ON THE PHENOMENON OF THE COLORED SUN, ESPECIALLY THE "BLUE " SUN

The phenomenon of a colored sun or moon can be explained by Mie's theory of large particle scattering, if the radius of such particles is of the order of the wavelength of light. Smoke generated by large forest fires in Alberta, Canada, traveled across the Atlantic. In many places in the eastern United States and especially over large parts of Europe, the sun, moon and stars were seen blue in color in September 1950. Using Mie's theory and some extinction measurements, the radius of the particles is calculated to be between 0.5 and 0.8 A and the concentration 175/cm3 and 127/cm3. The total amount in the smoke layer is about 4.7 to 6.5 X 107/cm 2 column. Three size spectra are assumed, case 1 deals with a uniform size, cases 2 and 3 with a Gaussian distribution of the size spectrum. The refractive index for water droplets is chosen, but it is also studied carefully for other substances. The radius becomes larger for ice at low temperatures and smaller for pure smoke; the total number varies inversely to the radius, but the extinction coefficient remains unchanged. Scattering by spherical particles is assumed, because the shape plays a very minor role. For anisotropic particles, the extinction coefficient increases by less than 10 percent. The intensity distribution of the solar spectrum is calculated between 3500 A and 7000 A after passin