This shader is part of a series of four shaders developed by Daniel Pelham. The two cloud shaders, BrightCloud and DarkCloud, generate animatable cloud and smoke. BrightCloud, simulates the effect of a light source behind the volume, making denser regions of the cloud darker than the thin areas. DarkCloud uses are a more traditional scatter model and has the effect of making the denser regions more bright. Take a look at the first two images on the left to see the difference. Both shaders support all light types as well as volumetric shadows.

other procedural shaders in this set
Fast Haze
Real Haze
Dark Cloud