Some kind of bloom effect would probably help get nearer what you want.
I think if you did that coupled with a fractal noise you could be well on the way.
So use a down-sampled version of the image to blur it based on light intensity, then overlay that onto the original image. I would also use that blur texture as a alpha map for some kind of noise (such as fractal) to get that "cloudy" look. Depending on how you built the initial image you may need to think about how a noise texture might swim as the camera moves and you will need to fix that, i.e. maybe sample a cube map there, just guessing at this point