Unusual sequence of weather today, as observed from the Dunbridge area
of Hampshire, northwest of Romsey. Started off fairly clear with
high-level cloud and relatively fresh.

Around 1400 I first noticed what looked like distant cumulus clouds
amongst a bank of high level cloud to the northwest from the Romsey
area of Hampshire. Was vaguely aware a weak front was coming through
(explaining the high level cloud) but was surprised at the cumulus.
Meanwhile overhead was clear blue skies and still fairly low humidity.

In the next hour the bank of cloud from the northwest didn't seem to
move much, but then I climbed a hill and at 1500-1530 noticed a
definite Cb cloud low to the northwest, with a huge anvil spreading
almost overhead - this view from a higher elevation revealed the true
nature of the cloud bank to the northwest.

In the next half hour, between 1530 and 1600 the Cb seemed to move very
rapidly southeastwards, so that by 1600 it looked like there was the
distinct possibility of overhead rain and thunder within the hour. Not
good given I was rather out in the sticks :-) The anvil also moved
overhead, obscuring the sun, and it became very humid.

In the next hour nothing changed much, the sky seemed to stay about the
same, the cloud seemingly not moving further SE. Indeed around 1700 the
cloud seemed to be tracking more due east, allowing a clearer area to
the southwest to apparently move overhead.

However after that the cloud moved rapidly southeastwards again, and it
became very oppressive. Around 1745 thunder was heard, twice, to the
northwest, then at 1815 quite a heavy shower began as the cloud finally
moved overhead... though no thunder by this time.

Quite an unusual sequence of events... firstly it's extraordinarily
rare for thunder clouds in warm/hot weather to move from the northwest
IMX. Indeed often in this area (south Hants) you see thunder clouds far
inland in the summer and you know - because they typically track S or
SW to N or NE - that the area will remain dry (e.g. third weekend in
June last year). Secondly the humidity arrived quite suddenly, thirdly
the cloud seemed to be moving quite erratically, and fourthly the
surface wind was apparently from the NW... not normally a thundery, or
even particularly warm direction. The high temperatures this weekend
themselves took me a little by surprise looking at the charts late last
week... I'd have expected only just above 20C, no more than 22-23 max.

After the shower passed the sky seemed to gradually clear again. Well
that didn't seem to last long.... :-)

Nick

wrote:

Around 1400 I first noticed what looked like distant cumulus clouds
amongst a bank of high level cloud to the northwest from the Romsey
area of Hampshire. Was vaguely aware a weak front was coming through
(explaining the high level cloud) but was surprised at the cumulus.

A weak front displays as high cloud?

Please explain.

"Weatherlawyer" wrote in message
oups.com...
|
| wrote:
|
| Around 1400 I first noticed what looked like distant cumulus clouds
| amongst a bank of high level cloud to the northwest from the Romsey
| area of Hampshire. Was vaguely aware a weak front was coming through
| (explaining the high level cloud) but was surprised at the cumulus.
|
| A weak front displays as high cloud?
|
| Please explain.
|

The Bracknell charts showed a high-level front (the symbols were hollow,
rather than solid for a surface front). From a few days back, the
"thickness" isopleths showed a trough along this line, corresponding to a
pool of cold air moving aloft. ["Thickness" in this context is the height
difference between the 1000 and 500 hPa pressure levels, and as pressure
falls more rapidly in colder air, being more dense, this "thickness" is a
rough measure of the temperature of the lower to middle troposphere.]

This pool of cold air running over the tropical air near the surface
resulted in the instability and resultant storms. This situation was
correctly forecast by the Met Office both on the day concerned and in the
previous two days "outlooks".

The pool was quite sharp edged. From Ashurst (location as sig below) at
16:00 BST the upper cloud could be seen to the north and east, but the sky
was clear with bright sunshine to the west. First indication were that the
active area would pass by to the east, but the upper winds veered more
northerly and two cells from the western edge of this area passed over
Totton, one producing a heavy downpour at around 18:45 BST, the second, less
vigorous, at around 20:00-20:15 BST giving me a wetting as I cycled home
from church. But the roads were noticeably less wet at the western edge of
Ashurst and the A35 was completely dry west of the railway bridge.

The high-level fronts can produce just as much active weather as their
surface counterparts. I am not aware of a symbol for a "weak" front as
such, although a "dissipating" front (precipitation and cloud fading away as
the temperature contrast across the front decays) is shown by the frontal
symbols on a broken line with strike-throughs in the gaps, rather than on a
solid line.
--
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