376 F. W. Very — Sky Radiation and Isothermal Layer. 



Blair suggests tentatively that the midday depression of tem- 

 perature at 3000 meters is due to the diminished transmission 

 of telluric radiation by the lower air at this time on account of 

 the dust and aqueous vapor carried upward in the local con- 

 vection, and that the maximum at 3 h a. m. at the same level is 

 caused by the nocturnal settling of dust and moisture which 

 clears the air and renders it more transmissive. A diurnal 

 variation in the transmissive quality of the atmosphere is cer- 

 tain, but whether it is competent to produce as wide a fluctua- 

 tion of temperature as the one at 3000 m with a total range of 

 3*7° C. and with the supposed lag in the phase, can not yet be 

 proved. Clayton's hypothesis accounts for the facts very well, 

 and I find little evidence of any marked difference in the 

 escape of telluric heat by radiation to the zenithal sky in the 

 daytime as compared with night, or at least the variations 

 which are observed are capable of simple explanation in chang- 

 ing cloudiness. 



Observations of Sky Radiation. 



During many years I have made occasional observations of 

 the radiation of a blackened bolometer or thermopile to the 

 sky. Perfectly pure skies are rare, and increasingly so in the 

 neighborhood of our large cities.* A transcript of all of the 

 measurements would therefore have little interest, as it would 

 record mainly the imperfections of the sky. Nevertheless, 

 such observations do tell us something in regard to the nature 

 of the atmospheric radiant processes. The experiments were 

 made with the heat-measuring apparatus pointing to a plane 

 metallic mirror reflecting to the sky, or to the ground, or to a 

 black card at air temperature. 



If the absorption of telluric radiation by the atmosphere 

 were complete, there would be no radiation to the sky. This 

 happens, indeed, when the sky is completely cloudy. But 

 actually, with a clear sky and normal vapor content, there are 

 extensive regions of the infra-red spectrum which pass readily 

 through the air, as the infra-red spectrum of the moon demon- 

 strates. These spectral regions of ready transmissibility are 

 but little affected in the following measures by any changes in 

 the vaporous contents of the atmosphere, except in one case of 

 a supersaturated air layer which will be specially noted. 



The effective temperature of the radiating earth in conse- 

 quence of the somewhat free transmission of its radiation to 

 space is not the mean temperature of the cool air at a consider- 

 able elevation above the surface, but depends largely upon the 

 temperature of the surface of the ground, which is much 

 * My present situation is 12 miles from the center of Boston. 



