Microphysical Structures of Stratiform and Cirrus Clouds

This chapter summarizes measurements of the physical and microphysical structures of stratus, stratocumulus, altostratus, and cirrus clouds, which are thought to have the greatest effect on climate because of the large areas they cover. These clouds can be loosely classified according to the phase(s) of the condensate: liquid, mixed-phase, and ice, respectively. Stratus clouds are layers or patches of low, often grey, clouds that hardly have definition and rarely produce precipitation. Altostratus clouds appear as gray or bluish sheets or layers, ranging from several hundreds to thousands of meters thick, often covering the entire sky. The layers are variously striated, fibrous, or uniform in aspect. Altocumulus clouds form in the mid-troposphere, in distinct layers often less than 100 m thick. From the ground, altocumulus clouds are identified by their sharp outlines, enclosing rounded often convective elements. Cirrus are detached clouds in the form of white, delicate filaments or mostly white patches or narrow bands, composed almost exclusively of ice crystals. Dense patches or tufts of cirrus may contain ice crystals large enough to acquire appreciable fallspeeds; extended trails of virga may sometimes be seen below such clouds. Wind shear and particle size affect the shape of the trails. One cirrus type displaying these features is cirrus uncinus; it is comma-shaped, terminating at the top in a hook or tuft, which is presumably an ice generation region.