WINTER
2007-8 part 3: January squalls and lightning superbolt
strikes Machynlleth!
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The first few weeks of 2008 have
seen predominantly rough, wet and often mild conditions -
the night of the 19th-20th January was the mildest on
record for January with several locations in the SE of
England seeing minima of 13oC.
The pattern so far this month has been "Zonal"
- meaning everything is coming at us from the Atlantic as
low after low develops beneath the influence of roaring
upper-level jetstreaks. Some interesting weather was in
amongst this lot: thunderstorms - including one that gave
a lot of positively-charged "superbolts", one
of which hit Machynlleth; some powerful squall-lines,
severe gales and hail. Not much snow or frost in other
words. I'll deal with the more interesting events
chronologically, starting with January 8th.....
This is a typical squall associated with a heavy
shower. We've all been caught in these from time
to time. But what can happen if such things get a
bit more organised? The evening of January 8th
was a classic example....
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An active cold front crossed the UK on that
evening. Such fronts can push under the warm
moist air ahead of them and lift it to a height
where it becomes highly unstable. Because fronts
are linear features, the resultant convection
occurs in a long thin line within which severe
wind-gusts, tornadoes, hail, thunder and
torrential rain can all occur. For fairly logical
reasons, such a feature is termed a squall-line.
This was the view from my house in Machynlleth at
about 7pm on the 8th. There was a sudden and huge
increase in wind and lashing rain and hail. This
lasted a few minutes and then all returned to
quiet and steady rain - a classic squall-line in
other words!
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Now, here's the Netweather Radar plot from a
little earlier. Heaviest rain is red, lightest is
blue. The squall-line can easily be seen as a
thin band of torrential rain. Machynlleth is the
red circle - it had yet to be hit.
The arrowed area is of interest. Look at the way
this section of the line has got ahead of the
rest, forming a bow-shape. Such bowing segments,
or bow-echoes, are focal points for severe
weather along squall-lines. In a more evolved
form, a whole series of these bows may develop,
giving an impression that a whole lot of waves
are running up the squall-line, often with breaks
to the line in between them - a feature referred
to by meteorologists as LEWPs - Line Echo Wave
Patterns in full.
Complex vorticity patterns develop in amongst
such features, with a consequent enhanced risk of
tornado development, but it is often difficult to
analyse damage patterns because surging
bow-shaped segments also bring with them the
threat of downbursts or microbursts (depending on
scale). These are surging downdraughts of cold
air that hit the ground and fan out across it,
and they can have 100mph+ winds within them -
sufficient to do major damage, such as this:
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...between Aberdyfi and Tywyn and associated with
that bowing segment on the radar above. These are
old, wind-gnarled trees in an exposed position in
the teeth of the prevailing SW wind, yet they
have been torn clean out of the ground, to
slide/roll to their current position. I was sad
to see this, as I have always loved old wind-bent
trees - they seem to have such character. Anyway:
debris has been flung forwards and not sideways
as one would expect with a tornado situation.
Plus, this is isolated damage. It is almost
certainly evidence that there was a damaging
microburst here in association with the bowing
segment, with winds of over 100mph destroying
these hardy old trees
Thunder was also reported during the
squall-line's passage with a friend living near
Forge observing sparks showering down from a
utility pole after a particularly close strike!
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Something of a breather followed, during which I
caught this bright rainbow over the Afon Leri
outfall into the Dyfi Estuary, with Aberdyfi in
the background....
Sunday 13th saw severe gales forecast for
Cardigan Bay, with a mid-morning high Spring tide
and a large ground-swell forecast, so I headed
off to Aberystwyth, parked by the Stone Jetty and
focussed my telephoto lens on the Harbour area,
where waves were hitting the end of the Prom.
Here are my three favourites - they reminded me
of fireworks, somehow....
