Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/130

Rh 120 METEOROLOGY [DEW. minimum occurs at the hours of the day when the surface of the land is most highly heated, the ascending current of heated air rising from it therefore strongest, and the resulting breeze from the sea towards the land also strongest. Now it does not admit of a doubt that the diminution in the amount of the aqueous vapour noted on board the &quot;Challenger&quot; near the shore points to an intermixture with the air forming the sea breeze of descending thin air-filaments or currents to supply the place of the masses of air removed by the ascending currents which rise from the heated surface of the land. At Batavia, on the north coast of Java, and at Bombay, the aqueous vapour is also subject to a secondary minimum during the warmest hours of the day. During the summer months this secondary minimum is best marked at inland places such as Peking, Nertchinsk, Barnaul, Tiflis, and Ekaterinburg, but the time of its occurrence is about two hours later than it is over the North Atlantic. Over all these places at this season the ascending current from the heated land in the interior of Asia is very strong. On the other hand the lowering of the amount of aqueous vapour scarcely if at all appears as a feature in the summer climate of St Petersburg, and not at all in that of Sitka, where the sea breeze is equally not a constant feature of the climate of the district. In the excessively dry, rainless, and hot climate of Allahabad, in April the diurnal minimum of the aqueous vapour occurs from 11 A.M. to 6 P.M., the time of absolute minimum being 2 and 3 P.M. During all other hours of the day the amount of the vapour is above the mean, a secondary minimum occurring from 1 to 4 A.M. At Allahabad, at this time, the absolute maxi mum vapour pressure occurs at 8 A.M. Quite similar to this is the diurnal distribution of the aqueous vapour in July at Lisbon and Coimbra, the minimum occurring from 10 A.M. to 8 P.M. At this time of the year the climate of this part of the peninsula is hot and dry and the rainfall insignificant in amount. As this region lies between the high atmospheric pressure so characteristic a feature of the meteorology of the Atlantic in summer and the comparatively low pressure over the continents southward and eastward, the winds are almost wholly north-westerly. In this connexion it is instructive to note that the time of maximum vapour pressure is from 4 to 7 A.M., when the velocity of the wind is near the minimum, and the chief minimum vapour pressure from noon to 4 P.M., when the velocity of the wind and ascend ing currents reach the daily maximum. These results show that the diminution in the vapour pressure during the hours when temperature is highest, which characterizes the climates of large tracts of the globe, is due to descending air-filaments or currents, which necessarily accompany the ascending currents that rise from the heated land. At Geneva during the summer months the vapour curve exhibits two daily minima very strongly marked, the one shortly before sunrise and the other from 2 to 4 P.M., and two maxima, one from 8 to 11 A.M. and the other from 6 to 10 P.M.; and with these the diurnal variations of cloud are in accordance. The peculiarly marked features of the vapour curve at Geneva are probably due to the size of the lake, which is large enough to give rise to a decided breeze during the day from the lake all round its shores and during the night to a breeze from the land all round upon the lake. On the setting in of the breeze, the mass of air composing it, having been for some time resting on the lake, is rather moist, and thus one of the daily maxima is brought about from 8 to 11 A.M. As the breeze continues the air supplying it is necessarily drawn from the higher strata of the atmosphere more copiously than in different situations ; and, having thus acquired increased dry ness in the descent, and having blown over the lake for too short a distance to materially influence its moisture, the air becomes constantly drier, till the minimum from 2 to 4 P.M. is reached. The lake breeze thereafter begins to diminish in force, and the air consequently becomes moister till the maximum vapour pressure of the day occurs when the lake breeze dies away and the land breeze has not yet sprung up. In the winter months, when these breezes do not prevail, the curve of diurnal vapour pressure shows only one maximum and minimum. The relative humidity of the atmosphere must not be confounded with its vapour pressure or absolute humidity. The relative humidity, or, as it is more frequently called, the humidity, of the air is the degree of its approach to saturation. Complete saturation is represented by 100 and air absolutely free of vapour by 0, the latter state of things never occurring in the atmosphere, a humidity of 10 being of rare occurrence even in such arid regions as those of Arabia. The great significance of this element of climate is in its relations to the diathermancy of the air, and consequently to solar and terrestrial radiation. It is supposed that perfectly dry air would allow rays of heat to pass through it with at most only a very slight increase to its temperature therefrom. Let, however, a little aqueous vapour be added to it, a partial obstruction to the passage of radiant heat is offered, and the tempera ture of the mixture, or common air, is sensibly raised. Hence, other things being equal, the less the amount of vapour the more are the effects of radiation felt, or the greater the heat of the days and the cold of the nights. The mere amount of vapour in the air does not determine the degree of radiation, but it is the amount of vapour together with a certain temperature in other words, the absolute and relative humidity of the air taken together that determines the heating power of the sun and the degree of cold produced by terrestrial radiation. The diurnal variation of the relative humidity is very different from that of the vapour pressure, and presents features of the simplest character. The following are the diurnal variations from the mean humidity 80 over the North Atlantic, from the &quot;Challenger&quot; observations in 1873 : 2 A.M. +2 4 ,, +2 6 ,, +1 8, 10 A.M. -1 Noon - 2 2 P.M. -3 4, -2 6 P.M. -1 8 10 ,, +1 Midnight + 2 Thus the maximum humidity occurs from midnight to 4 A.M., or when the daily temperature is at the minimum, and the minimum humidity at 2 P.M., when the tempera ture is at the maximum, the curve of humidity being thus inverse to that of the temperature. With two slight modifications this is the diurnal humidity curve for all climates and seasons. In the calm which intervenes in the morning between the land and the sea breeze the humidity continues high, or even increases, though at the time the diurnal increase of temperature has already set in. The other modification is seen in the humidity curves for Nertchinsk and Barnaul during winter, these curves being not inverse but coincident with the daily curves of temperature. In the climates of Central Asia in winter, the amount of vapour is very small, and the increase to the relative humidity during the day is probably occasioned by the more active evaporation from the snow during the day and the stillness of the air favouring the accumulation of aqueous vapour near the surface of the earth. Next to the winds, the aqueous vapour of the atmo sphere, in the diverse ways in which in different localities it is distributed through the hours of the day, plays the most important part in giving to the different parts of the globe its infinitely diversified climates. Dew. Dew is deposited over the earth s surface on comparatively clear and calm nights. As the cooling by terrestrial radiation continues, the temperature of objects on the surface is gradually lowered to the dew-point, and when this point is reached the aqueous vapour begins to be condensed into dew on their surfaces. The quantity deposited is in proportion to the degree of cold produced and the quantity of vapour in the air. Dew is not deposited in cloudy weather, because clouds obstruct the escape of heat by radiation, nor in windy weather, because wind continually renews the air in contact with the surface, thus preventing the temperature from falling sufficiently low. When the temperature is below 32, dew freezes as it is deposited, and hoar-frost is produced. The dew-point practically determines the minimum temperature