Iridescent colored bands on clouds

Iridescence on clouds is often considered to be closely related to the rings of the atmospheric corona. Both phenomena are caused by near-forward scattering of sunlight. The colored rings of the corona are caused by diffraction from water droplets of fairly uniform size. Iridescence is often seen as bands of color along the edges of clouds. Mie theory shows that near-forward scattering of sunlight is a complicated function of droplet radius <i>r</i> and scattering angle <i>θ</i>. It is difficult to analyze photographs of iridescence because most photographers deliberately exclude the Sun, and hence, we do not know the value of <i>θ</i>. This paper uses indirect techniques to determine the relative position of the Sun, thereby revealing <i>θ</i> and allowing estimates of the droplet radius <i>r</i> at different points in the iridescent clouds. The results demonstrate that variations in <i>r</i> produce the narrow bands of color aligned with the edges of clouds.