.... yesterday's (26th/Saturday) rain slowly faded away during the
evening - but enough humidity and cloud-cover to keep temperatures well
up: overnight minimum here 15.3degC ... now a fine morning with 2 oktas
Cumulus humilis (shallow), and using the rule that relates cloud cover
at max convection with RH at Cu initiation, we should only have
half-cover or less this afternoon. Pleasant W or WSW breeze &
temperature at 09Z was already 20.2degC (the 24hr max for the 26th).

Martin.


--
Martin Rowley: data via -
http://homepage.ntlworld.com/booty.weather/metindex.htm
Bracknell (Wooden Hill/Tawfield), Berkshire
NGR: SU 854 667 Elev: 80m
Lat: 51DEG23MIN30SEC(N): Long: 00DEG46MIN28SEC(W)

--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

Martin,
Interesting - what is the 'rule'?
Robin.

"Martin Rowley" wrote in message
...
... yesterday's (26th/Saturday) rain slowly faded away during the
evening - but enough humidity and cloud-cover to keep temperatures well
up: overnight minimum here 15.3degC ... now a fine morning with 2 oktas
Cumulus humilis (shallow), and using the rule that relates cloud cover
at max convection with RH at Cu initiation, we should only have
half-cover or less this afternoon. Pleasant W or WSW breeze &
temperature at 09Z was already 20.2degC (the 24hr max for the 26th).

Martin.


--
Martin Rowley: data via -
http://homepage.ntlworld.com/booty.weather/metindex.htm
Bracknell (Wooden Hill/Tawfield), Berkshire
NGR: SU 854 667 Elev: 80m
Lat: 51DEG23MIN30SEC(N): Long: 00DEG46MIN28SEC(W)

--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

"Robin Bryant" wrote in message
...
Martin,
Interesting - what is the 'rule'?

Only use in situations such as today: no (significant) airmass change
expected, no sea-breeze interference etc.

If U is the relative humidity at time of first 'puffy' cumulus seen,
then

CC = (U/6) - 6

where CC is the cloud cover IN TENTHS at the time of max convective
activity. (The rule was worked out in the days when cloud cover was
estimated and reported in tenths of sky covered)

So this morning, relative humidity at ~09Z (when Cu hum observed) = 60%
CC = (60/6) - 6 = 4 (tenths)

Note that the sky cover may well increase to much more than this for a
time; the 'rule' simply tries to work out what happens when the lower
part of the atmosphere that includes the cumulus cloud is well-mixed;
more humid, then more cloud, dry low-level air, then greater entrainment
of same and less (eventual) cover.

As with all these things, there will be exceptions (similar to the one
that couples (T-D) to cumulus base height, but it is not too bad a
guide. If there is a pre-existing layer of stratocumulus then be wary as
this implies higher humidity around this level - the cumulus may simply
'feed' this layer and lead to overcast skies.

Martin.


"Martin Rowley" wrote in
message
...
... yesterday's (26th/Saturday) rain slowly faded away during the
evening - but enough humidity and cloud-cover to keep temperatures
well
up: overnight minimum here 15.3degC ... now a fine morning with 2
oktas
Cumulus humilis (shallow), and using the rule that relates cloud
cover
at max convection with RH at Cu initiation, we should only have
half-cover or less this afternoon. Pleasant W or WSW breeze &
temperature at 09Z was already 20.2degC (the 24hr max for the 26th).

Martin.


--
Martin Rowley: data via -
http://homepage.ntlworld.com/booty.weather/metindex.htm
Bracknell (Wooden Hill/Tawfield), Berkshire
NGR: SU 854 667 Elev: 80m
Lat: 51DEG23MIN30SEC(N): Long: 00DEG46MIN28SEC(W)

--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

Pleasant W or WSW breeze &
temperature at 09Z was already 20.2degC (the 24hr max for the 26th).

This is one of the problems with a 24-hr max/min. I use 12-hr (08Z and
20Z) max's and min's, which would have avoided this particular anomaly, but
causes others, such as missing a night-time max or early evening min, nearly
always in winter. But one has to stick to one's declared régime however daft
it may occasionally seem.
Minimum overnight here 14.9°C.

Tudor Hughes, Warlingham, NE Surrey, 556ft AMSL