Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/135

Rh WIND FORCE.] METEOKOLOGY 125 in fig. 4, where the figures on the left are Beaufort s scale, and those on the right the equivalents in miles per hour. The solid line shows at the different hours of the day the mean force on the open sea, and the dotted line the mean force near land. As regards the open sea it is seen that the diurnal variation is exceedingly small, there being two apparent slight maxima, about midday and midnight respectively. On examining, however, the separate means for the North and South Atlantic, North and South Pacific, and the Southern Ocean, there is no uniform agreement observable among their curves, the slight variations which are met Avith being different in each case. It follows therefore that the force of the winds on the open sea is subject to no distinct and uniform diurnal variation. The difference between the hour of least and greatest mean force is less than a mile per hour. Quite different is it, however, with the winds encountered by the &quot;Challenger&quot; near land, the force of the wind there giving a curve as pronouncedly marked as the ordinary diurnal curve of temperature. The minimum occurs at 2 to 4 A.M. and the maximum from noon to 4 P.M., the absolute highest being at 2 P.M. The curves constructed for each of the five oceans from the observa tions near land give one and the same result, or a curve closely accordant with the curve of diurnal temperature. The differences between the hours of least and greatest force are as follows : Southern Ocean 6| miles, South Pacific 4 miles, South Atlantic 3| mifes, and North Atlantic and North Pacific 3 miles per hour. In the case of each ocean the velocity of the wind on the open sea is considerably in excess of that near land, but in no case does the maximum velocity near land, attained about midday, reach the velocity of the wind on the open sea. The 650 daily observations on the open sea give a mean hourly velocity of 17 miles, whereas the 552 near land give a velocity of only 12- miles per hour. The difference is greatest at 4 A.M., when it amounts to upwards of 6 miles an hour, but is diminished by the rising tem perature till at 2 P.M. it is less than 3 miles an hour. At Mauritius, which is situated within the south-east trades, the minimum velocity of the wind is 9 7 miles per hour, occurring from 2 to 3 A.M., from which it rises to the maximum 18 5 miles from 1 to 2 P. M., the influence of the sun being thus to double the wind s velocity. At Batavia, situated in a region where the mean baro metric gradient is much smaller, the differences are still more de cided. From 1 to 6 A.M. 85 per cent, of the whole of the obser vations are calms, whereas from noon to 2 P.M. only 1 per cent, are calms. In all months, the minimum velocity occurs in the early morning, when the temperature is lowest, and the maximum from 1 to 3 P.M., when the temperature is highest, the mean minimum and maximum velocities being to each other as 1 to 21. At Coimbra the mean maximum hourly velocity is five times greater than the minimum hourly velocity in summer, whereas in winter it is only about a half more. At Valentia, in the south west of Ireland, one of the windiest situations in western Europe, the three summer months of 1878 gave a mean hourly velocity of 13 3 miles per hour, the minimum oscillating from 10 to 11 miles an hour from 9 P.M. to 6 A.M., and the maximum exceeding 16 miles an hour from 11 A.M. to 5 P.M. The absolute lowest hourly mean was 10 miles at 11 P.M., and the highest 18 miles at 1 P.M., the velocity about midday being thus nearly double that of the night. Many observations might be added to these, including those published by Hann, Koppen, Hamberg, and others, which go to establish the fact that the curves of the diurnal variation of the velocity of the wind generally conform to the diurnal curves of temperature. The curves are most strongly marked during the hottest months ; and the maximum velocity occurs at 1 P.M. or shortly thereafter, being thus before the time of occurrence of the maximum temperature of the day, and the minimum in the early morning, or about the time when the temperature falls to the lowest. The rule also holds good with all winds, whatever be their direction. The exceptions to this rule are so few and of such a kind that they are probably to be attributed to causes more or less of a local character. Hann has shown, for a number of places in northern Europe, that with a clear sky the velocity is doubled from the minimum to the maximum, with a sky half covered the velocity is three-fourths greater, and with a sky wholly covered the velocity is only a half I with a clear sky the velocity is double, and with a sky half covered ! it is two-thirds greater, but with a covered sky the diurnal varia tion in the wind s velocity becomes irregular and faintly marked. Hann has also examined the winds at Vienna, and found that winds of a velocity not exceeding 30 kilometres an hour show a mean diurnal increase from 11 kilometres at 6 A.M. to 16 8 at 1 P.M., but that winds of velocity exceeding 30 kilometres an hour exhibit only a faintly marked and irregular increase of velocity during the day. In offering an explanation of this remarkable fact regard ing the diurnal variation in the velocity of the wind in all climates, it is to be remarked that the minimum velocity occurs when terrestrial radiation and its effects are greatest, but the increase of the velocity closely follows the sun, and the maximum is reached nearer the time the sun crosses the meridian than perhaps any of the other maxima or minima of meteorology which are dependent on the sun s diurnal course. It is also to be noted that the winds over the open sea are practically uninfluenced by solar and terrestrial radiation, for there the diurnal curve of j On nearing land, however, the wind s force exhibits a diurnal curve of variation as distinctly marked as, and bearing a close resemblance to, the analogous curve of temperature; while on the land itself these features become still more decidedly pronounced. Lastly, the amount of the diurnal variation of the temperature of the surface of the sea is less than a degree, whereas over all land surfaces the diurnal variation of the temperature is large, even where the ground is covered by vegetation, and enormously large over sandy wastes. From this it follows that, so far as concerns any direct influence on the air itself, solar and terrestrial radiation exercise no influence on the diurnal increase of the velocity influence at all, such influence must be altogether insig nificant, as is conclusively shown by the wind observations of the &quot; Challenger &quot; over each of the five great oceans of the globe. The same observations show that on nearing land the wind is everywhere greatly reduced in force. The retardation is greatest during the hours when the daily temperature is at the minimum ; and it is particularly to be noted that, though the temperature rises considerably, no marked increase in the velocity sets in till about 9 A.M., when the temperature has begun to rise above the daily mean. From this time the increase is rapid (see fig. 4) ; the maximum is reached shortly after the period of strongest insolation ; and the velocity falls a little (but only a little) during the next three to five hours, according to season, latitude, and position, and falls again to near the minimum shortly after the hour when the temperature is at the mean. Even at the maximum, the velocity near land falls considerably short of the velocity which is steadily maintained over the open sea by night as well as by day. The period of the day when the wind s velocity is in creased is practically limited to the hours when the tempera ture is above the daily mean, and the influence of this higher temperature is to counteract to some extent the re tardation of the wind s velocity resulting from friction and from the viscosity of the air. The increase in the diurnal velocity of the wind is in all probability due to the super heating of the surface of the ground and to the consequent ascensional movement of the air, tending to counteract the effect of friction and of viscosity between the lowermost stratum of the air and the ground. It is of importance in this connexion to keep in view the fact that in cloudy weather a temperature much higher than might have been supposed is often radiated from the clouds down upon the earth s surface, 1 which accounts for the phenomenon of the 1 Journal of Scottish Meteorological Society, vol. ii. p. 280.
 * more. On the other hand at the strictly inland situation of Vienna,
 * variation in the force of the wind is all but a straight line.
 * of the air with the increase of its temperature, or, if any