"Bernard Burton" wrote in message
...
Dave. I agree that the model has temp of -10 over the Dover Strait and
Kent
yesterday afternoon, and surface temperatures there were Langdon Bay 2C
and
Greenwich LV 2C, over a sea temp of 7C. The nearest ascent we have is the
midnight for Herstmonceux, which has a -9.9C at 850, and a 1000 mbar temp
of
0.0, equating to a max sea level temp of 2C at 1025 mbar. If the boundary
layer between the surface and 850 mbar were dry, the maximum temperature
difference between the surface and 850 mbar would be appx 14C. However,
that
layer is not dry, and from cloud base at about 300m (Herstmonceux at 00z)
up
to the inversion base at 850 mbar, the air is mostly saturated, with the
corresponding saturated temp lapse rate. For a saturated lapse rate from
850
mbar to the surface, and -10C at 850 mbar, the surface temp would be appx
+0.5 at 1025 mbar. So for an 850 mbar temp of -10, the msl temp would be
limited to the range +0.5 to +4, in the absence of other factors. I am not
sure what part of Essex you are in, but Stansted yesterday reported temps
generally in the range +1 to +2. They are 100m ASL, so you could add
another
1C if you are at sea level. It is also possible, and you do not say how
your
temp is measured, that your 4.6C is influenced by something local, like
buildings. Also, are you sure of the calibration of your instrument.
Lastly,
short-wave radiation can be significant, even on overcast days, and the
satellite imagery for 13th showed considerable breaks in the cloud over
the
SE. When the ground is heated by radiation, the boundary layer temperature
profile becomes out of balance, and over a shallow layer near the ground
can
exceed the dry lapse rate. Balance is only approached by convective
mixing,
which redistributes the surface heating over the entire boundary layer.

http://www.woksat.info/etcsb13/sb13-1330-b-uk.html

And a PS. The base of the capping inversion is at 850 mbar at
Herstmonceux.
Slight changes in this height can result in large changes in 850 mbar
temp.
Also, if there is cloud up to the capping inversion, long-wave radiation
from its top can produce the same sort of inbalance as heating from the
ground does, depressing the temperature at the inversion base so that the
lapse rate in the upper portion of the cloud exceeds the saturated rate.
Again, convective mixing tries to restore the balance, and the cooling is
redistributed over the entire boundary layer.
--------------------
Thanks for going to the trouble of explaining that in detail Bernard. I more
or less understand it! Generally speaking my max temperature calculations
using :-
The relationship between 850-1000 hPa thickness (h*) and the unadjusted
maximum temperature (Tu) is given by: Tu = -192.65 + 0.156h* ... Eq(g)
(Courtesy of our FAQ !) Then modified using table for month and cloud cover.

It gives reasonable results as a rule.
My temperature set up is pretty good I think. AWS housed in a home made
Stevensons screen 1.25m above grass/soil, away from house although slightly
sheltered by garden fence. Can sometimes get slight solar elevation with
strong low late afternoon sun but only about 0.5C. No sun at all that day
which is what surprised me. It can be calibrated and has been in the past
against an NPL calibrated mercury thermometer. (borrowed from work!).
Compares well with other local stations and usually very well with official
one at Gravesend which is 20 miles away across the river. Same time there on
Sat was 3.9C according to UKMO so not too different.

Just one of those little atmospheric weather quirks I guess, Bernard.

Thanks, Dave.
www.laindonweather.co.uk