In article ,
Tudor Hughes writes:
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.

Should that read "not have spread out"?

Another factor about the current situation is that the lower layers
are rather moist, more so than usual.

Tudor Hughes, Warlingham, Surrey.

I think this must be a tricky time of year for forecasters with this
set-up. A month ago they could have been confident that the cloud would
have been "burnt off", and in a couple of months time they could have
been confident that it wouldn't have been, but in late September and
through October it tends to be touch and go.
--
John Hall

"Distrust any enterprise that requires new clothes."
Henry David Thoreau (1817-1862)