Making it draw clouds
Caveman uses a particle system driven by the weather engine for clouds. The weather engine determines the number of clouds, and their direction and speed of movement. its will eventually control their color as well.
Clouds are draw in in a box 2000 units around the camera at an altitude of 500 to 700 units. Their altitude is lerp reduced from 100% to 50% as the range from the camera increases. Clouds are drawn as 3d alpha blended billboards. clip planes are set to high values before drawing, and reset to normal values after drawing. 100% cloud cover = 400 cloud particles.