Cumulus at Twilight

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.

Evening cumulus over the North Sea

These clouds represent the remains of much larger cumulus earlier in the afternoon when heavy showers took place. As the evening approached and temperatures fell, the clouds began to collapse, eventually producing a clear night.

The remains of larger cumulus congestus can be seen in the second photograph

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

Cumulus mediocris over the North Sea at Whitburn, Tyne and Wear

Cumulus congestus and mediocris over the North Sea at Whitburn, Tyne and Wear

Evening cumulus breaking up over Wolstanton

Evening cumulus at 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.

Cumulus at dawn in Snowdonia

The clouds below were photographed at dawn during an ascent of Pen-yr-Ole-Wen in North Wales. They are cumulis stratiformis and were being carried from the southwest in a returning polar maritime air stream. 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.

Cumulus stratiformis over Llyn Ogwen at dawn on a late October morning

Cumulus stratiformis over Tryfan at dawn on a late October morning

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 Lindisfarne Slakes, Northumberland

Stratocumulus over 'the harbour' at Lindisfarne

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.

Stratocumulus at sunset a lake in the Algonquin National Park, Ontario, Canada

Stratus clouds over the Algonquin National Park, Ontario, Canada, Early August 2007

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.

Stratus during mid-autumn 2007, appearing to have far more vertical relief than one might expect!

Underlit stratus clouds, London, Autumn 2007

Underlit stratus clouds, London, early November, 2007

A further example from early November, 2007