Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/126

Rh 116 METEOROLOGY [DIURNAL and tracts of sand are subject during the day to higher temperature and during the night to lower temperature near the surface than dense soils, and that frosts and extreme temperatures do not penetrate so far into loose as into dense soils. It is on these differences that some of the more striking features of climates depend. As snow is one of the worst conductors of heat, owing to the quantity of air filling the interstices among the ice crystals, it protects the soil it covers by setting a limit to the depth to which the severe frosts of the surface penetrate, and by arresting the escape of the heat of the soil upwards to the air. The communication of heat from one part of the earth to another by convection is seen on a grand scale in the winds and in the currents of the ocean. It is seen also in the ascending and descending currents of the atmosphere everywhere, which have their origin in the daily and unequal changes of temperature to which the surface of the earth is subject. The direct and beneficial effect which results from atmospheric and oceanic circulation is a more equable distribution of temperature over the globe, thus moderating the rigours of the polar regions and the heat of the tropics. An interchange of heat is constantly going on among bodies exposed to each other, whatever be their tempera ture. This mode by which heat is communicated from one body to another is called radiation. Radiant heat proceeds in straight lines, diverging in all directions from the source, is only in a limited degree influenced by the air through which it passes, and is not diverted from the straight course by the wind. The intensity is proportional to the temperature of the source, and is greater according to the degree of inclination of the surface on which the rays fall. If then a body be placed in the presence of other bodies, some colder and some warmer than itself, it will from this mutual interchange of temperature receive more heat from the warmer bodies than it radiates to them, and conse quently becomes warmer ; but it will receive less heat from the colder bodies than it radiates to them, and its temperature consequently falls. This is precisely the condition in which the earth is placed in space. When a part of the surface is turned towards the sun, that part of the surface receives more heat than is radiated from it ; and the temperature consequently rises most in that region which for the time is perpendicular to the sun s rays, and least round the annulus where the inclination of the surface is greatest. On the other hand, since the hemisphere turned from the sun radiates more heat than it receives from the cold regions of space, the temperature there falls. Owing to the essentially distinct conditions under which the earth is placed with respect to radiation, the subject falls naturally to be divided into two heads, solar radiation and terrestrial radiation. Solar Radiation. Of the sun s rays which arrive at the earth s surface, those which fall on the land and solid bodies generally are wholly absorbed by the thin surface layer exposed to the heating rays, the temperature of which consequently rises. Whilst the temperature of the surface increases, a wave of heat is propagated downwards through the soil. The intensity of the daily wave of temperature rapidly lessens with the depth at a rate depending on the conductivity of the soil, until at about 4 feet below the surface it ceases to be measurable. Part of the heat of the surface layer is conveyed upwards through the air by the convection currents which have their origin in the heating of the lowermost stratum of air in direct contact with the heated surface of the land. Altogether different is the influence of the sun s rays on water. In this case the sun s heat is not all, indeed very far from all, arrested at the surface, but penetrates to a considerable depth. The depth to which the influence of the sun is felt has been shown by the observations made during the cruise of the &quot;Challenger&quot; to be, roughly speaking, about 500 feet below the surface of the sea. The rate at which, in perfectly clear water, this heat is distributed at different depths is a problem that has not yet been worked out. Since water is a bad conductor, the heat thus distributed does not, as takes place with respect to land, penetrate to still lower depths by conduction, but only by different densities prevailing at the same depths, whether these different densities be due to different temperatures or different degrees of salinity. Thus one of the more important distinctions between land and water surfaces in their bearings on climate is that nearly all the sun s heat falling on land is arrested on the surface, whereas on water it is at once diffused downwards to a great depth. In examining temperatures of the sea taken at different depths, it is surprising to note the rapidity with which changes of temperature are felt at considerable depths, especially in cases when the temperature of the air rises rapidly, accompanied with strong sunshine. In shallow water the sun s heat raises the temperature much higher than that of deep water, this being obvious from the consideration that nearly the whole of the sun s heat which falls on the surface is utilized in raising the temperature of the shallow layer of water ; in other words, it is, so to speak, concentrated through a small depth of water instead of being diffused through a great depth. Surface Temperature of the Sea. The importance of a knowledge of this datum of meteorology will be at once recognized when it is kept in view that three-fourths of the earth s surface is water, that the temperature of the air resting on this surface is in close relation to the temperature of the surface, and that the latter has, through the intervention of the winds, direct and important bearings on the temperature of the air over large portions of the land surfaces of the globe. During the years 1859-63 Captain Thomas, while engaged on the survey of the islands on the north-west of Scotland, made observa tions of the temperature of the surface of the sea every hour of the day at all seasons, and with sufficient frequency for the determination of the diurnal range of the temperature of the surface. The daily minimum, 17 below the mean, occurred near 6 A.M.; the mean was reached about 11 A.M., the maximum, 13 above the mean, between 3 and 4 P.M., and the mean again shortly before 2 A.M. Thus the daily oscillation of the temperature of the surface of the sea amounted on the north-west of Scotland only to 3. In lower latitudes the amount of the daily fluctuation is somewhat larger, but everywhere it is comparatively small, if care be taken to make the observations properly, or at a distance from land, where the influence of the heated or cooled land is not allowed to vitiate the results. During the voyage of the &quot; Challenger &quot; a complete system of meteorological observations, including the tem perature of the surface of the sea, was made every two hours as part of the scientific work of the cruise. These are now being discussed, and the writer of this article is, by permission of the Lords Commissioners of H.M. Treasury, allowed to use such of the results as have been already arrived at. The diurnal march of the temperature of the surface of the North Atlantic has been determined from observations made on one hundred and twenty-six days from March to August 1873 and in April and May 1876, the mean latitude of all the points of observation being nearly 30 N., and the longitude 42 W. The following variations from the mean show the phases of this diurnal oscillation :