In message , Martin Rowley
writes
"Philip Eden" philipATweatherHYPHENukDOTcom wrote in
SNIP
1515z NNW F2-3 25km 2Cu060 24.7/08.4 1021.0
RMK: Also 2Ci300 ... a steady decrease in amounts and
thickness of Ci in the last 3h. Surprising drop in dewpoint.

1750z NNW F2-3 25km 1Cu035 22.4/14.8 1021.3
RMK: Also 2Sc050 and a trace of Ci plus COTRA. The
contrails are barely spreading and give the appearance of
white pencil lines drawn across the pale blue sky. That
15z dewpoint looks even odder now.
Whipsnade 214m

... if you are happy with the equipment, I would not rule out a
temporary drop as you have reported. Looking at the actuals and ascents
(such as we have), I suspect that you (and possibly we - haven't got any
autographic instrumentation to check) have been in the 'mixing zone'
between the high dewpoint stuff to the west, and lower dew point air to
the east. The slight ebb and flow of the wind field (broadly northerly,
but with confluence / diffluence evident) would be enough to cause
'folding' of the humidity envelopes defining the two air-masses, and
given your height (and a little helpful heat induced / convective
turbulence / overturning), then such DP variations are quite possible. I
note that somewhere in the immediate London area (can't tell which
station it is from my info) went down to 9C a couple of hours ago.

Martin.

The DP in Chalfont St Giles temporarily dropped to 10.9C at 1630z on the
5th, according to the Davis VP.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England