|Spring 2010 part 2: Cold:
the North Atlantic yin-yang!
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mid-May and not an awful lot to report on the weather front, apart from
the very dry April and the prolonged cool to cold conditions that have
continued right up to the present, of which I have attempted an
explanation further down this page.
The snowy late winter theme continued right up to Easter this year,
with snow-showers adding to the already present cover. I caught one of
the showers running across the Dyfi Estuary at Glandyfi one late March
afternoon, with the Tarrennau ridge in the background and the railway
bridge in the foreground, making for a nice image:
Later the same
afternoon I was in Borth and the skies were clearing so I went to the
upper part of the village to grab a couple of shots, showing the
almost-deserted beach despite the sunshine, and the wintry scene beyond:
Trum-Gelli, the Estuary and Borth: I waited
some time for the sunlight to make an appearance on the snowfields!
Easter Saturday, this was the view from the top of the
Machynlleth-Llanidloes mountain road with Plynlimon and Glaslyn and the
winding track into the wilderness. This is Plynlimon's best aspect by
April normally brings in the fishing season
on the local beaches, but this year the chilly weather has meant a very
slow start, with conditions feeling more like an endurance-test at
This was the only worthwhile fish caught
all month! Turbot of eating size are not abundant on our coast, but
most Aprils see a few. I guess the cold weather has kept the sandeels
out in deeper water: these little fish bring in the predators, but if
they stay away when cold nights chill the sand over low tide, then so
will those fish that hunt them....
On May Day
Monday, I went up to the rock-ledges on Bardsey Sound to see if any
mackerel had arrived yet - in warmer years I've caught them in April.
Not this year - just a few small pollack were caught from this normally
productive tidal rip...
Plenty of wildlife to watch though, as
Had I grabbed
the camera immediately I could have had some lovely shots as 30+
Bottlenosed Dolphins came through close to low tide in three pods - I
was close to the water reeling in my lure while the camera was stashed
in a niche 30ft above me. By the time I'd got up there, the third pod
was getting further away. Lesson learned for next time!
So, into May
and we have just seen a week of cold to cool days and very chilly
nights with ground-frosts reported in quite a few parts of the UK.
What's going on?
It's all down to the pressure-patterns at the moment (and not the
Iceland eruption - nothing's occurred on a big enough scale so far,
thankfully!). The map below
shows a typical Atlantic pattern, with low pressure over the North
Atlantic (red "L"s) and the Azores High sending a high pressure ridge
up across continental Europe (the red "H"s). Now, because the winds
around a high pressure anticyclone blow clockwise, high pressure to our
south and south-west gives us sou-westerly winds, bringing up air from
the tropical Atlantic - mild and moist in other words (red arrows), or
if the high pressure over Europe expands up over the UK, warm and dry.
In most years, this pattern dominates our weather.
However: the map below shows what has been going on this Spring. High
pressure ridging south from Greenland has created a persistent "block"
to the usual pattern, allowing warm air to push north into western
Greenland but also allowing cold air to flood south over us from the
depths of the Arctic, and in
place of the Azores High there are small areas of low pressure that
work their way up into the Mediterranean area, giving heavy thundery
downpours in that region.
So how does all that show up on weather-charts? Here is an air
thickness chart for the N Atlantic and Europe for May 10th:
chart shows what looks a bit like a yin-yang symbol, with low pressure
over E Canada and high pressure over Greenland and the N Atlantic then
low pressure again over Scandanavia. Mild 546+ DAM air (green line, red
arrows) is thus pushing
up around the W flank of the Greenland High, whilst chilly sub-528 DAM
air (pale blue line & arrows) is coming out of the Arctic and
straight down the Greenland High's
eastern flank, pushing down into the UK. The result? Western Greenland
gets mild whilst we shiver! So what does "thickness" mean? I'll try to
Thickness is a way of illustrating how warm or cold the air is in the
lower part of the atmosphere. The chart is plotted using the thickness
- or depth - between the 1000 millibar pressure level - i.e. close to
sea level, and the 500 millibar pressure level. The height of the 500
mb level above the surface varies depending upon how warm or cold the
air: in a bitterly cold polar airmass over Greenland, the 500 mb level
may be as low as 5000m, whereas over hot equatorial areas it might be
as high as 5800m. Hot air expands; cold air contracts - creating these
differing values. The resulting charts map out the airmasses by lines
(called isopleths) joining places with equal thickness values.
Thickness is measured in tens of metres - or decametres, abbreviated to
DAM. Traditionally, a number of key isopleth lines are plotted: 510 DAM
(plotted dark blue - none present on the above chart), 528 DAM (light
blue on the chart), 546 DAM (mid-green on the chart), 564 DAM
(yellow-green on the chart) and 582 DAM (yellow-orange, SE corner of
So how do these values translate into reality on the ground?
Sub-510 DAM air is rarely present over the UK in a harsh winter and
brings extreme cold. 510-528 DAM air is intermittently present over the
UK in all but the milder winters. Its presence on forecasts charts
hints at an airmass cold enough to give snowfall, although it should
not be used as a detailed snow forecasting tool. 528-546 DAM and
546-564 DAM airmasses are typically found over the UK all year round,
with an emphasis on 528-546 in winter and 546-564 in summer. Airmasses
thicker than 564 DAM air indicate a heatwave and are restricted to the
summer months. 582 plus DAM air is found only in the hottest areas of
Such blocked pressure patterns, once established, can be slow to shift,
as we have witnessed during this often chilly Spring. However, this
coming week, there are signs of a more typical pattern returning, which
will relieve a lot of gardeners!
The Tywyn Sea Defences project is coming towards completion. On a day
with rather poor, glaring light I happened to be in the town and
grabbed a few images:
These show the main Breakwater being completed. These huge boulders are
carefully slotted into position using the grab, the result being a
tightly-spaced rock armour exterior....
shot - despite the glare - shows the structure almost in full. I'll
post a better image when I get one...
cold and dry, non-gardener-friendly April, the veg garden is coming
along with shallots, onions and weeds all showing in good numbers!
point: the one broccoli plant (below) with a straight stem (as
seedlings they were mostly comprehensively trashed by a cat who
regarded their seed tray as a litter-tray) has grown to a mighty height
with huge leaves - but little else. I've hardly had a thing off it.
Compare it to the next image:
One of the
scrawny little ones, three sharp bends in its stem where it was
trampled as a seedling - and it has been firing these shoots out for
weeks now! Only a guess - it this productivity a natural response to
where the soil is very thin, I have left to see what comes up
naturally: lots of foxgloves which will be a great attractor for the
bees, plus some wild garlic and various other herbs...
But this is
what it's all about!
garden I have started on, a bit too late for this year really but in
these two beds I have planted seed potatoes. The deep couch-grass is
going to be difficult to keep at bay, but the soil's good underneath
and the turves make useful bed-boundary walls.
And that was Spring 2010! Slow, cold and often dry, but always with the
feeling that warmer times are getting closer and closer, and hopefully
too some decent storm-clouds to get to grips with - I can't remember
the exact date of the last decent thunderstorm here, it's been that
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