The British Climate
A temperate climate
The British climate is temperate, with no extremes of temperature and rainfall. Winters are usually fairly mild under the influence of the Gulf Stream, whilst summers are neither oppressively hot nor frustratingly cool. The average annual temperature is about 10°C, with a summer and winter average of approximately 15°C and 5°C respectively. The British climate is dominated by the tracks of frontal depressions which form in the Mid-Atlantic and pass across into Europe, bringing rain and frequent bad spells of weather.
A number of principal air masses that affect Britain have been identified, bringing with them characteristic patterns of weather. The more common air masses are shown below. These air masses are defined according to both their region of origin and their course travelled. Air from Arctic regions, for example, is classified as maritime arctic (mA). This air mass originates in the Arctic and travels across the relatively warm stretch of the North Sea. Maritime tropical air (mT), on the other hand, originates near the Gulf of Mexico and travels across the warm Atlantic before arriving in Britain. Other air masses include maritime polar (mP), continental polar (cP) and continental tropical (cT).
On an annual basis, the most frequent airflow is maritime, including polar and tropical air masses. When both cyclonic (low pressure) and anticyclonic subtypes are considered, maritime airflow accounts for a quarter of all climate patterns experienced in Britain, reaching a maximum of 35% during December and January. Maritime tropical air is mild in winter. In summer it is cloudy and rather cool but humid. When associated with frontal depressions rain is usually abundant; with anticyclones or high-pressure ridges, settled weather with warm sunny spells.
Maritime polar air streams produce cool, showery weather at all seasons, especially on windward coasts. The air has tracked over a relatively warm sea and becomes unstable as its lower layers are heated. Frequently during the passage of frontal depressions over Britain, a flow of maritime tropical air is followed by maritime polar air as the cold front passes. After a brief interlude of sunshine following the downpour of the cold front, frequent showers, associated with the moist unstable air may develop. These may be of snow during the winter months.
Continental polar air in winter is very cold and temperatures associated with this air stream are usually well below average. The air mass is basically very dry and stable but a track over the central part of the North Sea supplies sufficient heat and moisture to cause showers, often in the form of snow, over eastern England and Scotland. During summer, the airflow is usually warm, since even northern parts of Europe experience high temperatures during this time of year.
Continental tropical air reaches Britain from the Sahara; it is dry and in summer gives rise to heat waves, particularly in the southeast. The lower layers are stable, often capped by a temperature inversion, under which haze may build up. Sometimes, instability develops above the temperature inversion, giving rise to thunderstorms. In winter it gives pleasant mild weather.
Spatial variations in the British climate
The preceding sections described the general airflow that influences Britain, and the general patterns of climate that are experienced. Nevertheless, there are differences in the degree of oceanicity or continentality of the climate in particular areas. The figures below show the January and July surface temperature isotherms (lines of equal temperature) for Britain. Coldest temperatures during the winter occur in eastern Scotland and England. The temperature isotherms are orientated north-south, and reveal the warming influence of the maritime tropical airstream to the western half of Britain. The eastern half of Britain experiences greater continentality. In summer, the isotherms are orientated east-west and temperature variations due to latitudinal differences of solar radiation receipt are more evident, with highest temperatures in the south.
The British climate can be divided into four quarters. The northwest quarter is characterised by mild winters and cool summers, the northeast by cold winters and cool summers. The southwest experiences mild winters and warm summers, the southeast cold winters and warm summers. During winter, the western half of Britain experiences a more maritime climate, whilst the east receives influence from the cold air streams from the continent. In summer, climatic differences are more dominated by latitude.
The last figure shows the general pattern of precipitation in Britain. The western half, and in particular the higher ground, receives considerable rainfall, most of it frontal, but augmented by orographic uplift. Parts of highland Scotland can receive over 250 cm or 100 inches of precipitation per year. Precipitation amounts fall further to the east, particularly in the southeast of England, where certain areas may receive only 50 cm or 20 inches per year. The east of England, and to a lesser extent Scotland, lie in the rain shadow of the Welsh mountains, the Lake District and the Scottish Highlands.