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January 17th saw the next potentially interesting
set of events, with conditions supportive of
severe thunderstorm development. Behind a frontal
system that had brought overnight rain, there was
a cool polar maritime airmass, unstable to the
warm sea temperatures. Strong wind-shear - winds
moving at different directions and higher speeds
higher up in the atmosphere - ensured that any
storms that formed had the potential to become
fairly nasty - unfortunately in appearance too,
as the photo above, from just N of Tywyn looking
towards Rhoslefain, shows! Not pretty!
I stayed on the southern edge of this storm for a
time, listening to the loud booming thunder,
before heading north after it cleared, along the
coast towards Llwyngwril, passing a favourite
vantage-point layby high above the sea - where
the roadside was white with hailstones...
These had been melting for about 20 minutes and
were up to a centimetre across: allowing for
melting I'd guess they were originally
marble-sized. The "birdseye" texture is
common in larger hail and indicates multiple
growth stages, as a stone falls and then gets
lifted again by the updraught. Larger stones can
make this up and down circuit many times, before
they finally overcome the updraught strength and
fall to earth.
I continued to Fairbourne, then turned and drove
back over Friog, passing through the core of the
next storm. This was an alarming experience as
sheets of hail battered the jeep and visibility
dropped to no more than 10m. One of the most
violent storm-cores I have driven in!
Parking up in Llwyngwril at about 11.40, I
assessed the situation. Out to sea, yet another
storm was bearing down on me. It was equally
unphotogenic. My decision to run ahead of it, as
the roads were rapidly getting flooded in places,
was confirmed when a bolt of lightning hit the
nearby sea, with a long roll of deafening booms
of thunder like a series of bombs going off!
This very loud thunder was a strong sign that the
strike was positively charged. Positive lightning
makes up less than 5% of all lightning strikes,
but these "superbolts" are typically
six to ten times more powerful than negative
bolts. In most cases, positive lightning strikes
come right down from the thundercloud's
positively-charged anvil-shaped top. For a number
of complex reasons, winter thunderstorms produce
positive lightning much more than summer storms.
In essence, key factors are:
a) that winter thunderstorms are
"low-topped" - that is, their anvils
are typically a fair bit closer to the ground
than summer storms, because the top of the
Troposphere (where anvils spread out) is lower
down - so that lightning may be able to travel
more easily from the anvil to ground;
b) that since a developing winter stormcloud
contains a lot more ice, due to lower prevailing
temperatures, and ascending ice particles carry a
lot of positive charge to the anvil, the upper
region of the stormcloud can carry the bulk of
the electrical charge.
Never underestimate winter thunderstorms in
Northern latitudes, in other words!!
I fled ahead of this storm via Dolgellau and
Dinas Mawddwy. On my return I had my suspicions
confirmed - a colleague told me that all the
strikes as these storms crossed the area were
positive - interestingly once these storms had
reached the Midlands, they were dominated by the
more typical negative strikes! Thanks to Paul
Knighley of TORRO for that info!
Here's the midday radar (precipitation rates,
mm/hr, on the R):
When I arrived in Machynlleth at about 1pm, power
was off over 2/3 of the town, and was not to be
restored for over another hour. I made enquiries
- easily enough, as the deafening thunder was the
talking-point of the town - and it transpired
that a strike had hit somewhere near the railway
station. A neighbour reported seeing a spark jump
across her room at the time.
Positive lightning is particularly dangerous.
Since it originates in the upper levels of a
storm, the amount of air it must jump across to
reach the ground is much greater. As a
consequence, its electric field is far stronger
than a negative strike. Its duration is longer,
and its peak current and voltage can sometimes be
as much as 300,000 amperes and one billion volts.
One consequence of this is that it can do a lot
more damage: another is the deafening thunder, as
heard over Machynlleth and Llwyngwril at the
times of Thursday's strikes. This truly was a
case of a "superbolt"!
So no sign of Winter just yet, although this
coming Thursday (24th) carries a risk of some
wintry showers, especially for N Wales. It'd be
nice to see something more seasonal. People are
getting fed up of this perpetual gloom and
flooding!
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