Abstract: This paper presents a physically-based, visually-realistic interactive cloud simulation. Clouds in our system are modeled using partial differential equations describing fluid motion, thermodynamic processes, buoyant forces, and water phase transitions. We also simulate the interaction of clouds with light, including self-shadowing and light scattering. We implement both simulations - dynamic and radiometric - entirely on programmable floating-point graphics hardware. We use "flat 3D textures" - 3D data laid out as slices tiled in a 2D texture - to implement 3D simulations on the GPU. This has scalability advantages over the use of traditional 3D textures. We exploit the relatively slow evolution of clouds in calm skies to enable interactive visualization of the simulation. The work required to simulate a single time step is automatically spread over many frames while the user views the results of the previous time step. This technique enables the incorporation of our simulation into real applications without sacrificing interactivity.