abstracts: meteorology 77 



METEOROLOGY.— Some causes of variation in the polarization of sky- 

 light. Herbert H. Kimball, Weather Bureau. Journal of the 

 Franklin Institute, April, 1911. 



A summary is given of the observations made by the author in Wash- 

 ington between December, 1905, and May, 1910, and at various other 

 stations of the Weather Bureau between July and October, 1910. It 

 treats mainly of the relation of sky polarization to the meteorological 

 conditions, with special reference to atmospheric transmissibility for 

 solar rays. Most of the observations have been made in connection 

 with pyrheliometric measurements of the intensity of solar radiation. 



Accepting Rayleigh's theory that the polarized component of sky 

 light is due to the scattering of the solar rays by very small particles, 

 piincipally the gas molecules of the atmosphere, it follows that a deple- 

 tion of the solar rays due to scattering by large particles, such as dust, 

 smoke, and water drops, should be accompanied by a decrease in the 

 percentage of polarization. The observations show that this is the 

 case, but that loss of solar radiation intensity due to absorption by aque- 

 ous vapor has no effect upon the polarization. 



When air currents of different temperatures, or different degrees of 

 saturation, and, in consequence, of different densities, are in juxtaposi- 

 tion, discontinuous surfaces are formed which reflect and disperse light 

 incident upon them, resulting in a diminution of both the intensity of 

 the solar radiation and the polarization of sky light as measured at the 

 surface of the earth. The observations show that such conditions are 

 usually followed by cloudiness and rain. It would therefore seem that 

 a measurement of the polarization of sky light should give an early 

 indication of any disturbance that may exist in the atmosphere. This 

 would probably be the case were it not for the fact that sky polariza- 

 tion is modified by the amount of light reflected from the surface of 

 the earth and from clouds, and also from dust and haze in the lower 

 layers of the atmosphere. 



The distance of the neutral points of Arago and Babinet from the 

 anti-solar point and the sun, respectively, increase as the percentage 

 of polarized light decreases. Since these distances may best be measured 

 when the sun is just below the horizon, the irregularities due to reflec- 

 tion from the surface of the earth may be largely eliminated. However, 

 since pyrheliometric measurements are only influenced by atmospheric 

 conditions along the path of the incident solar beam, they are believed 

 to offer a more promising index to atmospheric conditions than do any 

 form of polarization observations. H. H. K. 



