In this paper, the problem of rendering a gas in real time is approached by placing specific constraints on the distribution of densities and color inside the gas. A multi-pass algorithm for real-time rendering of a gas defined by a three-dimensional boundary is presented. The density and the color of the gas inside the boundary may be constant, or the color of a dense gas can be defined by a texture. This algorithm is combined with an existing algorithm for rendering layered fog so that the density and color inside the three-dimensional boundary can vary along a single axis. Despite the constraints, the algorithms can be used to render a variety of volumetric phenomena in real time. The OpenGL implementation of the algorithms presented in this paper is used in SGI's Performer, Version 2.4, released in November 2000.