Like many natural phenomenon, clouds are often modeled using procedural methods, which may be difficult for an artist to control. In this paper, a freehand sketching system is proposed to control the modeling of volumetric clouds. Input sketches are used to generate a closed mesh, defining the initial cloud volume. Sketch analysis as well as the elevation at which the cloud is drawn is used to identify the cloud type and then generate a mesh with the appropriate characteristics for the determined cloud type. The cloud volume can then be edited using Boolean operations that allow for addition and removal of material from existing clouds. The proposed modeling system provides an intuitive framework for generating individual clouds and entire cloud fields, while maintaining the interactive rates necessitated by the sketch-based paradigm.