Cb

When I was a kid (yes, it’s one of those posts – please don’t interrupt.)

When I was a kid, living on RAF bases, I used to listen to the British Forces Broadcasting Service, (BFBS) in the mornings. One thing I remember from those days was the daily early morning weather report. BFBS did the equivalent of the shipping forecast for airmen. There would be a detailed weather forecast along with cloud types and heights to give the flight crews an idea of what to expect that day. I remember the strange sounding names, Cumulus, Stratus and Cirrus and their variations, and the figures that gave cloud cover and cloud base height.

Just over two years ago, Rufus and I got caught in a thunder storm while I was training for a trek. Ever since, I’ve taken an interest in weather prediction and in particular the early warning signs of thunder storms. We had a heavy storm here yesterday, with a lot of lightning and very heavy rain preceded by hailstones. It was well predicted and before the weather changed, I decided to read up on the cloud types. I wanted to try to identify them as they built up and so see first hand the early stages of a thunder storm.

Classic thunder clouds are generally Cumulonimbus clouds, (abbreviated to Cb). They are instantly recognisable as massive and billowing. They can form quite quickly, within 20 minutes sometimes, by warm air rising within the cloud and drawing cooler air in from below. The billowing part is sharply defined while it is formed of water droplets, although this sharpness may fade as the water freezes at higher altitudes. There will almost certainly be rain beneath this cloud, and more often than not hailstones and lightning.

I watched these kinds of clouds forming to the north of the house yesterday. They were so massive and high that it was hard to judge how far away they were. A quick check on the weather radar ‘app’ I have showed they were about 10 miles north, and they were indeed producing lightning. Later that night, the clouds formed over the house and we had our own storm.

This morning was bright and clear of cloud and I decided an early start was in order. There was still some humidity in the air and although the forecast said no clouds or rain for us, there was a lot of lightning activity in Europe and we often get their weather. So I read a little more from the cloud book and found out that there are a couple of early warning cloud species to keep an eye out for.

Altocumulus Floccus (small tufts of clouds) indicate humidity and unstable conditions at high altitude. These conditions can feed and energise cumulonimbus clouds, an already energetic cloud system. They can indicate a coming storm. Altostratus Castellatus clouds also reveal instability at higher altitudes but the clouds are more dense and usually result from more energetic conditions. Again, these clouds herald a coming storm (or at least the conditions necessary for one to form).

Armed with that information, Rufus and I headed north to Mynydd y Gwair. Yesterday, this seemed to be lightning central according to the website I’d been watching, with several dozen strikes recording in the area. I almost expected to see smoking craters but there were none – I guess that only happens in movies. The sky was clear and the morning was warm as we set off over the moorland north of the Upper Lliw reservoir. Sheep parted before us as we squelched through the surface water. Here at least was evidence of last night’s storm.

At the little river that feeds the reservoir, Rufus jumped in and paddled upstream while I walked the bank looking for little waterfalls to photograph. I’d forgotten about checking the weather until I noticed the sun had disappeared. I looked up and saw a few puffy clouds dense enough to obscure the sun. Nothing to worry about according to my new found knowledge, so I went back to setting the tripod up. I was using a very dense filter so exposure times were in the order of a minute or so. The next time I looked up into the sky I saw some familiar clouds; Altocumula Floccus.

I decided to move out of the river valley as it was hiding the horizon and most of the sky. I wanted to see how widespread the clouds were and what was coming up. I moved downstream and saw that it was a very isolated patch of cloud which was clearing to the west. So I went back to photographing waterfalls again. Rufus, uncaring of the cloud types, splashed and paddled and bobbed his way downstream. We played in the water and I threw stones for him to catch and dredge. In a deep part of the river, I threw dead bracken stems for him to swim after.

I looked up again and saw more Floccus. But now, to the south, a larger bank of cloud was forming beyond the reservoir. It had the appearance of an early thunder cloud and I decided, given the conditions, that we start heading back to the car. Out of the valley, there was a breeze blowing towards the reservoir. One of the signs of Cumulonimbus is that as the warm air rises within it, it drags the surrounding air towards it, causing a breeze. It often leads to people thinking the cloud is moving against the prevailing wind. A wind in the direction of the cloud is a warning sign.

The breeze also made the walk back pleasant and Rufus ranged far and wide, unconcerned about any coming storm. And after a few minutes, although the cloud was growing, I wasn’t so concerned either. By the time we’d reached the car, the cloud had grown but hadn’t moved and rather than jumping in and driving off, I left most of the kit in the boot and we walked off onto a man made bank on the opposite side of the moor. We spent another 10 minutes or so exploring the surroundings before finally making our way home. Ahead, over Morriston, the clouds were thick and dark but as we neared home, they broke up and as I write this, the sky is full of larger Cumulus clouds (‘fair-weather clouds’), normal for the time of day and year.

Which means I have no excuses for not finishing off the lawn, tidying up the boarders and cutting down a couple of dead bushes.

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