The earth in turn sends out radiation, whose wave-length is appropriate to its temperature. The maximum intensity of the terrestrial radiation is in the neighbourhood of io ,u and such radiation is partly absorbed by the water vapour in the atmos phere, which also in turn sends out long wave radiation.
In any discussion of the effects of radiation upon an isolated element of air we must therefore take into account the radiation coming upward from the lower layers of the atmosphere, and the radiation coming downward from the upper layers of the atmos phere. Unfortunately these do not include all the possible sources of heat affecting the element of mass under consideration. As we have seen in the section Dynamical Aspects, one of the effects of turbulence is to produce a vertical flow of heat, which is upward or downward according as the lapse rate is greater or less than the adiabatic. In addition to this, there is a latitude transfer of heat.
The equator being warmer than the poles, any motion of air across the circles of latitude carries warm air pole-ward or cold air equa tor-ward. Hence in considering the radiative effects over any par ticular zone, we must allow for the fact that it receives heat not only by direct radiation from the sun, but also by horizontal convection from other latitudes.
In computing the total amount of the incoming solar radiation, a deduction of 43% is made from the theoretical value, to allow for albedo or loss by reflection.
Abbot has recently discovered a persistent period of isi months in the variation of the solar constant, and there is evidence of a number of other shorter periods. It has been suggested by Ameri can writers that the variations of the solar constant are effective in varying terrestrial weather (H. H. Clayton. World Weather).
The Direct Effects of Incoming Solar Radiation.—The process of convection of heat does not become effective until the lapse rate has surpassed the adiabatic limit, but temperature at some distance above ground begins to rise before the lapse rate has reached the adiabatic limit. This phenomenon is probably due to the effect of direct radiation from the ground. The com bined effects of radiation and turbulence have been discussed by Chapman (Q.J.R. Met. Soc., 1925) and Brunt (Proc. Roy. Soc., 1929, and 193o). Over land, the lapse rate near the ground may amount to several hundred times the dry adiabatic on sunny afternoons, while at night the cooling of the ground by radiation to clear skies builds up large inversions. The effect of night cool ing on the formation of fog and mist has been discussed by G. I. Taylor (Q.J.R. Met. Soc., 1917).
Over the oceans more incoming radiation is lost by direct reflection, some is used up in evaporation of water, and a consider able depth of water is necessary to absorb completely the re mainder. Further the specific heat of water is about four times that of dry soil. Hence the temperature of the surface of the sea is far less variable than that of a land surface, and observations indicate that the diurnal range of temperature over the sea is of the order of I °F. For the same reasons the seasonal variation of temperature is much less over the sea than over land, and so oceanic climates are more equable than land climates.
Efforts have been made to explain the existence of the strato sphere as a direct effect of balance of incoming and outgoing radiation. Reference should be made to the original memoirs of Gold (Proc. Roy. Soc., 1909), Humphreys (Astrophysical Jour nal, 1909), Emden (Sitzber. bayr. Akad. Wiss., 1913) and Milne (Phil. Mag., 1922). Estimates of the amount of radiation leaving the earth in different latitudes have been given by Mugge (Zeit schrift fur Geophysik, 1926) and Simpson (Memoirs R. Met. Soc., 1928, 1929).