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.
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 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.
The clouds below were photographed at dawn during an ascent of Pen-yr-Ole-Wen
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 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
It is often difficult to decide whether clouds belong to one group or another.
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, perhaps the most uninteresting cloud formation during the normal
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