“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

• Skybox: Good when the sky can be viewed from all directions. For example in a space game. It is basically a box with inverted faces.
• Skydome: Probably the best choice when rendering outdoor scenes with terrain. For example a driving game. It is basically a half sphere (dome) with inverted faces.
• Skyplane: Sometimes an option when rendering outdoor scenes and you should not be able to see the horizon. It is just a plane with the normal pointed down and a sky texture attached to it.

Information and tutorials about skyboxes:
http://www.toymaker.info/Games/html/skybox.html
http://www.gamasutra.com/features/19981023/bell_01.htm

Information and tutorials about skydomes:
http://www.spheregames.com/index.php?p=templates/pages/tutorials
http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series4/Skydome.php
http://www.flipcode.com/archives/Sky_Domes.shtml

Link to more details from the author (he says a demo should be available soon).
http://www.gamedev.net/community/forums/topic.asp?topic_id=511184