Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/13

Rh to differences of absolute height, though of the greatest possible practical importance, yet leave untouched a whole field of climatological research a field embracing the mean temperature of different hours of the day at different heights, for an explanation of which we must look to the physical configuration of the earth s surface and to the nature of that surface, whether rock, sand, black soil, or covered with vegetation. Under this head by far the most important class of con ditions are those which result in extraordinary modifica tions, amounting frequently to subversions, of the law of the decrease of temperature with the height. This will perhaps be best explained by supposing an extent of country diversified by plains, valleys, hills, and table-lands to be under atmospheric conditions &quot;favourable .to rapid cooling by nocturnal radiation. Each part being under the same meteorological conditions, it is evident that terres trial radiation will proceed over all at the same rate, but the effects of radiation will be felt in different degrees and intensities in different places. As the air in contact with the declivities of hills and rising grounds becomes cooled by contact with the cooled surface, it acquires greater density, and consequently flows down the slopes and accumulates on the low-lying ground at their base. It follows, therefore, that places on rising ground are never exposed to the full intensity of frosts at night ; and the higher they are situated relatively to the immediately surrounding district the less are they exposed, since their relative elevation provides a ready escape downwards for the cold air almost as speedily as it is produced. On the other hand valleys surrounded by hills and higu grounds not only retain their own cold of radiation, but also serve aa reservoirs for the cold heavy air which pours down upon them from the neighbouring heights. Hence mist is frequently formed in low situations whilst adjoining eminences are clear. Along low-lying situations in the valleys of the Tweed and other _rivers of Great Britain laurels, araucarias, and other trees and shrubs were destroyed during the great frost of Christmas 1860, whereas the same species growing on relatively higher grounds escaped, thus showing by incontestible proof the great and rapid increase of temperature with height at places rising above the lower parts of the valleys. This highly interesting subject has been admirably eluci dated by thenumerousmeteorologicalstationsof Switzerland. It is there observed in calm weather in winter, when the ground becomes colder than the air above it, that systems of descending currents of air set in over the whole face of the country. The direction and force of these descend ing currents follow the irregularities of the surface, and like currents of water they tend to converge and unite in the valleys and gorges, down which they flow like rivers in their beds. Since the place of these air-currents must be taken by others, it follows that on such occasions the temperature of the tops of mountains and high grounds is relatively high because the counter-currents come from a great height and are therefore warmer. Swiss villages are generally built on eminences rising out of the sides of the mountains with ravines on both sides. They are thus admirably pro tected from the extremes of cold in winter, because the descending cold air-currents are diverted aside into the ravines, and the counter-currents are constantly supplying warmer air from the higher regions of the atmosphere. Though the space filled by the down-flowing current of cold air in the bottom of a valley is of greater extent than the bed of a river, it is yet only a difference of degree, the space being in all cases limited and well defined, so that in rising above it in ascending the slope the increased warmth is readily felt, and, as we have seen, in extreme frosts the destruction to trees and shrubs is seen rapidly to diminish. The gradual narrowing of a valley tends to a more rapid lowering of the temperature for the obvious reason that the valley thereby resembles a basin almost closed, being thus a receptacle for the cold air-currents which descend from all sides. The bitterly cold furious gusts of wind which are often encountered in mountainous regions during night are simply the out-rush of cold air from such basins. The two chief causes which tend to counteract these effects of terrestrial radiation are forests and sheets of water. If a deep lake fills the basin, the cold air which is poured down on its surface having cooled the surface water, the cooled water sinks to a greater depth, and thus the air resting over the lakes is little if at all lowered in tem perature. Hence deep lakes may be regarded as sources of heat during winter, and places situated near their outlet are little exposed to cold gusts of wind, while places on their shores are free from the severe frosts which are peculiar to other low-lying situations. The frosts of winter are most severely felt in those localities where the slopes above them are destitute of vegetation, and consist only of bare rock and soil, or of snow. If, however, the slopes be covered with trees, the temperature is warmer at the base and up the sides of the mountain, the beneficial influence of forests consisting in the obstacle they offer to the descending currents of cold air and in distributing the cold produced by terrestrial radiation through a stratum of the atmosphere equalling in thickness the height of the trees. Hence as regards strictly local climates, an intelligent knowledge of which is of great practical value, it follows that the best security against the severity of cold in winter is afforded where the dwellings are situated on a gsntle acclivity a little above the plain or valley from which it rises with an exposure to the south, and where the ground above is planted with trees. When it is borne in mind that in temperate climates, such as that of Great Britain, the majority of the deaths which occur in the winter months are occasioned or at least hastened by low temperatures, it will be recognized as of the most vital importance, especially to invalids, to know what are the local situations which afford the best protection against great cold. In truth, mere local situations may during periods of intense cold have the effect of maintaining a temperature many degrees above that which prevails close at hand a difference which must mitigate suffering and not unfrequently prolong life. In addition to mere elevation and relative configuration of surface, the land of the globe brings about important modifications of climate in the degree in which its surface is covered with vegetation or is a desert waste. Of all surfaces that the earth presents to the influences of solar and terrestrial radiation an extent of sand is accompanied with the most extreme fluctuations of climate, as these are dependent on the temperature and moisture of the air ; whilst on the other hand, extensive forests tend to mitigate the extremes of temperature and distribute its daily changes more equably over the twenty-four hours. As regards the influence of the sun s heat on the tempera ture of the air, attention is to bo given almost exclusively to the temperature of the extreme upper surface of the earth heated by the sun with which the air is in immediate contact. Badly conducting surfaces, such ^ as sand, will evidently have the greatest influence in raising the temperature of the air, for the simple reason that the heat produced by the sun s rays being conveyed downwards into the soil with extreme slowness must necessarily remain longer on the surface, in other words, remain in immediate contact with the atmosphere. Similarly at night, the cooling effects of terrestrial radiation being greatest on sandy surfaces, the climate of sandy deserts Ls characters; by nights of comparatively great cold. These daily 