When there's a ring around the moon,
Rain or snow is coming soon.
A ring around the moon is caused by the refraction of moonlight against ice crystals in the upper atmosphere. The halo is always white or pale in colour, but sometimes a narrow band of colour can be seen. The brightest parts of the halo are immediately above, below, and to either side of the moon. Incomplete halos show up as arc sections or patches of hazy refraction on either side of the moon. Such arcs are sometimes called moonbows, while the bright patches are known as moondogs.
The size of haloes is constant, determined by the angle of refraction through six-sided ice crystals. The primary halo is always 22 degrees from the moon. If colour is visible, it will be red on the inside and blue on the outside. Sometimes a secondary halo, twice as wide, will also be seen, with reversed colours.
Because of refraction, the sky between the moon and the halo is distinctly darker than the night sky around it. This phenomenon is more clearly visible with the moonbow's daytime equivalent, the ring around the sun, where the sky between the halo and the sun becomes a much darker azure compared to the pale blue outside the ring.
Defraction of light through very small ice crystals or even tiny liquid droplets in the atmosphere can also cause a rainbow-like effect with a red inside border. Where there is a clear disc of colour and the ring or series of concentric rings are very close to the moon, the halo is properly referred to as an airy disc or corona. The smaller the droplets, the brighter the corona.
In the summer, the presence of such ice crystals indicates the presence of stratospheric cirrus clouds, which can indicate an approaching cold front and strong lines of thunderstorms ahead of that front. As the cold air behind the cold front displaces summer heat, the temperature can drop so quickly that spring or summer heat can be followed by a flash freeze. This combination has led to the folklore that a ring around the moon indicates bad weather to come. The halo is not a 100% reliable predictor, but its presence does come close to the reliability of many 7-day forecasts.
Brightness does not itself correlate with the probability of bad weather, since there can be many different reasons for increased or decreased brightness, many of them having nothing whatsoever to do with atmospheric conditions. In contrast, the solidity of the ring indicates that large ice crystals are evenly scattered through a large section of sky, which suggests that a similarly large portion of land below is in for a serious storm.