photo #8899171

Glenn Johnson (Cloud Appreciation Society Member 55,100) was heading home from an evening walk in Selly Oak, Birmingham, England when he noticed a big, bright halo around the almost full Moon. ‘Despite it being a fairly murky evening,’ he said, ‘and there being a lot of light pollution from the city streets, it was still a breathtaking phenomenon.’Known as a 22-degree halo, this optical effect can form whenever the sky is hazed with Cirrus or, as here, Cirrostratus cloud. Such high-altitude formations are made of ice crystals, and when these have developed in simple, hexagonal shapes that are clear, they can act like microscopic prisms, bending the sunlight or moonlight shining through them. 22-degree halos are likely formed by crystals in the shape of hexagonal columns – like minuscule unsharpened pencils of ice. The light is bent, or refracted, most at an angle of 22 degrees. If Glenn had pointed one arm towards the Moon and the other to any point on the halo, his arms would have made an angle of 22 degrees.You might notice that the ring of the halo doesn’t look quite circular here. It would, in reality, have been precisely so for Glenn. His wide-angle camera lens had merely distorted the geometry. When it comes to the 22-degree halo, the compass of the sky never fails.