This paper presents a new animation framework for the modeling of gaseous phenomena. We combine particle and grid based techniques in an innovative way. Based on this framework, our system allows for an incremental design of animations. In the first stage, an animator uses particle methods to model the evolution and appearance of gases. Grid based techniques are subsequently employed to compute high quality animations. Central to the success of our technique is a new algorithm to efficiently advect densities on grids. To demonstrate the effectiveness of our approach, we have included many animations.