Abstract: We propose a model for representing the shape of cumulus clouds. We draw on several approaches that we combine and extend: We store a hierarchy of quasi-spherical particles (or blobs) living on top of each other. The shape of these particles is defined by an implicit field which deforms under the influence of neighbor particles. We define a set of shaders to simulate a volumetric appearance in the spirit of Gardner's textured ellipsoids, to make the shape appear continuous, and to account for dedicated shading effects. In this paper we deal only with the definition of the shape. However, we believe that this model is well suited to be integrated with particle animation and advanced rendering.