The circumhorizontal arc is an impressive, large, colored arc, which usually lies close to the horizon when visible. It is closely related to the circumzenith arc; it is caused by the same plate-ice crystals (and Parry-oriented pencil-shaped crystals). Light enters one of the vertical side faces of the crystal and exits at the bottom face. The circumhorizontal arc can only be visible when the sun is higher than 58 deg. above the horizon, making it a rare observation for observers far north (e.g. 50 deg. lattitude and more).
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