Cumulus at Twilight

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As the day draws to a close, convection usually begins to die down. The sun is no longer heating the surface very much and the rising thermals of air that produce cumulus clouds start to die away. The clouds fragment and evaporate, often leading to clear skies during the night.It is during the evening, perhaps, that the most beautiful cumulus skyscapes occur. Often the bases of the clouds are now hidden from the sun and appear particularly dark, in contrast with the cloud tops which are still sunlit.

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Evening cumulus over the North Sea

These photographs show typical evening skies on the east coast, after a showery day in summer, in a cool polar maritime airstream.

The first photograph show Cumulus congestus and mediocris over the North Sea at Whitburn, Tyne and Wear, and the second, just Cumulus mediocris

As the temperature falls convection begins to die. In the first photograph, some large cumulus congestus still survive, with ragged pannus clouds beneath, still suggesting the potential for a shower.

The second photograph was taken only ten minutes later, by which time convection had declined, and was only just enough to support much smaller cumulus mediocris clouds, giving the sky a much flatter and benign appearance.

As the evening progressed, all the cloud evaporated giving a clear start to the night.

Evening cumulus at Wolstanton

Evening cumulus breaking up over Wolstanton

This photograph was taken in June 2008 during a showery spell, again in cool polar maritime air. Only the remnants of earlier cumulus clouds survived by late evening, forming Cumulus fractus.

Stratocumulus at dawn in Snowdonia

The clouds below were photographed at dawn during an ascent of Pen-yr-Ole-Wen in North Wales. They were being carried from the southwest in a returning polar maritime air stream forced to rise over the mountains. The presence of a much higher broken layer of altocumulus stratiformis is perhaps responsible for reducing the intensity of the light and producing rather more subdued colours thans those normally associated with sunset.

In fact, the high cloud was part of the leading edge of a weak occluded front that went on to produce dank drizzly weather for much of the rest of the day.

The lake in the first photograph is Llyn Ogwen and the mountain in the second is Tryfan, arguably the rockiest mountain in Wales.

Stratocumulus at twilight

Lindisfarne sunsets

Stratocumulus cloud sheets are possibly one of the most uninteresting cloudscapes to look at. We normally associate them with dismal cloudy days perhaps with the odd outbreak of light rain or drizzle.

When they begin to break up, and at sunset, in particular, they can present a beautiful scene, especially where there is some reflection from a water surface.

Stratocumulus at sunset over the 'Slakes' and the 'Harbour', Lindisfarne, Northumberland

Algonquin sunsets

It is often difficult to decide whether clouds belong to one group or another. This especially true of stratus and stratocumulus. It the clouds are really flat and layered then clearly they are stratus, but what if there is evidence of slight cumuliform growth on parts of their upper surfaces?

The photographs below illustrate the problem. The clouds just above and to the right of the setting sun seem to be clearly stratocumulus, but only the lower surfaces of the clouds in the foreground can be seen making them very difficult to classify.

In the photograph on the right, which was taken after the sun had set, the clouds are more clearly of the stratus type. Any rising columns of warm air have now dissipated as the evening has cooled down, removing the source of cumulus growth.

Stratus clouds over the Algonquin National Park, Ontario, Canada, Early August 2007. The first photograph show Stratus undulatus.

Stratus at twilight

London sunsets

Stratus, perhaps the most uninteresting cloud formation during the normal daylight hours can look quite spectacultar when underlit at sunset. The next four images are were taken at South Woodford, London, during November 2007.

The first picture shows a degree of low level turbulence and a cloud formation that approaches the newly defined Stratus asperitas

Underlit stratus clouds, London, early November, 2007