The natural consequence of a volumetric material is (quite obviously) clouds.
A simple way to create clouds in Cycles is by modeling the desired shape and then assigning an appropriate volumetric material. In the following screenshot, you can see this method applied to the usual Suzanne mesh:
Start Blender and open the
9931OS_09_cloud_start.blend file, where there is the Suzanne_cloud object with two Emitter mesh-lights and a bright World set with a Sky Texture.