22 



KIMBALL: NORMAL ATMOSPHERIC TRANSPARENCY 



sun, the proportion of the radiation that is received diffusely 

 from the sky is relatively great. 



Skylight polarization is intimately connected with atmospheric 

 transparency, since an increase in the primary diffusion is ac- 

 companied by an increase in the secondary scattering in the lower 

 atmosphere. This increases the intensity of the horizontally or 

 negatively polarized component of the light, and neutralizes to 

 some extent the postive or vertical polarization of skylight. In 

 Table 3 are summarized skylight polarization observations made 



Fig. 1. Variations in solar radiation intensities and in the percentage of sky- 

 light polarization. Circles indicate data for Mount Weather, Va.; crosses, data 

 for Madison, Wis. 



at Mount Weather when the sky was free from clouds and there 

 was no snow on the ground. In the last two columns are given 

 the departures of the monthly values for 1912 and 1913 from the 

 averages for the respective months derived from observations 

 taken previous to June, 1912. These departures are plotted in 



