This book is designed primarily for readers with an active interest in meteorology -- meteorologists, both professional and amateur, weather observers, and all those concerned with aviation in its various phases. It is also intended for that considerable number of persons whose chief interest in meteorology is the study of clouds. The author deals in turn with classification of clouds; the materials of which clouds are composed; the process of cloud formation; the nuclei of condensation and sublimation; the problem of precipitation release (including discussion of rain-making experiments); the methods and equipment employed in observing clouds; the relation of clouds to flying; their significance with regard to forecasting; the special forms of cloud found at very high levels in the atmosphere; and the optical and electrical phenomena associated with clouds. For the benefit of non-technical readers, a glossary of technical terms is appended. There is a bibliography, and subject and author indexes have been included. This book is illustrated with diagrams and photographs; the latter are from a number of different sources, including the author's own cloud photographs.