91Èȱ¬

Bob Faraway from Eastington - not far off the M5 near Stroud, Gloucestershire -Ìýhas e-mailed me a photograph of clouds he watched passing above his home on Monday morning.

"Please can you advise whatÌýmight cause theseÌýcloud formations," Bob asks, "as they look unusual to me?"

Well, they're not actually unusual per se. Bob hasÌýphotographed the higher levels of cloud building-up well ahead of the frontal system that ultimately broughtÌýyesterday's rain.

The cloud itself is predominantlyÌý, forming at mid-levels in the atmosphere and signalling the eventual arrival ofÌýthe occluded front discussed in Tuesday's blog.Ìý ButÌýthis featureÌýwas still a long wayÌýwest of Eastington when Bob snapped the shot.

The elongate clear slot visibleÌýinÌýhis skyscapeÌýis likely to signify where drier air is descending; therebyÌýdisrupting the otherwise extensive cloud cover. All manner of changes to atmospheric pressure, temperature, humidity and winds aloftÌýcan create sudden gaps in large areas of cloud. Quite often,Ìýthey're created by the wave-likeÌýdisruption of airflow passing across hills, mountainsÌýand other terrain (a type of standing atmospheric wave ).Ìý

With each increasing hour after Bob tookÌýhis photo, the cloud cover above Eastington would have steadilyÌýincreased, lowered and thickened.Ìý

ThroughÌýFriday into Saturday, we will see a similar story emerging in the skies above the West Country, as another Atlantic depression sweeps eastwards towards the British Isles, heralding further wet and rather windy weather -Ìýespecially on Saturday itself.