Winter 2008-9 part 2 - The Atlantic strikes back!


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The second half of January 2009 was markedly stormy compared to the first, featuring the Atlantic in all its wrath. The weather "peaked" on January 17th, with a sub-950mb low  situated just to the NW of the UK. This storm brought winds gusting to 94mph (at Capel Curig) and 70-80mph quite widely around the Welsh coast. Belmullet in Co. Mayo topped out with 108mph! The storm struck Wales in the early evening, with the most intense winds lasting just a few hours, but followed by squally thunderstorms that persisted through the night - I enjoyed a lightning-show at about 3 o'clock on Sunday morning, which made a nice change. I normally sleep through such things, only to find out about them the next day - "did you see the lightning last night? It was amazing" etc!

This page deals with the storm and the weather that followed. It also features me getting to grips with a digital SLR - a Nikon D300, which is a steep but enjoyable learning-curve because it gets me out there taking as many photos as possible as I get familiar with the squillions of options on the thing. I reckon there is a subtle improvement in the images as you go down this page - they are in order of taking - as I gradually figured out how to set the thing to aperture-priority (as I always shot with the Canon A1s) and get the ISO locked onto 200. The lens is a Nikkor 18-200mm (27-300mm film equivalent) which covers almost everything - good to avoid too many lens-changes in adverse conditions - but Mifsuds in Devon found me a nice used Tokina 12-24 mm ultrawide for really big skies. What is impressive about these modern lenses is the vibration reduction technology they deploy - it goes a long way to eliminate camera-shake which in turn means a bit less messing about with tripods. All good stuff in fast-moving situations......

Storm damaged telegraph poles

The image above is a tiny crop from a much bigger shot, taken on Sunday 18th following the storm. Machynlleth fared reasonably well but towards the coast, more damage was evident, in this case well on the way to being fixed. A friend at nearby Llancynfelyn had a large branch land on her woodshed and car (amazingly not doing TOO much damage) and down towards Ynyslas, houses were missing slates and ridge-tiles. Luckily the tide was a small neap, so that coastal damage was minimal!

Aberystwyth Stone Jetty

Down at Aberystwyth, the sea was still in an angry mood....

Aberystwyth Stone Jetty opportunities for playing around with different zoom lengths! I was glad to have a UV filter covering the lens, as in between shots a lot of salt had to be removed!

Slush on Machynlleth-Llanidloes mountain road

Although stormy, the weather was not especially mild. This Westerly-dominated setup is known as "cold (or cool) zonality" i.e. the weather's coming in off the Atlantic, but the air is not from the warm sub-tropics but from colder northern parts. This means lots of showers, especially in coastal areas and over adjacent hills, with hail falling widely and snow over higher ground. Here, the top of the Machynlleth-Llanidloes mountain road is decidedly slushy and either side there is a good coating of snow....

Aberdyfi Jetty

Tuesday January 20th brought a promising outlook of clear skies and local convective storms, enough to warrant an afternoon trip to the coast. Arriving at Aberdyfi a large thunderhead dominated the skyline. I pushed up the coast a bit but it fell apart before making landfall so I popped up onto the hill above Aberdyfi to mess about with the telephoto end of the zoom....

Aberleri from above Aberdyfi

I'll revisit this spot when the light's a bit better - an afternoon high tide might make for an effective shot. Still, the original when viewed at full size has a great deal of sharp detail!

In the meantime, something interesting on the SE horizon caught my eye....

Dyfi Estuary with cumulonimbus in background

Back at Aberdyfi beach, I could see a distant storm's anvil.....

Cumulonimbus cloud with overshooting top

Telephoto shot. Classic overshooting top - the "bulge" of cloud rising from the middle of the flat top of the anvil. Thunderstorm anvils form at the Tropopause - the boundary between the Troposphere (where virtually all weather goes on) and the Stratosphere above. Here, there is a monster temperature inversion - it gets warmer into the Stratosphere, so the buoyant warm air rising through the cold Troposphere meets its match at the Tropopause, loses its buoyancy and spreads out beneath it, to make the familiar anvil-shape. Overshooting tops occur when particularly powerful updraughts push through the Tropopause by dint of their momentum, although the air cannot convect any further upwards. Imagine a fountain - this is simply a cloud version!

The diagram below, in which the thick black line on the RHS plots temperature change with height (temperature scale along bottom), has the Tropopause marked by the red arrow:

Valentia ascent 20th Jan 2009 1200

Valentia, in S Ireland, is "upstream" of mid-Wales in a SW wind, so this atmospheric sounding, taken from the ascent of a weather-balloon two hours before the storm was photographed, gives a good picture of the atmosphere over Wales at the time. On the left-hand side we have air pressure (L) and altitude (R). We can thus see that the Tropopause was, at this time, situated at about 8300m (Everest is 8848m high), where the air pressure was just 330mb. The Tropopause is
always a long way up but the exact height varies from day to day. Let's just go back to the photo:

Annotated thundercloud showing tropopause

(note the Tropopause is in reality flat at any one point - the apparent slope is just perspective)

So basically we are looking at a cloud-top at almost the height of Everest. How far away was it? About 80km away. It went over a friend's place in the western Brecon Beacons giving thunder and 10mm hailstones. This is where weather-forums like UK Weatherworld come in handy - it was easy to check this out and a look at the rainfall radar correlated the info. Quite a significant thunderstorm for the time of year!

Gnarled Hawthorn trees

During the same day I was drawn to these trees, stark against the white sky (a high cloud-veil was moving in from the west now).....

That was to presage two low pressure systems midweek that brought lots of heavy rain, so on Thursday 22nd, with a 2-hour clear weather-window identified using satellite images, I headed for the Elan Valley to intercept blue skies and water cascading over the dams (thanks to Howard Kirby for the shout on this!).....

Caban Coch Dam

I was not to be disappointed - here's Caban Coch dam....

Caban Coch Dam

....impersonating Niagara!

Caban Coch Dam

View from above, looking down, as it were....

Moving on to Claerwen, a huge reservoir up a side valley:

Claerwen Dam

Claerwen Dam

On up the Elan Valley, I rounded a bend and found this huge Western Hemlock that had blown over. Almost certainly a victim of the previous Saturday's windstorm (the rootball was unweathered and there have been no other severe storms recently), this thing was huge! My jeep for scale!

Fallen Western Hemlock, Elan Valley

Up on the moorlands above the reservoirs, moor-grass adorned fences where the winds had torn it from the hillside.....

Moor-grass caught in fence

Sunday 25th saw another chance of some good convection early in the morning, so I arrived at Borth not long after sunrise:

Cumulonimbus at Borth

The distant bank of cumulonimbus clouds were heading this way.....

Surf at Borth

Telephoto of the powerful surf...

Cumulonimbus at Borth

As the clouds neared, precipitation shafts appeared - snow at that height, lit up by the low-angle sunlight....

Cumulonimbus at Borth

First shot with the Tokina ultra-wide as the decaying showers came over, giving just a little rain in their death-throes....

Kelvin-helmholtz Waves

Behind, another line of cells was moving in but light conditions had deteriorated and the winds backed southerly, steering the cells up through the Irish Sea. Before leaving, I took this unattractive but interesting shot showing a Kelvin-Helmholtz wave-cloud on the upper LHS of the anvil that dominates the view here.
All in all a productive spell of weather and an enjoyable experience in learning a new skill, a learning curve that will last for quite some time, I suspect!

Looks as though things will settle down now, with colder dryer conditions on the cards as we head into February. Boy, am I looking forward to Spring!


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