Cumulus clouds have a variety of forms and sizes, depending on the amount, strength and depth of convection:
Cirrostratus and Cumulus humilis at Aberffraw, Anglesey, Wales, July 2002
These clouds occur where there is only very slight uplift within a small vertical range; They occur on otherwise sunny days; In this case, above the cumulus clouds is a high level veil of cirrostratus (which gives the sky a milky appearance), and a mid level stratus cloud (top left). The cirrostratus here, forms the leading edge of a warm front.
The photograph shows the estuary of the Afon (river) Ffraw at Aberffraw
Cumulus mediocris stratiformis at Perranporth, Cornwall, April 2002
These occur when there is a little more uplift; They are not big enough to produce rain, but given time and continued growth they can develop into Cumulus congestus.
Each cloud lies within a 'column' of rising air; The clear air between the clouds is descending.
In the photograph of Perranporth beach, Cornwall, the tops of the cumulus are rather flattened, suggesting that they are having difficulty gaining further height; This often happens at at a temperature inversion.
Cumulus mediocris over Westport Lake, Burslem, Stoke-on-Trent, June 2000
The next photo, of Westport Lake, Stoke-on-Trent, was taken on a day of more vigorous cumulus growth; In this case the clouds show no evidence of flattening at the top and could almost be classified as congestus except that their vertical extent is not great enough; The picture was taken during the morning, and as the day went on much larger cumulus congestus did eventually develop, giving heavy showers.
On both occasions, the cumulus clouds were developing in cool polar maritime air, on a WNW wind, following the passage of a cold front; This air mass is usually unstable, especially in coastal areas during the winter half of the year, and in inland areas during the summer.
Throughout the year, however, polar maritime air is associated with good visibility and low levels of air pollution!
Flattened Cumulus medicris over the Ring of Brodgar, Mainland, Orkney, showing some evidence of cloud street formation
Cumulus clouds require convection before they can develop. This happens when pockets of air near the surface become warmer, expand and become lighter than the air around them. They then begin to rise in much the same manner as a hot air balloon. The height to which these pockets of air can rise determine the vertical extent of the cloud that is likely to be produced. With Cumulus congestus there is considerable scope for uplift so the clouds can look quite dramatic.
Flattened Cumulus mediocris over Scapa Flow, Orkney
Very often, however, the vertical extent of uplift is much smaller and may indeed be terminated by a temperature inversion, an altitude beyond which the temperature increases with height for a time. This leads to a flattening out of the tops of the cumulus clouds. Any rising pockets of warm air attempting to breach this layer, suddenly find that they are no longer warmer and lighter than the surrounding air, so they quickly sink and flatten out.
Cumulus mediocris in a northerly airstream over the North Sea, off the north coast of Lindisfarne, Northumberland
There is a very blurred margin between Cumulus mediocris and Stratocumulus clouds. Quite often we find discrete cumulus of medium vertical extent but with flatted tops. If the cloud sheet was continuous the cloud could clearly be described as stratocumulus, but often discrete clouds occur. Examples of this type are common place and are usually associated with dry weather and sunny intervals. Several examples are shown below here from the Orkneys, Northumberland and Anglesey, but such clouds are common place in Britain and can be seen anywhere in the right conditions.
Cumulus mediocris and humilis over the Irish Sea at South Stack, Anglsey, Wales
There would appear to be a case for having a special stratiformis species of cumulus, to better describe discrete cumulus clouds with flattened tops, but the stratiformis species is officially limited to cirocumulus, altocumulus and stratocumulus by the World Meteorological Organisation, so technically there is now such description as cumulus stratiformis. Time for a change?
Cumulus congestus at Wolstanton; eventually produced a very heavy shower
By this stage the clouds have reached quite a great height (usually over 3km) and have bright sunlit sharp edges near their summits and dark bases. They can produce showery rain, and occasionally, hail. Although the cloud tops endure temperatures well below zero, the clouds are still formed by minute liquid water droplets. Water still in a liquid state at temperatures below zero Celcius is described as supercooled. At these heights the air is very pure, usually lacking dust particles, salt, etc. Water only freezes at zero Celcius when there particle are present.
Cumulus congestus at Wolstanton, August 2004. This was the leading edge of a cloud system which very quickly developed into a violent thunderstorm.
Note that the edges of the cloud in the next picture are particularly sharp. This indicates that the cloud is growing rapidly, and is a sure warning that heavy showers are on the way .
Cumulus clouds can produce some of the most dramatic 'cloudscapes', often causing a marked contrast between light and shade. In the picture opposite, the sun is hidden behind thick cloud and its rays have been channeled between towering cumulus clouds. Such bright rays of sunlight are described as crepuscular rays.
They are usually most pronounced when the sun is low in the sky, e.g. towards evening.
Cumulus congestus at Wolstanton, August 2004. In this case, the cumulus was dwarfed by an immense Cumulonimbus cloud, which cast a shadow on all below it.
While the upper parts of cumulus are usually brightly lit, where the cumulus is below even higher denser clouds they can be distinctly grey and ominous. The example shown here, was a precursor to a heavy thunderstorm, which quickly developed in a hot humid south easterly flow over Wolstanton on August 2004, the wettest month on record in North Staffordshire, with a total of over 230mm of rain at Wolstanton.
In the British Isles, Cumulus congestus develop best in three particular air masses:
Polar Maritime - over land areas in the summer and sea and coastal areas in the winter. In both cases the visibility is usually good and the temperature slightly on the cool side.
Arctic - over land areas in the summer and sea and coastal areas in the winter. Again visibility is usually excellent, but the temperature is decidely cold. During the winter half of the year, snow and hail showers often result.
Tropical Continental - over land areas in particular, during the summer half of the year, usually in heatwave situations, when the wind is light and between south and east, pulling air from the continent. Thundery showers usually result.
Altocumulus castellanus during a heatwave at Wolstanton July 2015
Altocumulus is a middle-level cloud of limited vertical extent. In this case there is quite strong convection which produces an effect resembling the turrets in a castle, hence the descriptor: castellanus
When castellanus occurs and shows signs of rapid growth, it is the sign if instability in the atmosphere and is often associated with showers and thunderstorms. In this case, a major thunderstorm developed NW of Wolstanton and went on to produce flooding and lightning damage in Manchester, some 30 miles to the north.
Altocumulus floccus during on the evening of a summer's day at Wolstanton
This photo was taken on the same day as the one above. The convection in the castellanus clouds had diminished but they had started to produce trailing virga - streaks of precipitation falling towards the surface, but evaporating at the same time. Floccus clouds also indicate instability and the liklihood of shower and storm development nearby.
Altocumulus stratiformis lacunosus during later afternoon on a summer's day at Wolstanton
More often convection is very limited producing flattened clusters of altocumulus that are given the suffix stratiformis to indicate their stratiform nature. Sometimes they are arranged in lines producing an affect that is usually referred to as mackerel sky because of their resemblance to the pattern on the fish. In this case they have the additional suffix lacunosus.
From these descriptions you may have noticed that altocumulus is very much a summer-time cloud. During the winter half of the year, altocumulus is not so common as there is rarely enough differential heating in the atmosphere to produce middle level convection. When the cloud does occur in cooler times of the year, it is usually of the stratiformis type.
Cumulus and Cumulomibus clouds often have a variety of interesting features. You can see a sample of them here. They can also look particularly beautiful at sunset and sunrise. Visit the twilight clouds page for some examples.