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An actinoform or actiniform describes a collection of marine low clouds that takes a distinct shape. They are named after the Greek word for "ray" due to their radial structure. Actinoform clouds can spread out over 300 km (190 mi) across and thus cannot be easily seen with the naked eye. In addition, actinoform clouds can form "trains" that are up to six times the length of the original cloud field, yet they maintain their own distinct identity.