I remember watching one of Mark's tutorials on how to render a scene in the sky, of flying airplanes. There was a trick he used so that the background of a scene would only show the clouds of the realistic sky and not that murky brownish-grey ground colour. Does anyone remember how to do that?

I've switched off the ground plane in this render, but the clouds seem to start much higher up, and all I get for background is this ugly brown.