We consider a cloud to be a collection of points which collect light from the sun and scatter it back to the viewer. I represent the cloud as a particle system with cloud particle billboards and light it using equations based on Harris' paper.
Images (work in progress):
On a related note, I've probably spent more time (four days) really making a good, flexible design than the actual coding (refer to the software design article). After all, coding is about thinking, not typing.