Climate and Weather

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 notoriously variable. The climate of a place or region, on the other hand, represents the average weather conditions through the year. In every part of the British Isles obvious changes are taking place as winter passes into spring, spring into summer, and so through autumn to winter.

The position of the British Isles within latitudes 50° to 6I°N is a basic factor in determining the main characteristics of the climate. Within the limits of the general climatic type - maritime, temperate, with no dry season and with summers only moderately warm - there is, however, room for considerable variation between one region and another.

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. These factors must be distinguished from the actual features of the climate such as temperature, precipitation, wind, sunshine, fog, the humidity of the air.

Britain has agenerally mild and temperate climate, which is dominated by marine influences and is rainy and equable. Britain's climate 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 which begins as the Gulf Stream, in the Gulf of Mexico, crosses the Atlantic Ocean, and so reaches the shores of Europe as a warm current, and partly to the fact that north-west Europe lies in a predominantly westerly wind-belt. This means that not only do marine influences warm the land in winter and cool it in summer, but also that the winds blowing over the Atlantic have a similar effect and at the same time carry large amounts of moisture which is deposited over the land as rain. Britain's climate is generally one of mild winters and cool summers, with rain throughout the year, although there are considerable regional changes.

Latitudes determine 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 greater the angle of the sun above the horizon, the greater is the heat received and the length of the period between sunrise and sunset. The length of day at London ranges from 16 hours 35 minutes on 21 June to 7 hours 50 minutes on 21 December.

The sea greatly modifies the climate of the British Isles, for their relatively small area and the indented nature of the coastline allow maritime influences to penetrate well inland. The sea, whose waters have a higher specific heat than the rocks of the Land surface, warms up more slowly, but also cools down more slowly than does the land. Consequently, in summer the land tends to be warmer than the sea, and in winter the converse is true. This moderating effect of thesea is, in fact, the cause of the relatively small seasonal contrasts experienced in Britain.

The prevailing winds in the BritishIsles arewesterlies.They are extremely moist, as a result of their long passage over the warm waters of the North Atlantic. On theirarrivalover Britain, the winds are forced upwards,and as a result large-scale condensation occurs, clouds form and precipitation follows, especially over the mountainous areas.

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 having far more rainfall than the lowlands of the south and east. The western Scottish Highlands, the Lake District, the Welsh uplands and parts of Devon and Cornwall receive more than 2,000 mmof rainfall each year. The greatest annual rainfall recorded inBritain was 6,527 mm at Sprinkling Tarn (Cumbria) in 1954. Much of this precipitation takes the form of snow, and on some of the highest summits of the north a layer of snow may persist for several months of the year.

In contrast, the eastern lowlands, lying in a rain-shadow area, are much drier, and usually receive little precipitation. Much of East Anglia has a rainfall of less than 700 mm run each year, and snow falls on only 15 to 18 days on the average. The lowest annual rainfall was recorded at Margate (Kent) in 1921 (236 MM).

Rainfall is fairly well distributed throughout the year, but, on average, March to Juneare the driest months and October to January the wettest.

Ireland is in rather a different category, for here the rain-bearing winds have not been deprived of their moisture, and, although low-lying, much of the Irish plain receives up to 1,200 mm of rainfall per year, usually in the form of steady and prolonged drizzle. Snow, on the other hand, is rare, owing to the warming effects of the North Atlantic Drift.