If you want to follow Kadri's idea on clouds, it shouldn't be too difficult. Just create a cloud layer, make it a localised cloud and move the center to the place you want to have it. Uncheck the density shader and increase the cloud density (for temporarily better visibility). Then adjust the radius. If I'm not completely wrong, you should get a perfect round cloud if you use the radius's value for cloud depth. Do some test renders until you have the right size. If you like you can then use a cloud fractal or a power fractal as a density shader to make it more irregular.
