Sundog. Colorful sundog (centre) against a blue sky, seen over silhouetted buildings. Clouds are seen across top. Sundogs, or parhelia, are luminous and sometimes colourful areas which appear on both sides of the sun at the same elevation in the sky (one one side seen here). They are caused by the refraction of sunlight in hexagonal ice crystals present in the atmosphere. This example of a sundog may be due to the presence of ice crystals in the contrail of an airplane. Photographed in Brittany, France.