Weather Banter - View Single Post - Thunder from the northwest... Dunbridge Hants

Yokel · June 5th 06, 11:50 PM posted to uk.sci.weather

"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.
--
- Yokel -
oo oo
OOO OOO
OO 0 OO
) ( I ) (
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Yokel @ Ashurst New Forest
SU 336 107 17m a.s.l.

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