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madeinatlantis
LONDON TRAVEL GUIDE
THE CLIMATES OF BRITAIN
CLIMATE is average weather, and although the weather of Britain is everywhere changeable, yet the average result of the changes is not the same for all parts. The southeast is as markedly contrasted with the north-west in the matter of climate, as in position, outline, relief, and structure. In their passage off Ireland and Scotland the storms frequently carry their rainy south-eastern quadrants over the land, and the oceanic border is, in consequence, the most rainy portion of Britain. Anti-cyclones, on the other hand, which tend to produce drought, are rare along the shores of the ocean, although not infrequent, either in summer or winter, over the portion of Britain which lies opposite to the continent. But there is a second reason why the north and west should be rainy: they are the districts which contain the British uplands. When wind, sweeping over the plain of the sea, strikes a mountain slope, it must diverge from its former path either round the flanks or over the summit of the mountain. In so far as it is forced upward, the air, being relieved of pressure, expands a little and is chilled, and since it is saturated with oceanic moisture, it drenches the slope opposed to it with torrents of rain. Thus western Scotland and Ireland are moist, both because they are frequently traversed by the most rainy quadrants of cyclones, and because they are mountainous. For a similar double reason the English plain is relatively dry: it is a lowland district, and it lies beyond the most usual path of the cyclones.
The leeward side of mountains is drier than the windward side, for the air which has passed over a ridge, having shed much moisture on the upward slope, is compressed and slightly warmed on the downward slope. This dryness to leeward of the heights has been termed their rain-shadow. In Britain, where so much of the rainfall is due to cyclonic influences, even the leeward slopes receive abundant moisture; yet the rain-shadows to eastward and north-eastward of the hills are distinctly indicated upon the map. They are evident in the form of areas of smaller precipitation to the east of Dartmoor and of Wales, and in the neighbourhood of Dublin; but the most conspicuous is on the east coast of Scotland, and especially in Buchan and in the Straths of Dee, Don, and Spey, which lie under the lee of the Grampians, the highest and most massively continuous of the British uplands. To the west of the Grampian summits the rainfall is probably the heaviest in Britain, and it is here, in consequence, that denudation has trenched the deepest and most numerous glens.
Temperature is distributed over Britain, through the seasons, according to a somewhat different law. In July the isotherm, or line of equal temperatures, indicating an average of 60° Fahrenheit for the month, follows a sinuous course from west to cast, through Ireland, Wales, and the North of England. At this season of the year, direct sunshine counts as an important determinant of air-temperature even in Britain, and as a consequence, heat diminishes in the normal poleward direction. In January, however, the isotherm of 40° Fahrenheit for the month strikes almost due southward along the west of Scotland and the east of Wales; all points to the east of it have an average January temperature of less than 40°, while Cornwall, Wales, Ireland, and the Western Isles of Scotland have a temperature of more than 40°. The winds from a relatively warm ocean, and the latent heat set free by the frequent rains are the causes of the mildness of the western winter. Britain may be divided into four climatic provinces, or quadrants, by means of the July isotherm of 60° and the January isotherm of 40°. In the south-eastern quadrant, round London, the summers are warm and the winters cold, the average temperatures of January and July differing more than 20°, and the climate being, therefore, relatively extreme and continental in type. In the north-west, on the oceanic border, the winters are mild and the summers cool: they differ in average temperature less than 20°, the climate being equable and oceanic in type. The north-eastern and south-western quadrants have intermediate characteristics. At Aberdeen, in the north-east, the winter is cold and the summer relatively. cool, the average temperature for the whole year being comparatively low. At Waterford and Plymouth, in the south-west, the summer is warm and the winter mild, the average temperature for the whole year being relatively high. If a statement of the rainfalls be added to the variation of temperature, the four provinces may be thus defined:
The south-east, containing London, Norwich, and Lincoln, is relatively extreme and dry. The north-east, containing Aberdeen, Edinburgh, and Newcastle, is dry, but somewhat cooler and less extreme than the south-east. The north-west, containing Stornoway, Londonderry, and Galway, is very equable and moist. The southwest, containing Waterford and Plymouth, is equable and moist, but, on the whole, warmer than the north-west. The isotherms drawn across Britain exhibit characteristic curves in correlation with important physical features. The July isotherm of 60° bends northward over Ireland and England, and southward over the Irish Sea, for in summer the land is warmer than the sea. The January isotherm of 40°, on the other hand, exhibits an eastward bend over the English Channel, for at this time the sea is warmer than the land. These lines, however, which indicate the average temperatures throughout the twenty-four hours, give smaller indication alike of the contrast between south-east and north-west, and of the influence of the channels and the uplands, than do other lines drawn according to the mean minimum temperature of the January nights and the mean maximum of the July days. On an average January night the Land's End is ten degrees warmer than is the region of the East Midlands in the neighbourhood of the Fens, whereas the highest temperature of an average July day in this midland district is ten degrees warmer than on the coasts of northern Scotland at the same season. Moreover, the charts which bear these lines of mean minima and maxima make apparent, with almost diagrammatic clearness, the influence not merely of the Irish Sea and the English Channel, but of the constrictions of Great Britain at Glenmore, the Midland Valley of Scotland, and Solway Firth, and they also show the influence of the Cheshire Gap and the estuaries of the Severn and Thames.
