1 22°-halo, 2 Parhelia (Sundog), 3 Sun pillar, 4 Parhelic Circle, 5 Circumzenithal Arc, 6 Tangent arcs and Circumscribed Halo, 7 46°-halo, 8 Subsun
Cloud of the Day – Halo
A meteorological halo, also called nimbus, icebow and gloriole, is a product of sunlight or moonlight being refracted by ice crystals in the atmosphere. Although a halo can often be seen around an artificial light, such as a street lamp, the real ones form in cirrostratus clouds 5-10 kilometers above the ground. Due to the refractive index of the ice crystals, the halo forms 22 degrees away from the light source. Sometimes a second one can be seen at 46 degrees. As long as the ice crystals extend far enough and the Sun or Moon is at least 22 degrees above the horizon, the halo is always circular.
In addition to their beauty, haloes are also used in weather lore. Since the presence of cirrostratus is often a harbinger of lowering cloud, they can be used to forecast approaching weather.