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Old June 27th 04, 12:25 PM posted to uk.sci.weather
Martin Rowley Martin Rowley is offline
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First recorded activity by Weather-Banter: May 2004
Posts: 2,309
Default [WR] Warm night here in Bracknell


"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.


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Martin Rowley: data via -
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Bracknell (Wooden Hill/Tawfield), Berkshire
NGR: SU 854 667 Elev: 80m
Lat: 51DEG23MIN30SEC(N): Long: 00DEG46MIN28SEC(W)

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