Artificial clouds play an important role in the computer generation of natural outdoor scenes. Realistic modeling and rendering of such scenes is important for applications in games, military training simulations, flight simulations, and even in the creation of digital artistic media. We propose a model for simulating cloud formation based on an efficient computational fluid solver. We combine the fluid solver with a model of the natural processes of cloud formation, including buoyancy, relative humidity, and condensation. This allows us to simulate the formation and growth of clouds at interactive rates.