Winter Halo Above Eastern Massachusetts
July 06, 2009
While I was sweeping a fresh snowfall from my overburdened roof in eastern Massachusetts this past winter, a dash of color and a familiar rounded shape caught my eye – a 22 degree halo or icebow. Halos are formed when sunlight interacts with randomly oriented hexagonal ice crystals, most often found in cirrus type clouds. Sunlight that passes though pencil-shaped hexagonal crystals is essentially passing though a prism having a 60 degree angle. The average angle of the deviation of refracted sunlight in such prisms is just under 22° (21.84°). The deviation is actually 21.54° for red light and 22.37° for blue light. This slight difference in refraction angle explains why the inner side of an icebow is tinted red and the outer side is tinged with blue. Photograph taken on a frigid day in January 2009.