On 29 Nov, 18:59, Hawkeye23 wrote:
Apolgies from a newbie if this turns out to be an old chestnut.
I have noticed this before in general - but in particular up here
(N.Yorks) in the last few days.I imagine there is a scientific name
for the phenomenon - which I hope someone will tell me.But more
generally I am hoping that someone will explain how/why it happens.
Layman's observations as follows.When I get up and take dogs out just
before dawn (c 7am now) I notice just traces of frost but nothing much
An hour later after it is light- I notice the frost is much more
intense - white over - and ice on the roads.
I would be most grateful for an explanation of why this happens and
what it is called.Many thanks.

Dear Mr Hawkeye,

Further to Dave Cornwell's explanation, there is the added input of
the westerly katabatic flow in this part of NE England.
You do not say where you are situated but if you are in hill country
and above the valley floor you could be experiencing mixing of warmer
upper air.
During clear, calm periods, the cold downflow from the Pennine slopes
begins around sunset and stops around sunrise. Our lowest temperatures
are often during the calm periods at these two times. The rest of the
night is often quite breezy, mixing the warmer air aloft.
A good example was last night here in Copley. The Lead Mill area in
the valley was not disturbed by the mixing and fell to -7.0C while the
upper village had two minima times with -2.1C just after sunset and
-2.7C after sunrise. The wind fell calm again at 10Z with a consequent
drop from -0.5C to -2.0C in full sun.

Hope that helps.

Ken