Neutrality of Extinction by Atmospheric Clouds in UBVR Spectral Regions

K. Serkowski
1970 Publications of the Astronomical Society of the Pacific  
The (U-B) and (B-V) color indices can be measured accurately through clouds using a multichannel photometer. These color indices change proportionally to the total extinction by clouds, becoming more negative with increasing extinction. The change in cloud extinction by l m does not change the color indices by more than 0 ,! '01. The observed wavelength dependence of extinction in clouds is explained by the scattering of light on water droplets about twelve microns in diameter. The sun usually
more » ... oes not change its color perceptibly when seen through a thick layer of clouds. This indicates that the extinction of light by clouds is approximately neutral for visible light. The observations of third-magnitude early-type stars were made through clouds using the ten-channel photometer-polarimeter designed by Serkowski and Mathewson (1970) . The UBVRI spectral regions are separated in this instrument by dichroic filters. The star can be measured simultaneously with sky background. The results are shown in Figure 1 where the change in color index caused by clouds is plotted against the visual extinction by clouds. Each symbol is a result of a 20-second integration with the 24-inch telescope. The observations made on five nights during December 1969 and January 1970 (denoted by open circles) are in good agreement giving for the rate of change in color index tu-b = d(U-B)ldV = -0.007 , fß_v =d(B -V)ldV = -0.007 , (1) r V -R = d(V-R)ldV = -0.011 . 0 Present address: Lunar and
doi:10.1086/128983 fatcat:yxqrmqi62zhqhkr32kteosutwy