Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/136

Rh 126 diurnal variation in the wind s velocity occurring frequently also in cloudy weather. On the other hand, during the night, when terrestrial radiation is proceeding, the tempera ture of the surface falls greatly, and instead of an ascensional movement in the lower stratum of the air there is rather a tendency towards a descensional movement (if the wind be light there is an actual movement) of the lowest air stratum down the slopes of the country ; and since the friction between the wind and the surface of the earth is thereby increased the diurnal velocity of the wind falls to the minimum during these hours (see also p. 156). Among the most marked exceptions to the general rule of the diurnal distribution of wind force may be cited the bitterly cold furious blasts of wind encountered in narrow valleys in such mountainous regions as the Alps during clear and comparatively calm nights. These are simply the out-rush of the cold air poured into the upper basins of the valleys by the descensional currents from the slopes which the chilling effects of terrestrial radiation set in motion. On the other hand, the air of the valleys becomes heated and expands during the day, thus giving rise to a warm wind blowing up the valleys, which, on account of the vapour it carries with it from the lower levels, fre quently covers the higher slopes and tops of the mountains with cloud and drizzling rain. Diurnal Variation in the Direction of the Wind. In all climates near seas and other large sheets of water, where the distribution of atmospheric pressure is tolerably equable, or the barometric gradient small, and the sun heat moderately strong, land and sea breezes are of daily occurrence. In such places a breeze from the sea gradually sets in in the morning, which gradually rises to a stiff breeze during the heat of the day and again towards evening sinks to a calm. Soon after this a breeze sets in from the land, blows strongly seaward during the night, and dies away in the morning, giving place to the sea breeze as before. These breezes are occasioned by the surface of the land being heated in a much higher degree than that of the sea during the day ; the air over the land being thereby made lighter ascends, and its place is supplied by the cooler air of the sea breeze drawn land ward, and partly also by descending currents, as shown by the humidity observations of the &quot; Challenger,&quot; which indicate increasing dryness when the sea breeze is strongest. Again during the night the temperature of the land and of the air over it falls below that of the sea, and the air of the land thus becoming heavier and denser flows over the sea as a land breeze. As the best-marked and most fre quently occurring cases of the sea breeze begin some distance out at sea and gradually approach the land, it is very probable that, as suggested by Blanford, the ascend ing heated air flows seaward as an upper current, and that the increased barometric gradient thus caused largely accounts for these breezes. Sea and land breezes are thus determined by the relative posi tions of the land and its coasts, subject to a further modification arising from the rotation of the earth. Thus on the coast of the Gulf of Lyons the sea breeze from the south veers to south-west and dies away as a west wind, while the land breeze from the north gradually veers to north-east and dies away as an east wind. On the coast of Algeria, on the other hand, the sea breeze veers from north to north-east and dies away in the east, whereas the south land breeze veers to south-west and dies away in the west. Sea breezes also occur in such unsettled climates as that of Scotland, when the weather conditions are favourable. These conditions are presented when an anticyclone overspreads the country, with its accompanying fine settled weather, small variation in the distri bution of atmospheric pressure, clear skies, and consequently strong sunshine. Under these conditions the following are the veerings of the wind off the coast of Berwickshire. In the morning the wind is north-west till about 10 A.M., when it veers to north, falling all the time till finally it sinks to a calm. A little before noon it springs up [WIND DIRECTION. from north-east or east, veers to south-east from 2 to 3 P.M., where it continues till 7 P. M., about which time it veers to south and then south- west, diminishing in force and finally sinking to a calm. About sunset it springs up from west, veering to north-west during the night, where it continues till the following morning. The wind thus virtually makes the round of the compass, is strongest from north west and south-east and weakest at north-east and south-west, being thus strongest when its course is perpendicular to the line of coast. The observations made by the &quot; Challenger &quot; in the region of the north-east trades in 1873 show a small diurnal variation in the direction of the wind, the variation being from E. 47 5 N. at 2 to 6 A.M. to E. 56 N. at 10 A.M. to 2 P.M., the variation being thus 8 55 towards north during the hottest hours of the day. At Mauritius observatory, which is several miles from the sea, the daily period in the direction of the wind is from E. 22 15 S. at 4 A.M., being the most southerly point, to E. 7 S. at 1 P.M., and thence back to E. 22 15 S. at 4 A.M. The diurnal varia tion is 15 15, and thus the influence of the sun impresses on the wind at this observatory a more truly easterly character. At the Austrian naval station at Pola, near the head of the Adriatic, the daily variation in the direction of the wind is well- marked. Starting from a point east of south at 5 A.M., it gradually veers round to westward, the most westerly point, almost due west, being reached at 5 to 6 P.M., after which it gradually shifts back to its starting point in the morning. Here we have evidently a diurnal wind-system different from that of the land and the sea breeze. Pola is situated near the south-western extremity of the peninsula of Istria, and the direction in the early morning of east by south is the direction the wind would take if a small anticyclone overspread the peninsula ; and the direction from the west in mid afternoon is the direction the wind would have at Pola if the peninsula were occupied by a small cyclone with the lowest pressure in the centre. Now the influence of solar radiation is to form, through the ascending current from the heated land, a diminution of pressure over the land, in other words, what is essentially a cyclone. On the other hand, during the night the influence of terrestrial radia tion is to generate, through the cooling of the land and the air resting above it, a relatively higher atmospheric pressure in the interior of the peninsula with its characteristic system of out- blowing winds. At Coimbra, in July 1878, the diurnal variation of the wind s direc tion was from W. 49 37 N. at 2 to 6 A.M. to W. 33 15 N. at 4 to 6 P.M., the amount of the variation being thus 16 22 in the direction of west. At Valentia, in the south-west of Ireland, during the summer months of 1878 the diurnal variation of the wind s direction was from W. by S. at 7 to 9 A.M. to S.W. by W. at 5 to 7 P.M. The variation was thus from a point nearly south to a point nearly south-west, or through nearly 45 in the direction of west. On the other hand, at Aberdeen during the same months of 1878, the diurnal variation of the wind s direction was from S.W. at 6 to 7 A.M. to S. by E. at 12 to 4 P.M., the variation being thus 56 from south-west in the direction of east through south. Attention is here drawn to the exactly opposite ways in which the diurnal veering or shifting of the wind takes places at Valentia and Aberdeen, but particularly to the important circumstance that in each case the diurnal changes in the wind s direction which actually occur are precisely those that would take place on the supposition that during the hottest hours of the day an ascensional movement of the air sets in from the heated lands of the British Islands, and that an in-draught takes place all round, which with the descending currents makes good the loss caused by the up-draught. Thus then both the diurnal increase in the wind s velocity and the change in its direction which observation shows to take place during the hottest hours of the day are traced to the same cause, viz., the heating of the surface by the sun, the heating of the lowest stratum of air resting on the surface, and the ascensional movements which are the neces sary result. It is instructive to note that at Nukuss, at some distance to the south of the Sea of Aral, where the summer direction of the wind is northerly, the north component is at the daily maximum at 4 p. M. , having shifted into this direction from north-east, where it is at 9 A. M. Much or nearly everything remains to be done in working out this problem in its practical details as one of the important elements of climatology, with the view of arriving at some definite knowledge of the influence of physical configuration and different vegetable coverings of the surface on radiation and on the velocity and direction of the wind. Diurnal Variation in the Amount of Cloud. Mists and fogs are visible vapours floating in the air near the surface of the earth, and clouds are visible vapours at a consider able height. These forms of visible vapour are all produced by whatever lowers the temperature of the air below the dew point, such as radiation from the molecules of the atmosphere towards the cold regions of space, the simple