4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. OQ 



at the earth's surface, in the vicinity of the p water-vapor band, a 

 indicates the depth of the p band, and /' shows the intensity of the 

 radiation of the band itself as compared with no solar radiation. In 

 other words, b represents the minimum value of radiation in the water- 

 vapor absorption band, p, and /; + a is the so-called " smooth-curve " 

 value as given in Smithsonian publications, or the intensity of the 

 radiation of the infra-red region adjacent to the p band, as compared 

 with no solar radiation. The ratio " minimum/smooth-curve " is a 

 measure of the precipitable water vapor prevailing in the path of the 

 solar beam through the earth's atmosphere. By plotting " log ' mini- 

 mum/sniooth-curve ' versus air mass " a curve similar to the corre- 

 sponding " log pyrheliometry calories versus air mass " is obtained. 



^<^^f^^'o^ \ 



Fig. I. 



Hence by such a plot the uniformity of the water vapor prevailing 

 in the path of the solar beam can be determined, and by comparing the 

 plot with the corresponding pyrheliometry plot, it can be concluded 

 whether any nonuniformity in the pyrheliometry was caused by chang- 

 ing amounts of water vapor or by other agencies such as haze, dust, 

 and smoke. 



In order to ascertain the amount and uniformity of the haze, dust, 

 and smoke prevailing in the atmosphere, the Angstrom pyrheliometer 

 was again employed, but this time with the direct sunlight shaded off, 

 so that the radiation from a concentric ring of sky around the sun of 

 approximately 17° outer radius was measured. During the observa- 

 tions on Fogo Peak and those in Southwest Africa including part of 

 the first visit to Ganzberg Mountain, a much larger area of sky was 

 seen by the instrument. Throughout the remainder of the observa- 

 tions in Southwest Africa and in all of the Sinai observations, an 



