A special hemispheric mesh that has a special texture setup and can be set as background of a scene. Rendered after clearcolor/skybox. Special effects are possible using material and shaders. Skydome needs its own sun color, as sometimes the visible sun might differ from what you use for the scene.
Alternatively you can render a custom mesh, which might be better for your needs. The cloudmatrix effect is very costly as it generates an infinite sky, a custom mesh is a cheap solution.
o-+ l3dlist o-+ skydome