1. The process by which various types of clouds are formed. In most cases, cloud formation involves cooling by expansion of ascending moist air. In rare exceptions, the cooling may occur as a result of other processes such as mixing, as in a contrail. The ascent of air may result from vertical instability, as in most cumulus clouds; from undulatory motions at inversion surfaces, as in certain undulatus species; from orographic lifting; and at a frontal surface, as in many altostratus and other stratiform clouds. 2. A particular arrangement of clouds in the sky, or a striking development of a particular cloud.