm of rain falls. This is due to the RAIN SHADOW as air that has lost a lot of moisture over the hills will sink back down, warm up, and as warmer air can hold more moisture it is less likely to generate rainfall.

Britain is affected by FRONTAL RAINFALL:

• This occurs when warmer air is pushed up over a wedge of cooler air where two air masses meet in a frontal system. This occurs where cooler polar air undercuts warmer tropical air. As the warmer air is forced to rise it cools, water vapour condenses and forms cloud and rain over a wide area. Especially in winter months when successive low pressure systems or depressions approach the western shores of Britain. 

CONVECTIONAL RAINFALL:

• Rainfall due to extreme localised heating of the ground. (Summer months). 
• Occurs when air above ground is warmed becomes less dense than the surrounding air & rises. When it reaches dew point condensation occurs & clouds develop but very strong heating produces highly unstable air which continues to rise creating cumulonimbus clouds. Creates intense sudden rainfall affecting South East Britain especially.