Weather is not the same as climate. The weather at a place is the state of the atmosphere there at a given time or over a short period. The weather of the British Isles is greatly variable.
The climate of a place or region, on the other hand, represents the average weather conditions over a long period of time.
The climate of any place results from the interaction of a number of determining factors, of which the most important are latitude, distance from the sea, relief and the direction of the prevailing winds.
The geographical position of the British Isles within latitudes 50o to 60o N is a basic factor in determining the main characteristics of the climate. Temperature, the most important climatic element, depends not only on the angle at which the sun’s rays strike the earth’s surface, but also on the duration of daylight. The length of day at London ranges from 16 hours 35 minutes on June to 7 hours 50 minutes on 21 December. British latitudes form the temperate nature of the British climate, for the sun is never directly overhead as in the tropical areas.
Britain’s climate is dominated by the influence of the sea. It is much milder than that in any other country in the same latitudes. This is due partly to the presence of the North Atlantic Drift, or the Gulf Stream, and partly to the fact that north-west Europe lies in a predominantly westerly wind-belt. This means that marine influences warm the land in winter and cool in summer. This moderating effect of the sea is in fact, the cause of the relatively small seasonal contrasts experienced in Britain.
The moderating effect of the ocean on air temperature is also stronger in winter than in summer. When the surface water is cooler than the air above it – as frequently happens during the summer months – the air tends to lose its heat to the water. The lowest layers of air are chilled and become denser by contradiction, and the chilled air tends to remain at low levels. The surface water expands because it is warmed, and remains on the surface of the ocean. Unless the air is turbulent, little of it can be cooled, for little heat is exchanged.
Opposite conditions apply in winter. The air in winter is likely to be cooler than the surface water, so that the heat passes from water to air. Air at low levels is warmed and expands and rises, carrying oceanic heat with it, while the chilled surface water contracts and sinks, to be replaced by unchilled water from below. This convectional overturning both of water and of air leads to a vigorous exchange of heat.
The prevailing winds in the British Isles are westerlies. They are extremely moist, as a result of their long passage over warm waters of the North Atlantic. On their arrival to Britain, the winds are forced upwards, and as a result large-scale condensation takes place, clouds form and precipitation follows, especially over the mountainous areas.
North and north-west winds often bring heavy falls of snow to north Britain during late October and November, but they are usually short-lived. Continental winds from the east sometimes reach the British Isles in summer as a warm, dry air-stream, but they are more frequently experienced in winter when they cross the north sea and bring cold, continental-type weather to eastern and inland districts of Great Britain.
Relief is the most important factor controlling the distribution of temperature and precipitation within Britain. The actual temperatures experienced in the hilly and mountainous parts are considerably lower than those in the lowlands. The effect of relief on precipitation is even more striking. Average annual rainfall in Britain is about 1,100 mm. But the geographical distribution of rainfall is largely determined by topography. The mountainous areas of the west and north have more rainfall than the lowlands of the south and east. The western Scottish Highlands, the Lake District (the Cumbrian mountains), Welsh uplands and parts of Devon and Cornwall in the south-west receive more than 2,000 mm of rainfall each year.
Реферат опубликован: 12/04/2006