A very fast technique to render clouds as imposters which are only updated when the viewport has changed enough to make a visible error. Here’s the abstract from the paper:

“This paper presents a method for realistic real-time rendering of clouds for flight simulators and games. It describes a cloud illumination algorithm that approximates multiple forward scattering in a preprocess, and first order anisotropic scattering at runtime. Impostors are used to accelerate cloud rendering by exploiting frame-to-frame coherence in an interactive flight simulation. Impostors are particularly well suited to clouds, even in circumstances under which they cannot be applied to the rendering of polygonal geometry. The method allows hundreds of clouds with hundreds of thousands of particles to be rendered at high frame rates, and improves interaction with clouds by reducing artifacts introduced by direct particle rendering.”

Link to a webpage with much more information about this technique:
http://www.markmark.net/clouds/

Link to the paper:
http://www.markmark.net/PDFs/RTCloudsForGames_HarrisGDC2002.pdf