The diminution of winter temperature in Britain, from west to east rather than from south to north, is a part of one of the most striking climatic phenomena on the globe. In the month of January a great northward spread of oceanic warmth covers all the eastern half of the entry to the Arctic Basin. It is due to a wind-driven set of the oceanic surface in a poleward direction, moving on the average perhaps four and a half knots a day. The limit of the consequent warmth in the atmosphere may be assumed conveniently at the January isotherm of 32° Fahrenheit. This line leaves the North American coast south of the latitude of Newfoundland, and, running just east of Iceland, attains to its northernmost point in the neighbourhood of the North Cape of Scandinavia. Thence it turns sharply southward and follows the Norwegian coast, cutting through the marginal fringe of islands and peninsulas, and, continuing still southward through Denmark, and across Europe almost to the head of the Adriatic, turns finally south-eastward and eastward along the Balkans. As seen from another planet in an average January, the earth north of the fortieth parallel of north latitude -- north, that is to say, of New York, the Pyrenees, and the Balkans -- would present a generally shining surface, indicative of snow and ice, and into this would be thrust north-eastward to the seventieth parallel a dark pointed gulf, wherein the waters would be free of ice and the lands of snow; but the northern extreme of this gulf would be hidden in the midwinter night. Britain is placed not far from the centre of the area of abnormal warmth of winter, and the British winters are, in consequence, milder than those of any other region under the same northern latitudes.
Other minor, but not unimportant, determinants of climate exist, and of these the influence of elevation is the most significant. Alike in summer and in winter temperature tends to decrease upward. The winds also, being less impeded by friction, are on the whole stronger on the peaks than on the plains, and bright sunshine -more effective for the growth of plants than the diffused light from a grey sky -- is rarer on the clouded uplands. In all these respects of relative temperature, wind, and sunshine, the oceanic border would be less favoured, by virtue of its mere position, than the continental angle of Britain, but the contrast is accentuated by the uplands of the north and west, and the plain of the south-east. On the eastern or corn-growing side of the country the greater length of the summer day in the north is not without appreciable agricultural value.
Local causes of still more restricted operation are aspect, the character of the soil, and the nature of the vegetation. In a latitude so northerly as that of Britain a hill-slope facing southward receives the sun's rays more directly than does a flat surface or a slope tilled northward. As a consequence, the southern faces of hills are more sunny -- in the winter time more genial, in the summer time more burnt; and this influence of aspect is the more important in proportion to the amount of sunshine -- more effective, therefore, in the English plain than among the uplands of the oceanic border. Moreover, a slope, of whatever aspect, is exposed to less rigour in a frosty night, because the heavy cold air glides away from it into the valley bottoms. A sandy surface is the cause of greater extremes of temperature than is a heavy clay soil, for the air in the interstices of the sand is a bad conductor of heat, and temperatures, either high or low, are accumulated in the surface layer, whereas the water in clay, having a high specific heat, prevents a rapid rise of temperature, and checks a fall. Vegetation tends to equability, because of the stratum of air which is detained by the foliage, and by reason, also, of the evaporation of moisture from the vast surface of the total leafage.
