On Monday, 23 September 2013 17:07:22 UTC+1, Steve Wood wrote:
I was out in the countryside from dawn this morning. As we under a big high pressure area, there was mist in the valleys that had formed overnight as air in contact with a cooling surface flowed downhill and it's water vapour turned to droplets. All straight out of a text book. What I do not understand is why cloud formed as the sun came up and soon blocked out the sun. So my question - how does cloud (not ground level mist) form in sinking air? -- Steve Wood

The air in which the cloud is formed is not sinking. The base of the subsidence inversion rarely descends to below about 3000 ft. If it had been in midsummer the cooler air beneath the inversion could have been warmed out enough for the inversion to disappear. There would still have been convective cloud, maybe quite large, but it would have spread out in the way it has done in the last few days. Another factor about the current situation is that the lower layers are rather moist, more so than usual.

Tudor Hughes, Warlingham, Surrey.