Climate of the Alps
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The climate of the Alps is the climate, or average weather conditions over a long period of time, of the exact middle Alpine region of Europe. As air rises from sea level to the upper regions of the atmosphere the temperature decreases. The effect of mountain topography on prevailing winds is to force warm air from the lower region into an upper zone where it expands in volume at the cost of a proportionate loss of heat, often accompanied by the precipitation of moisture in the form of snow, rain or hail.
The position of the Alps in the central European continent profoundly affects the climate of all the surrounding regions. The accumulation of vast masses of snow, which have gradually been converted into permanent glaciers, maintains a gradation of very different climates within the narrow space that intervenes between the foot of the mountains and their upper ridges; it cools breezes that waft to the plains on either side, but its most important function is to regulate the water supply of the large region which is traversed by the streams of the Alps. Nearly all the moisture that is precipitated during fall, winter, and spring is stored in the form of snow and gradually diffused in the course of the succeeding summer; even in the hottest and driest seasons the reserves accumulated during a long preceding period of years in the form of glaciers are available to maintain the regular flow of the greater streams. Nor is this all; the lakes that fill several of the main valleys on the southern side of the Alps are somewhat above the level of the plains of Lombardy and Venetia, and afford an inexhaustible supply of water, which, from a remote period, has been used for that system of irrigation to which they owe their proverbial fertility.
Six regions or zones, which are best distinguished by their characteristic vegetation, are found in the Alps. It is an error to suppose that these are indicated by absolute height above sea level. Local conditions of exposure to the Sun, protection from cold winds, or the reverse, are of primary importance in determining the climate and the corresponding vegetation.
The Subalpine is the region which mainly determines the manner of life of the population of the Alps.
Roughly one quarter of the land lying between the summits of the Alps is available for cultivation. Of this low country, about one half may be vineyards and grain fields, while the remainder produces forage and grass. Of the high country, about half is utterly barren, consisting of snow fields, glaciers, bare rock, lakes and stream beds. The other half is divided between forest and pasture, and the product of this half largely supports the relatively large population. For a quarter of the year the flocks and herds are fed on the upper pastures, but the true limit of the wealth of a district is the number of animals that can be supported during the long winter, and while one part of the population is engaged in tending the beasts and in making cheese and butter, the remainder is busy cutting hay and storing up winter food for the cattle.
The larger villages are mostly in the mountain region, but in many parts of the Alps the villages stand in the subalpine region at elevations varying from 1200 m to 1700–1800 m (4000–6000 ft). The most characteristic feature of this region is the prevalence of coniferous trees that, where they have not been removed, form vast forests that cover a large part of the surface. These play a most important part in the natural economy of the country. They retain the soil by their roots, protect the valleys from destructive avalanches and mitigate the destructive effects of heavy rains. In valleys where they have been cut away, waters pour down the slopes unchecked; every tiny rivulet becomes a raging torrent that carries off the grassy slopes and devastates the floor of the valley, covering the soil with gravel and debris.
In the conifer forests of the Alps the prevailing species are the Norway spruce and the silver fir; on siliceous soil the European larch flourishes. The Scots pine is chiefly found at a lower level and rarely forms forests. The Swiss pine is found scattered at intervals throughout the Alps but is not common. The mountain pine is common at higher altitudes, often forming a distinct zone of Krummholz above the level of its congeners on the higher mountains. In the Northern Alps the pine forests rarely surpass 1800 m (6000 ft) elevation, but on the south side they commonly attain 2100 m (6900 ft), while European larch, Swiss pine and mountain pine often extend above that elevation.
The Alps are eponymous of the Alpine climate typical of the Alps between the tree line up to the permanent snow line, roughly between 1800 m and 2500 m. This alpine region contains the full beauty and variety of characteristic vegetation of the Alps.
The region contains many shrubs:
- Three species of rhododendron have masses of red or pink flowers;
- The common junipers grow at elevations above the rhododendrons.
