BRACKNELL RAIN & HAIL STORM: 5th AUGUST, 2004
1. With local surface temperatures of ~27 or 28degC (labelled Tx above: the effective maximum temperature of the day) and dew-points here of ~17 or 18degC (labelled Td above), the convective condensation level (CCL) was around 860mbar (~1480m), with parcel tops (PTOP) around 220mbar (~11½ km); note that with such vigorous ascent (large CAPE), there would have been a huge element of 'overshoot' and tentatively, I suggest that tops would have reached at least 150mbar (>14 km).
2. Note the high values of CAPE: in the region of 1300 J/kg .. well into the category (potential) of 'severe' thunderstorms.
3. Note the value of Lifted Index (LIFT) well below -5; anything below -4 is reckoned 'good' for large hail.
4. Note the large wind shear elements outlined, not only in speed but direction too: ideal conditions for multicellular development of great vigour.
These notes were posted into the newsgroup uk.sci.weather, and added to from contemporay notes:
"distant thunder heard ~1425Z"
"rain (with distant thunder) ~1440Z"
" Now (1445Z) ... thunder getting louder (closer): no lightning seen yet. AirTemp: 26.8degC, DewPoint: 17.9degC (Max: 28.2degC) 5 oktas TCU, 2 oktas Cb cal .... Cb activity (TS) appears to be concentrated in the NE quadrant: I say 'appears', because the sky is (has been) so hazy that cloud development has been difficult to observe. Wind getting gusty (Bft 5 at the moment), and sky greyer."
"squally wind to Bft 6 around 1500Z ... very threatening sky 1510Z ... rain now moderate, squally wind."
"1512Z ... now moderate thunderstorm, with moderate, ocnl heavy rain: hail also observed - no great quantity. Visibility 1515Z: < 1km" [later noted as being towards the northeast, i.e. towards Bracknell town centre; the hail duration was noted (see below) as less than 1 minute. Diameter of hail ~6mm, a few ~7mm. Hail element a small part of the precipitation total.].
"1525Z ... rain now light, no hail for some time - a few CG strokes in VCNTY. Emergency service sirens now evident all about, and have been for some 10 - 15 minutes."
"1610Z ... thunder still rumbling around: nothing very frequent, much of it CC (some quite lengthy rumbles), but the occasional CG strike visible - frequency of these about 1 every 5 minutes. We seem to have daughter cells developing in-situ, rather than storms advected in from elsewhere. No extreme rainfall - enough to wet the garden etc., quite well but no rivers of water or anything like that.
"Thunderstorm ceased here ~1650Z (5.50PM) with rain having finished about 10 minutes prior to that. The hail element was dia. >5mm, but only lasted for less than 1 minute. Some cracking cloud-ground (CG) strokes in the immediate area, but of low frequency. Total rainfall this event 6.5mm . . . "
Subsequent to my reports above, Graham Davis (towards the town centre) wrote " It was heavy enough here Martin. Visibility dropped to about 300M in heavy rain and hail. Footpaths were turned into fast-flowing streams. Heaviest rain I've seen for about thirty years."
But in a street around 400m away, a similar report to mine, i.e. 'hail' fell, but nothing out of the ordinary.
"A summer storm which hit Bracknell last week left a series of flash floods in its wake. Firefighters from Bracknell and Wokingham Road battled to clear up the aftermath of the storm, which saw huge hailstones fall on the town. Shop workers from Currys in the Peel Centre had to be evacuated last Thursday after water gushed into the front of the store and safety fears were heightened when police noticed electrical power points had been soaked . . . . After the storm several reports claimed hailstones measuring 4.5cm had fallen across Bracknell but the Met Office denied such large chunks of ice had fallen"
My comments on above (re: hail):-
We have four possibilities ...
(a): The hail diameter was 'as reported' of diameter 4.5cm.
(b): Chunks of ice were observed, composed of aggregates of individual hail stones.
(c): The depth of hail fall was 4.5cm.
(d): The hail diameter was some other figure.
