The most usual form of halo is that of 22° radius. This is a luminous ring of light surrounding sun or moon, with the inner edge red and sharply defined and the spectral colors proceeding outward in order; red is frequently the only color visible, the remainder of the ring appearing whitish. Since the halo is produced by refraction of light by ice-crystals, which exist in clouds of a certain type gathering at high altitudes, it is always a very good indicator of an approaching storm.