332 CARNEGIE INSTITUTION OF WASHINGTON. 



atures are lower. The molecules of the nitrogen group thus serve as tem- 

 porary pockets for the heat absorbed by water-vapor also. 



But the account of the water-factor does not close when the vapor passes 

 back to droplets of water or floating ice crystals. The resulting clouds play 

 an important part by throwing back into outer space about 70 per cent of the 

 solar rays that fall upon them. There is some compensation, however, in the 

 terrestrial rays they throw back toward the earth. The clouds are themselves 

 absorbers and radiators. A part of the atmospheric vapor, when condensed, 

 takes the form of ice crystals, which, after serving their functions in the air, 

 form mantles of snow on the earth's surface that usually persist longer than 

 the clouds and thus add much to the total reflection of the icy products of 

 the vapor. All these factors are to be equated and charged against the 

 account of water-vapor, for they are the offspring of the vapor. According 

 to Dines's estimate, about one-half of the energy of the solar rays is reflected 

 and takes no part in the heating of either the earth or the atmosphere. This 

 probably includes the scattering of light by the molecules of the atmosphere 

 and by dust suspended in it; but discounting liberally for this, there remains 

 a heavy charge to be carried to the account of water-vapor because of the 

 reflection caused by its products. 



The falling of rain, hail, and snow through the atmosphere has a cooling 

 effect, which, though apparently a minor matter, must be added to the 

 adverse side of the thermal account of water-vapor. 



In view of these heavy items on the debit side, the question naturally 

 arises whether the climate of the earth would not be warmer than it is if 

 there were no water on its surface and no water-vapor in its atmosphere. 

 The question, however, is premature at this point, for the water in the atmos- 

 phere is inseparably connected with that of the hydrosphere, and the climatic 

 influence of this should be considered as well as that of the land surface. 



THE THERMAL FUNCTION OF THE LAND SURFACE. 



Thus far, for convenience, the surface of the earth has been treated as 

 though it were a unit absorbing insolation and either imparting it to the 

 atmosphere directly by contact or radiating it skyward subject to further 

 absorptions and radiations or to escape into outer space. But the land surface 

 and the water bodies, especially the ocean, play very different parts, which 

 involve very different time-factors; indeed, in the oceanic function, the time- 

 factor rises to climacteric importance. 



The land has relatively small competency to detain heat, for it is readily 

 taken in and readily given out, both by contact and by radiation. The pene- 

 tration is relatively slight and the capacity for thermal storage small. The 

 ordinary seasonal variations of temperature cease to be appreciable at very 

 moderate depths. Even such cumulative effects as have been inherited from 

 the last glacial period seem to be inappreciable below 1,000 or 1,200 feet.^ 



THE THERMAL STORAGE OF THE OCEAN. 



The case is radically different, however, with the ocean, in several vital 



respects. The surface of water reflects more of the solar rays than does the 



surface of the land when neither is covered with snow or ice. There is also 



a deeper penetration of the rays retained and a more distributive effect. 



' The degrees of extension and of relief of the land are more remotely very important climatic 

 factors, but are here excluded for lack of space, though they have time-functions of moment. 



