Folklore has it that a ring around the moon signifies bad weather is coming. The ice crystals that cover the halo signify high altitude, thin cirrus clouds that normally precede a warm front by one or two days.

It is believed that the number of stars within a moon halo indicate the number days before bad weather will arrive. Give it a try the next time you observe a moon halo.

In meteorology, a corona is produced by the diffraction of light from either the Sun or the Moon by individual small water droplets (and sometimes tiny ice crystals) of a cloud.

The corona consists of small number of concentric colored rings around the celestial object and a central bright aureole. The angular size of the corona depends on the diameters of the cloud droplets - small droplets produce bright coronae. Coronae differ from haloes in that the latter are formed by refraction from comparatively large rather than small ice crystals. Reddish colors always occupy the outer part of a corona's ring. (This is an atmospheric Airy disc.)

A halo is an optical phenomenon that appears near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of optical halos, but they are mostly caused by ice crystals in cold cirrus clouds located high in the upper troposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colors because of dispersion, similarly to the rainbow.

Sometimes in very cold weather optical halos are formed by crystals close to ground level, called diamond dust. The crystals behave like jewels, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.

Atmospheric phenomena such as halos were used as an empirical means of weather forecasting before meteorology was developed.

Other common optical phenomena involving water droplets rather than ice crystals include the glory and the rainbow.

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