>> The ascent (above) would support the growth of large hail, with a >5degC 'gap' between the environment and parcel curve around 500hPa - often cited as a requirement. Also CAPE and shear values are large, again pointing to the potential for large hail. However, despite some research (see below), I can find no mention of damage to cars, property, conservatory roofs etc., in the region - just one mention of a single pane of glass broken in a greenhouse. There is one large car sales area (Inchcape-Ford), a large open-air car park, and an open-air 'Garden Centre' area behind Focus DIY. NONE of these mentioned damage - which with hailstones around 2 inches in diameter I would expect.
>> I said in the newsgroup at the time that 'aggregates' of hail might just be possible, but I have not been able to pin down any first-hand accounts of same. I do not consider this is likely.
>> 4.5cm is suspiciously close to 2 inches. It may be that the hailfall was sufficient, particularly with drifting on the undoubtedly high surface water levels, to accumulate to a depth of "2 inches". People, even now, don't talk to reporters in terms of cm; they use inches. I am inclined to think that the phrases used were " we had two-inches of hail here ", meaning that as well as the flooding, hail was noted to an appreciable depth - 1 inch doesn't sound much, 2 inches is better! However, after research, I have not been able to find anyone to repeat such a statement.
Based upon the reports below, and consideration of the upper air ascent, I suggest that hail with diameter 10 to 15mm did fall widely along the path, with local falls of 20mm diameter stones. In places, considerable amounts of hail accumulated, which took several hours to disappear. There is a 20% chance (1 in 5) from the limited reports that hail diameters up to 25mm occurred, but not widely. I would be unhappy, given the almost non-existent damage reported to go any higher, and the hail of this size would almost certainly be 'slushy' or 'spongy' hail - high liquid water content coating with temperatures around zero: using the TORRO hail categories I assess this event as:-
H2, size code 3 (isolated 4)
Following a newspaper letter asking for information about this event, I have
had the following replies:
1. Crown Wood (CW)(time 4 to 4.30): "huge hailstones .. smaller
than a ping-pong ball .. but otherwise didn't / couldn't relate size to
'ordinary' objects .. didn't go out .. "lumps of ice" .. very short
fall .. no damage .. accompanied by 'torrential' rain". (My comment: must
have been significant as she made a definite diary note. If they didn't cause
damage, then suggests 'spongy' hail - aggregates?)
2. Crown Wood (CW)shops (time not given): " Leaving shops to return
to Martins Heron (MH)(i.e. travelling east) .. 200 yds from shops, hail started
suddenly then rain (diameter of hail became smaller with further travel east)
.. mothball size (!) .. (when pressed, gave size 3cm, definitely not 4.5cm) ..
thought damage would be done to car (but none occurred) .. 'frightening' ..
when returned home (MH), damage to greenhouse / single pane of glass broken
". (My comment: not exactly sure about the 3cm - mothballs are about the
size of a small marble, say circa 1.5cm, which would tend to fit with the
report at 1.)
3. Great Hollands (GH) (Yardley): (time not given): "hail driving
against windows (not as here in TF) .. sound of hail so loud couldn't hear
television (but she is 82 years old) .. just like a snowstorm ..
"frightening" .. lay on ground for several hours
(unlike ours in TF which disappeared within minutes) .. no damage noted .. but
didn't go out."
4. I note from the August COL summary [5th](seen by me 20th September) the
following: ... " 3 cm diameter hail was observed at Elkstone
(Gloucestershire). " This is obviously a long way away from us here, but
was part of the same complex - pushing us towards something of the order ~2cm
at least as a potential. Hail was also from Bracknell (Harmanswater) but no
details give except to note that it was the first recording of hail in August
since local records began in 1971. (Also noted that at Alton, Hampshire:
"Large hail, 2-3cm diameters, reported" BUT NO DATE!!!!)
5. Warfield (Priory Lane) (PL): (time not given): " Golf ball would
be TOO LARGE .. in discussion with husband suggest ~2cm diameter .. 'noisy' ..
white .. bouncing on impact .. no damage .. NOT lingering after fall (in
contrast with 3. above) .. suggests rain and/or high temperatures worked well
on what I am now thinking was 'slushy' or 'spongy' hail .. this is the first
report from north of Bracknell (still within my estimate of cell swathe
though).
