"Ken Cook" wrote :
On 15 Feb, 09:38, "Philip Eden" philipATweatherHYPHENukDOTcom wrote:

RMK: Min temp reached between 19 and 20z yesterday
when breeze picked up and disrupted inversion.
09-09: Tx 7.9 Tn -0.4 Tg -3.6 RR trace(w) sss 2.9

Evening minima then mixing seem to be a feature of your site. The same
often happens here with light winds and minima two or three hours
after sunset on clear evenings. During the rest of the night the
temperature graph fluctuates with the katabatic wind which then
disappears just after sunrise when another dip in temperature occurs.
I assume it's a feature of hill country. Min last night was at 2030Z
yesterday evening before the katabatic and again at 0750Z after it
ceased.

The katabatic penetrates down to the Lead Mill for less time and
sometimes not at all.

Met O site 09-09 Tx 8.1 Tn -0.3 Tg -5.7 RR trace sss 7.3
Lead Mill 09-09 Tx 7.7 Tn -1.5

Ken ... yes, it's been a revelation since I moved here. I think
I've read most of what's been published on frost hollows
(although I'm not sure how much of it I've retained!) but I
can't recall detailed comparisons of the characteristics of
different types of hollow.

If you stick Whipsnade into multimap (or whichever is your
map of choice) and look up the NGR square 00-01 18-19
you'll get a rough feel for the sort of country it is. I'm just 300m
walk from the top of the scarp slope which is 228m amsl, about
1km from the highest point hereabouts (243m), in a shallow
dry valley on the dip slope (the valley is no more than 10m
deep and the slopes have a gradient of about 1:10). The
katabatic catchment is probably less than 0.5km^2 but it
seems to work extremely efficiently for a couple of hrs after
the sun sets.

When the gradient wind is from the western half of the
compass the breeze picks up after only 2 or 3 hours
(katabatic assisted by gradient?) but on one evening last
week when the gradient wind was N to NEly, I didn't
reach my min until 21-22z - that was the -9°C which was
lower than most of the regular cold spots in Beds/Herts/
Bucks.

I'm still in temporary mode re instrumentation (no data-
logging AWS yet) but some time this year I aim to have
AWSs both in my garden and at the summit (and perhaps
eventually at the bottom of the scarp slope at the Gliding
Club too).

Endlessly fascinating, this climate stuff, isn't it?

Philip