AUTUMN
2005 - PART 2: First snow - November 25th:
Coast to Coast on the Cb-conveyor!
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Well,
winter's now upon us (December 10th) and the
weather, with its own apparent sense of irony,
has turned mild, grey, damp, anticyclonic and
tedious. Perhaps this is a reaction to the media
hype about the severest winter since 1947, 1963
or whatever other date springs to the
copywriter's mind....
The thing is, winters of late have been above
average in terms of temperatures, so that an
"average" winter now would feel cold to
many people and therefore demand for heating
energy would go up sharply. The reality is that
it's sensible with EVERY oncoming winter to be
prepared for possible cold weather, even if at
the end of the day it turns out to be a mild damp
squib!
Here I had the firewood stash nicely built up in
time. In fact, there are now a few dents in it as
a consequence of the late November-early December
cold snap, which saw snow falling to sea-level on
Friday 25th.
The heaviest snow of the 25th was localised with
westernmost parts of Wales and Cornwall seeing
significant problems, inland Wales seeing some
problems and some other areas hardly seeing a
flake. Why?
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Well basically it's down to a couple of things.
Firstly, at this time of year, if low pressure
moves into the North Sea it can drag down a cold
polar airflow. This manifests itself as a
northwesterly round to northeasterly wind. Its
orientation is critical.
Unstable polar air flowing south over our
relatively warm seas leads to convection
occurring, as is well-known and illustrated
elsewhere on this site for example. But, and it's
a big but, the length of the track of this air
over the sea is critical. Since temperatures over
the land in cold weather are not supportive of
convection, showers only tend to brew up over the
sea. The ones that move in overland are mature or
decaying cells, unless a strong extra forcing
mechanism such as a low-pressure trough is
present.
Between Scotland and Northern Ireland is a narrow
sea-channel - the North Channel, crossed by the
Stranraer-Larne ferry. A NNW wind will transport
cold air over the warm sea, picking up evaporated
moisture as it goes, all the way down from
Western Scotland, through the North Channel, past
the Isle of Man, though outer Cardigan Bay and
across the outermost parts of the Bristol
Channel, with hardly any encounters with land -
the NE coast of Northern Ireland, the Lleyn
Peninsula, Pembrokeshire and Cornwall being the
only obstacles to its convective progress. None
of these areas are extensive enough to kill off
the convective process. Therefore, what we end up
with is a continuous narrow line of vigorous
convection through the area. Because of the role
of the North Channel in supporting such a weather
setup, this is known, appropriately enough, as
the North Channel Effect.
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Here is the Net-weather subscription radar for
0730 on the 25th November. Convective showers are
initiating just south of the North Channel and
moving down the narrow line - note how the line
is not dissimilar in width to the channel itself.
They are becoming heavy to the south of the Isle
of Man, and then as they move south one after
another they are clipping the tip of the Lleyn
Peninsula, affecting Pembrokeshire and running
into SW England where they are losing their
intensity. Thus, a long narrow conveyor-belt of
cumulonimbus clouds is present - also known as a
shower-train. The showers are moving along it,
but it (the conveyor, that is) hardly changed its
position all day.
Note also a second train of showers is developing
SE of the Isle of Man and running into the N
Wales coast. This air, however, has had a much
shorter run over the sea and thus the showers are
less intense and shorter-lived. The distance
travelled by the air over the sea is critical.
Incidentally, a NE wind would NOT support the
North Channel Effect and all we would see in that
instance would be a few snow-flurries forming
over Liverpool Bay and drifting onto the North
Wales coast.
Another important point for the aspiring weather
photographer is that polar airflows are
beautifully clear and crisp - which makes the job
an awful lot easier!
So, onto the fun stuff. I had an early start on
the 25th and well before it started to get light
I had identified the convective line on
radar/infrared satellite imagery. This was worth
a look I decided, what with cumulonimbus clouds
to the west and an impending sunrise, so I set
off down to the coast at Borth through light
snow-showers: no more than 1-3cm had fallen
overnight and the roads were fine so long as
steady progress was made. All the way down to
Borth I could make out the convective conveyor
and anticipation of something good started to
grow and grow!
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The view upon arrival, with the rising sun
clipping the tops of the cumulonimbus clouds, and
a sprinkling of snow on the shingle at the head
of the beach. The conveyor stretched across
Cardigan Bay from coast to coast - from Bardsey
and the SW Lleyn Peninsula in the N to Strumble
Head and Pembrokeshire in the S....
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Looking back inland to a cold dawn and remnant
snow-showers over the Cambrian Mountains...
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This is looking west a while later (some of these
are taken with a 28mm lens, hence the slight
curvature of the horizon).....
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Meanwhile, light levels were increasing and I was
distracted by the second inland line of
snow-showers referred to above. You can see that
they are weaker affairs (no North Channel
Effect!) but are still producing small and quite
photogenic cumulonimbus cells...
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...perfectly aligned into a line again, but the
foreground needed to be better...
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...that's
better! Borth's wet sand comes into its own once
again!
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Back to the main Cb-conveyor anyway! At this
point some particularly powerful cells started to
develop upstream: these would pass me within
about 15-30 minutes so it was telephoto-time....
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What struck
me from my vantage point was how solid-looking
these cells were - like glaciated walls of ice on
this side - the eastern side of the North Channel
Effect's sphere of influence - with clear air
between them and myself....
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The conveyor's eastern side was some 15 miles to
my west, so that the snow and hail could be seen
as distant precipitation-shafts, and from time to
time C-G lightning stabbed down into the sea.
This is, therefore, an outbreak of thundersnow,
as it's popularly called. Notably, later on the
same day, four people were struck by lightning
from a cell on the same conveyor. Their
misfortune occurred in the Prescelli Hills of
Pembrokeshire, an area in which the snow caused
considerable traffic problems - the BBC reported
that "Dyfed-Powys Police advised drivers in
Carmarthenshire and parts of Pembrokeshire to
travel only if their journey was essential on
Saturday." These drivers were right under
the conveyor so they got hit by every snow-shower
that ran along it over a period of more than 24
hours....
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This cell looked particularly magnificent and was
indeed the best of the trip....
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The incredibly solid anvil reminded me of an
Independence Day spaceship!
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All of the time new convection was activating in
this boiling, busy mass of thunderclouds...
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However, by 0900, as the light levels increased,
so the sharp definition within the clouds
decreased a little: I was now a) cold and b)
hungry, so happy with my "haul" I
headed home for a seriously large breakfast,
leaving the empty beach to the gulls and the
oystercatchers.
In its various guises, this conveyor persisted
all through the following weekend as the
northerly blast continued, although as
temperatures crept back up it began to give hail
and sleet showers instead of snow...
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...with a final shot from this remarkable spell
of weather, taken on the Dyfi Estuary at sunset
on Monday 28th November - still with the odd
cumulonimbus popping up on the conveyor!
Cumulonimbus clouds make great subjects for
photography, and this was one of the most
satisfying "shoots" I have had with
them! Now, I wonder what the rest of the Winter
will bring???
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