I reckon - those MrPR would be more scientifically and meterologically correct about it - that it's a result of the sun so low in the sky it lights the 'lid' of clouds above us from below, but at such an angle that the light passes through all the muck in the air but coming out with the beautiful result. I may be wrong. Great pics steveo! Looking out from my flat on the hill, I love seeing the cloud above the lights of the city - Broadgate building, Shard, Gherkin and soon to be coming the Building With More On Top, and they light the cloud above them whilst all else is dark.