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The tiny ice crystals that make up this cloud refract the sun or moonlight, splitting it into the 7 colours of the rainbow; red, orange, yellow, green, blue, indigo, violet, giving a ring with an angular radius of 22 degrees. Most often the dim light of the moon will only produce a faint white ring.
But, how does the sun or moon produce the halo? The haloes form best when the cloud is cirrostratus, a thin veil of cloud found in the icy reaches of the upper atmosphere.
Why should the halo be a harbinger of rain? Well, cirrostratus cloud is usually found near the leading edge of a front, and fronts spreading in from the west bring much of our rain here in Britain. The cloud is very likely to thicken to dark, deep layers of nimbostratus from which persistent drizzle, rain and in winter snow are likely to fall.
If the cloud cover is not complete and only part of the halo is visible we may see a mock sun of ‘sun dog’ and at night a ‘moon dog’. This is a brighter disc of cloud away from the sun or moon. Unfortunately, rain within 24 to 48 hours is still the usual prognosis!
A rainbow in the morning is always seen in the west, since the sun rises in the east, and the west is the direction from which most of our weather comes. So a rainbow in the west means rain is on its way as this old saw indicates: