You can create two ways.
One by creating cloud layer and set height and altatude range.
Second by creating Sphere, squishe it little bit and apply volumetric material to it.

Rest in fine tune settings on volumetric material.

check this :
http://www.geekatplay.com/tutorials/groundfog/
http://www.geekatplay.com/tutorials/insidecave/
http://www.geekatplay.com/tutorials/epiclandscapes/greenvalley/
http://www.geekatplay.com/tutorials/epiclandscapes/marshes/
http://www.geekatplay.com/tutorials/epiclandscapes/lavapath/

hope this help you.