Instrumental observations permit of the tabulation of the different factors which go to form climate, but the general character of the vegetation is the clearest and most subtle index of their combined effect. Only five per cent. of the total weight of the wheat crop is said to be derived from the soil, the remainder wholly from the atmosphere. The approximate conditions of active vegetable growth in British latitudes appear to be a temperature of not less than 42° Fahrenheit, and a rainfall of at least eighteen inches. If the number of degrees be set down by which, on the average of the twenty-four hours, the temperature of any day exceeds the minimum of 42°, and the total of such numbers for all the days in the year be added together, a sum of "day degrees" is obtained which expresses roughly the vegetative potentiality of the climate. When we bear in mind how late in the warm season British wheat is harvested, it will be seen that there is no large margin of climatic power available. Maize will nowhere ripen in Britain, but it can be grown as a fodder crop. On the other hand, owing to the longer summer days and to the late season of the maximum rainfall, the conditions suffice for the maturing of both wheat and barley even in parts of Scotland.
The excess of moisture along the oceanic border causes oats to be generally substituted for wheat and barley in all but the eastern counties of England and Scotland. Wheat, being deeply rooted, can withstand both the frosts and the droughts of the English plain, but it has there always been considered a precarious crop, owing to the occasional defect of adequate summer heat and to the frequency of thunderstorms. In those portions of Scotland and the north of England in which wheat can be grown at all, it is perhaps more secure on the average of years than in the south, both on account of the comparative rarity of thunderstorms and of the more marked equability of the seasons. Barley may be raised under rather less favourable conditions of temperature than wheat, and will endure a greater amount of moisture, but it matures best in the wheat districts of the east of England, although in dry season it suffers more from drought, being less deeply rooted than wheat. Pasture grass is, however, by far the most characteristic crop of Britain, and the best suited to its climate. The humidity of the air maintains its verdure: the frequent rains supply its growth. Neither the frosts nor the droughts of the English plain permanently injure it, although they may check its productivity, but it grows with greatest luxuriance in the lowland portions of the west and north, where frosts and droughts are rare and short. It is probably less dependent than the cereals upon direct sunshine, and thus flourishes along the oceanic border, where the annual total of day degrees of temperature is high, but the heat is largely dark heat, brought by the winds from the sunshine of more southern latitudes. On the uplands even grass is displaced by bogs of peat moss, and moors of heather, gorse, and bracken.
According to the actual vegetation of to-day Britain belongs to three zones of climate. The lowlands of the east and south-east are largely agricultural and devoted to cereals. The lowlands of the west and north are pastoral, or given to the cultivation of oats; while the uplands, especially in the north and west, are clothed with bog and moor. Like the continental areas immediately to east and south of it, Britain seems to have been densely forest-clad in pre-historic times, and tree trunks are frequent in the peat bogs of Scotland and Ireland. All the world over, however, forest has been found to thrive under conditions which do not permit of an easy restoration of tree growth should it once be removed. Trees afford shelter to one another and to the undergrowth, but on naked, wind-swept uplands the saplings are bent, the soil is washed away by torrential rain, and the young shoots are eaten by cattle and sheep. In the populous lowlands of Britain few relics remain of the primeval forest, but there are many small woodlands of recent plantation.
In a highly civilised land, with artificial and luxurious modes of life, climate acquires an added geographical significance by determining the position of the health resorts. In the region of the Channel-entries a place of southern exposure, in the neighbourhood of Torquay or Penzance, has an exceedingly mild but moist winter. Further to eastward such places as Brighton and Folkestone have winters a little colder, marked, however, by more sunshine, and by a less humid air, for the temperature of the Channel surface is maintained, not by a wind-driven set, as is the case further to westward, but by the periodic intermingling of oceanic water, due to tides sweeping in upon the submerged platform. In the north-east of Scotland are conditions of a very different nature. Owing to the high barrier of the Central Grampians about Cairn Gorm, Ben Macdhui, and Lochnagar the rainshadow is very pronounced, and it is possible to obtain summer coolness at high levels without the excessive rainfalls which characterise the heights almost everywhere else in Britain: hence the popularity of Braemar and other resorts in the glens of the Dee, the Don, the Avon, and the Spey.
Thus it is true in utmost detail, as in the large, that the climates of Britain are related to the features of its configuration. The connection may be traced with equal facility in the maps illustrative of the distribution of pressure, of temperature, of storm tracks, of rainfall, and of sunshine. From end to end the land is divided -- by
the belt of greater uplands, extending northward and southward from Sutherland to Devonshire -- into western windward, and eastern leeward sides, but each transverse gap in the high ground admits the western influences through to the east. The broadest generalisation, however, is that, based on the frequency of cyclones in the north-west and of anti-cyclones in the south-east, which accumulates yet another contrast between the continental angle and the oceanic border.
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