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 or on a foggy glass surface. 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 large coronae. For the same reason, the corona is clearest when the size of the droplets is most uniform. Coronae differ from haloes in that the latter are formed by refraction (rather than diffraction) from comparatively large rather than small ice crystals. Reddish colors always occupy the outer part of a corona's ring. A corona is essentially an Airy disc caused by the atmosphere.