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 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?

Low-level turbulent mixing caused by convection underneath the subsidence
inversion is likely to be the main cause. When the cloud tops reach the
inversion they spread out resulting in a layer of stratocumulus.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.

Low-level turbulent mixing caused by convection underneath the subsidence
inversion is likely to be the main cause. When the cloud tops reach the
inversion they spread out resulting in a layer of stratocumulus.

Mostly found over Weston Coyney and Tideswell


Graham

Low-level turbulent mixing caused by convection underneath the subsidence
inversion is likely to be the main cause. When the cloud tops reach the
inversion they spread out resulting in a layer of stratocumulus.

Mostly found over Weston Coyney and Tideswell


Graham

Low-level turbulent mixing caused by convection underneath the subsidence
inversion is likely to be the main cause. When the cloud tops reach the
inversion they spread out resulting in a layer of stratocumulus.

Mostly found over Weston Coyney and Tideswell


Graham

On 23/09/2013 22:10, Graham wrote:



Low-level turbulent mixing caused by convection underneath the subsidence
inversion is likely to be the main cause. When the cloud tops reach the
inversion they spread out resulting in a layer of stratocumulus.

Mostly found over Weston Coyney and Tideswell


Graham

And also Wolverhampton ! ;-)

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.

On 23/09/2013 22:40, Joe Egginton wrote:
On 23/09/2013 22:10, Graham wrote:



Low-level turbulent mixing caused by convection underneath the
subsidence
inversion is likely to be the main cause. When the cloud tops reach the
inversion they spread out resulting in a layer of stratocumulus.

Mostly found over Weston Coyney and Tideswell


Graham

And also Wolverhampton ! ;-)

And Cheltenham...

--

Paul Hyett, Cheltenham

Thanks for your replies - this does make sense and agrees with what I observed. (Though it is still annoying when I want to take a photograph of valley mist lit by the sun!).

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)