September 18, 1914] 



SCIENCE 



419 



Ordinarily when we speak of the trans- 

 missivity of the atmosphere without further 

 specification, we mean the vertical transmis- 

 sion through the entire atmosphere. This can 

 be obtained from the zenithal sky radiation, 

 which is all that needs to be considered in 

 what follows. 



The transmission of terrestrial radiation by 

 the atmosphere relates entirely to rays of long 

 wave-length, and is effected by processes quite 

 different from those which govern the trans- 

 mission of solar radiation. Large variations 

 may occur in the transmission of terrestrial 

 radiation, and indeed quite suddenly, so that 

 the eye scarcely appreciates the approach of 

 new atmospheric conditions from any change 

 in the appearance of the sky. It is only 

 rarely that moderately smooth transmission 

 curves can be obtained from the zenith to the 

 horizon, because the conditions are apt to 

 change before the observations can be finished. 



The transmission of soil radiation to space 

 is continually fluctuating between zero and 

 an upper limit which seldom exceeds 60 per 

 cent, of the maximum theoretical value for 

 unimpeded radiation. At night, the diurnal 

 convection diminishes greatly, and on land 

 the wind is apt to fall as the sun goes down. 

 Hence on many nights the surface air is 

 approximately calm, and a thin quiescent layer 

 of air forms in contact with the soil, in which 

 the temperature is apt to fall below the dew 

 point as a result of nocturnal radiation. 

 Such a layer of nearly saturated air close to 

 the ground, though exceedingly shallow, 

 develops an extraordinary absorptive power 

 for infra-red radiation in broad diffuse bands 

 as a result of the production of the hydrols, 

 and these bands may eventually extend so 

 widely as to produce practically complete ob- 

 struction of terrestrial radiation. An ex- 

 ample of this is given in my paper on " Sky 

 Eadiation and the Isothermal Layer." ^ After 

 1 American Journal of Science, Vol. XXXV., 

 April, 1913, pp. 377 to 378 and 380 to 381. 



some hours of cooling, further diminution of 

 temperature is prevented in such cases by the 

 obstruction offered by this very thin air layer; 

 and if we compare the loss of radiation from 

 the earth's surface by night and by day, the 

 former is much the smaller on such nights as 

 are here considered. In spite of a phenom- 

 enally clear sky, the temperature of the ground 

 as morning approaches often remains almost 

 stationary, partly from the giving up of latent 

 heat of evaporation in the condensation of 

 aqueous vapor, and partly from this increase 

 in the absorptive power of moist air as satura- 

 tion becomes imminent.^ If, however, instead 

 of observing the superficial radiation through 

 this closely adherent air layer, we take the 

 radiation from a surface surrounded by rela- 

 tively dry air in the room of an observatory, 

 and let this radiation pass out through an 

 aperture either directly to the sky, or, as is 

 more convenient, allow the rays to pass to the 

 sky after reflection from a mirror, placed out- 

 side the aperture, but far enough above the 

 surface of the ground to be above the layer of 

 adherent soil-chilled air, very little difference 

 is to be found in the transmission of radiation 

 from sources at terrestrial temx)€ratures 

 whether measured by night or by day, such 

 differences as exist being those which may 

 always be expected from changing cloudiness, 

 or from variations in the general conditions 

 as to moisture, etc. 



It is common for writers on terrestrial 

 radiation to assume that the earth as a whole 

 radiates at a mean temperature of about 



2 Those who are much in the open air know that 

 on frosty mornings in winter a much more com- 

 fortable temperature is experienced on passing 

 from the open into woods. The friction of moving 

 air against the innumerable stems of the forest- 

 cover helps to retain the absorbent layers of moist 

 surface air in the woodland, and escape of radia- 

 tion is impeded. Under exceptional circumstances 

 the excess of temperature in the woodland may 

 reach 20° or 30° C. See G. A. Pearson, "A 

 Meteorological Study of Parks and Timbered 

 Areas in the Western Yellow-pine Forests of Ari- 

 zona and New Mexico, ' ' Monthly Weather Beview, 

 October, 1913. Cf. especially Pigs. 6 and 7 (pp. 

 1620 and 1621). 



