It is something I learned from cyphyr. It is a localized, "empty" cloud layer. By empty, I mean it has no fractal attached to it. When you use it, it acts like haze depending on the density of the layer. You will also need to play with the edge sharpness, depending on how "hard" you want the cloud layer to be.
If you can't figure it out and I will post a .tgd for you.