Beside the basic cloud types, there are subgroups and some unusual cloud formations. Terms such as humulus (fair weather), and congestus (rainshower) are used to further describe the cumulus genus. Fractus (jagged), castellanus (castle shaped), and uncinus (hook shaped) are other descriptive terms used together with some basic cloud types. In mountainous regions, lenticular clouds are a common sight. They form only over mountain peaks and resemble a stack of different layers of cloud matter. Noctilucent clouds form only between sunset and sunrise, and are only seen in high latitude countries. Contrails (condensation trails) are artificial clouds formed from the engine exhaust of high altitude aircraft.
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Shafer, Vincent J., and John A. Day. A Field Guide to the Atmosphere. Boston: Houghton Mifflin Co., 1981.