- Three species of bilberry are associated with the junipers.
- Several dwarf willows grow near the snow line.
On the higher parts of lofty mountains in the Alps more snow falls in each year than melts. A portion of this is carried away by the wind before it is consolidated, but a large portion accumulates in hollows and depressions of the surface and is gradually converted into glacier ice which descends by slowly flowing into the deeper valleys where it help swell perennial streams.
Mountain snow does not lie in beds of uniform thickness and some parts are more exposed to the sun and warm air than others. Beds of snow commonly alternate with exposed slopes covered with brilliant vegetation without an obvious boundary of perpetual snow. A seemingly clear boundary seems visible when a high mountain chain is viewed from a distance: Similar conditions are repeated at many different points, so that the level at which large snow beds show themselves can be seen as approximately horizontal. But this is true only as far as the conditions are similar. On the opposite sides of the same chain the exposure to the sun or to warm winds may cause a wide difference in the level of permanent snow, though the increased fall of snow on the side exposed to moist winds may more than compensate for the sun's rays.
Still, the "line of perpetual snow" is not fixed. The occurrence of favorable meteorological conditions during several successive seasons may and does increase the extent of the snowfields and lower the limit of seemingly permanent snow, while the opposite may cause the limit to rise higher on the flanks of the mountains. Hence all attempts to fix accurately the level of perpetual snow in the Alps are fallacious. At best, local accuracy might be established for a particular district. In some parts of the Alps the limit is about 2400 m (7900 ft) elevation, while in others it cannot be placed much below 2900 m (9500 ft). As very little snow remains on rocks angled more than 60°, this is soon removed by the wind, some steep masses of rock remain bare even near the summits of the highest peaks, but as almost every spot offering the least hold for vegetation is covered with snow, few flowering plants are seen above 3350 m (11,000 ft).
There is reason to think, however, that it is the lack of soil rather than climatic conditions that checks the upward extension of the alpine flora. Increased direct effect of solar radiation compensates for the cold of the nights, and in the few spots where plants have been found flowering up to a height of 3650 m (12,000 ft), nothing has indicated that the processes of vegetation were arrested by the severe cold which they must sometimes endure. The climate of the glacial region has often been compared to that of the polar regions, but they are very different. Here, intense solar radiation by day, which raises the surface when dry to a temperature approaching 27 °C (80 °F), alternates with severe frost by night. There, the Sun, which never sets is only able to send feeble rays that maintain a low temperature, rarely rising more than a few degrees above the freezing point. Hence the upper region of the Alps sustains a far more varied and brilliant vegetation.
The great plain of upper Italy has a winter climate colder than that of the British Isles. The olive and the characteristic shrubs of the northern coasts of the Mediterranean do not thrive in the open air, but the valuable tree ripens its fruit in sheltered places at the foot of the mountains, and survives along the deeper valleys and the shores of the Italian lakes.
The evergreen oak grows wild on the rocks around the alpine lake, Lake Garda, and even lemons are cultivated on a large scale, with partial protection in winter. The olive has been known to survive severe cold when of short duration, but it cannot be cultivated with success where frosts are prolonged or where the mean winter temperature falls below 5.5 °C (42 °F). To produce fruit requires at least 24 °C (75 °F) during the day through four or five months of the summer and autumn.
The grape vine is far more tolerant of cold than the olive, but to produce tolerable wine it demands, at the season of ripening, a degree of heat not much less than that needed by the more delicate tree. These conditions are satisfied in the deeper valleys of the Alps, even in the interior of the chain, and up to a considerable height on slopes exposed to the Sun. The protection afforded by winter snow enables the plant to resist severe and prolonged frosts that would be fatal in more exposed situations. Many wild plants characteristic of the warmer parts of middle Europe are seen to flourish along with the vine. A mean summer temperature of at least 20 °C (68 °F) is considered necessary to produce tolerable wine, but in ordinary seasons this is much exceeded in many of the great valleys of the Alps.