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?