HUMIDITY. METEOROLOGY 119 Spring. Summer. Autumn. Winter. Year. Batavia -0-3 -0-2 -0 2 -0 2 9 Calcutta 1-0 07 07 9 8 Peking 6 0-6 6 07 6 Nertchinsk 01 3 6 I O 5 Barnaul 5 5 07 8 6 Ekaterinburg o-f. 07 07 6 6 Tiflis 0-5 5 0-5 4 5 St Petersburg 6 6 3 O l 4 Valentia 0-4 5 - 1 - O l 2 Greenwich 07 8 6 O O 4 Rothesay 0-4 3 3 0-3 3 These results show remarkable uniformity, and it may be inferred from them that mean temperatures deduced from maximum and minimum observations are about half a degree above the true mean temperature. In general climatological inquiries, observations with these thermo meters have the strong recommendation of supplying from observations taken once a day the data for the determina tion of the mean temperature and mean daily range of localities ; to which falls to be added the further advantage of giving results more uniformly comparable for different places than could be afforded by observations made with a common thermometer at any single hour or pair of hours daily. Daily Variation of the Humidity of the Air. The gaseous envelope surrounding the earth is composed of two atmospheres, quite distinct from each other, an atmosphere of dry air and an atmosphere of aqueous vapour. The dry air, which consists of oxygen and nitrogen, is always a gas, and its quantity remains constant ; but the aqueous vapour does not continue permanently in the gaseous state, and the quantity present in the air is, by the ceaseless processes of evaporation and condensation, constantly changing. If the aqueous vapour remained permanently and unchanged in the atmosphere, or were not liable to be condensed into cloud or rain, the mixture would become as complete as that of the oxygen and nitrogen of the air. The equilibrium of the vapour atmo sphere, however, is being constantly disturbed by every change of temperature, by every instance of condensation, and by the unceasing process of evaporation. Since dry air further materially obstructs the free diffusion of the aqueous vapour, it follows that the law of the independent pressure of the vapour and of the dry air of the atmosphere holds good only approximately. The aqueous vapour, however, constantly tends to approach this state. Since, then, the independent and equal diffusion of the dry air and the aqueous vapour is, owing to these disturbing causes, never reached, the important conclusion follows that the hygrometer can never indicate more than the local humidity of the place where it is observed. Hygrometric observa tions can therefore be regarded only as approximations to a true indication of the quantity of aqueous vapour in the atmosphere over the place of observation. It is, however, to be added that, while in certain cases the amount of vapour indicated is far from the truth, yet in averages, particularly long averages, a close approximation to the real amount is reached if the hygrometer be at all tolerably well exposed and carefully observed. Aqueous vapour is constantly being added to the air from the surfaces of water, snow, and ice, from moist surfaces, and from plants. The rate of evaporation increases with an increase of temperature, because the capacity of the air for vapour is thereby increased. The atmosphere can contain only a certain definite amount of vapour, according to the temperature ; when therefore the air has its full complement of vapour, or when, in other words, it is saturated, evaporation ceases. Thus the rate of evaporation is greatest Avhen the air is driest or freest from vapour, and least when the air is nearest the point of saturation. Since currents of air remove the moister and substitute drier air over the evaporating surfaces, evapora tion is much more rapid during wind than in calm weather. As air expands under a diminished pressure, its temperature consequently falls, and it continues to approach nearer to the point of saturation, or become moister; and, as it contracts under an increased pressure, its temperature rises and it recedes from the point of saturation or becomes drier. Hence ascending currents of air become moister with every addition to the ascent, and descending currents drier as they continue to descend. Thus as winds ascend the slopes of hills they become moister, but when they have crossed the summit and flow down the other side they become drier in proportion to the descent, and all the changes may be experienced from extreme dryness to saturation in the same mass of air, which all the time has practically had its amount of aqueous vapour neither added to nor diminished. In an atmosphere of air and aqueous vapour perfectly mixed, the elastic force of each at the surface of the earth is the pressure of each. In this case the elastic force of aqueous vapour would be the pressure of the whole vapour in the atmosphere over the place of observation. This pressure is expressed in inches of mercury of the barometer. If we suppose the total barometric pressure to be 30 000 inches, and the elastic force of vapour to be O745 inch, the pressure or weight of the dry air, or air proper, would be 29 255 inches, and of the aqueous vapour 745 inch. From this it follows that the elastic force of vapour may be regarded as indicating the quantity of aqueous vapo*ur in the air at the place of observation, or it may be desig nated the absolute humidity of the air. The diurnal variation in the elastic force of vapour in the air is seen in its simplest form on the open sea. Grouping together all the hygrometric observations made on board the " Challenger " on the North Atlantic at a distance from land, from March to July 1873 (eighty-four days), we have for that time a mean elastic force of 659 inch, and the following diurnal variation : Inch. 2 A.M. - 015 4 ,, - 020 6 ,, - 016 8 ,, - 007 Inch. 10 A. M. -f 004 Noon + 017 2 P.M. + -020 4 +-017 Inch. 6 P. M. + 007 8 +-002 10 - -005 Midnight -I- -003 Hence the minimum ( - 020 inch) occurs at the hour when the temperature of the surface of the sea and air resting over it falls to the daily minimum ; it then rises to the mean a little after 9 A.M., and to the daily maximum ( + 020 inch) at 2 P.M., when the sea and air are also near the daily maximum, and falls to the mean shortly before 9 P.M. Treating the observations made near land by the "Challenger" during the same months, the following is the diurnal variation disclosed : Inch. 2 A. M. - 003 4 ,, - 009 6 ,, - -010 8 ,, - -003 Inch. 10 A.M. + 014 Noon + -011 2 P. M. + 007 4 ,, +-015 Inch. 6 P.M. 000 8 ,, - -004 10 - -005 Midnight- -007 The disturbance induced by proximity to land in the distribution of the aqueous vapour in the lower strata of the atmosphere is very striking. The maximum and minimum no longer follow the corresponding phases of the temperature of the surface of the sea and of the air. The disturbing agents are the sea and land breezes and their effects. Under the influence of the land breeze the time of the minimum humidity is delayed till about 6 A.M.; and under the influence of the sea breeze and its effects the amount of the aqueous vapour shows a secondary minimum
from noon to 2 P.M. It is to be here noted that tliis midday