Abstract

A circular region of increased brightness is often observed surrounding the sun and the moon. The outer edge of the pattern is often slightly reddish in color and on occassion one or more faint rings can be seen surrounding the inner pattern. This phenomena is caused by the diffraction of light around cloud or aerosol particles and is called the aureole. Practiced observers of atmospheric optical phennomena realize that the solar or lunar aureole can be used to estimate the diameter of the particles. The angular diameter of the aureole generated by small particles is larger than that generated by large particles. For optically thin clouds, theory, based on the wave nature of light, shows that the angular diameter of the inner lobe of the aureole,Θ, can be related to the particle diameter, D, and the wavelength of the light,λ, by the following approximation: The wavelength dependence of Θ explains why the outer edge of the aureole often appears reddish.

© 1997 Optical Society of America