In the British Isles and elsewhere in the world, there are a number of cloud types that only occur when specific rare atmospheric conditions allow:
Cumulus humilis is the smallest of the cumuliform clouds with little vertical extent as the convection is limited to narrow layer. Their cauliflower form is not fully developed. Cumulus fractus is also present, consisting of even smaller wisps of cloud that have yet to develop any particular form. When observed they show swirling movements caused by air currents associated with convection. Given time they may develop into humilis but the may also simply evaporate and disappear.
Collectively, these small scale cumulus clouds are often loosely referred to as fair weather cumulus because they will not produce any precipitation.
Noctilucent Clouds at Carlton-in-Coverdale
21st June, 2019
© Mike Cinderey
These clouds 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 static or even descending.
In the photograph 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 Perranporth beach, Cornwall
April 2002
© Paul Swinhoe
The flattened top of this example is even clearer. It may appear to have an anvil form, but this is definitely not Cumulonimbus incus, as the cloud is of small vertical extent and shows no evidence of glaciation producing a fibrous top!
Cumulus mediocris, Harriseahead, Staffordshire
15thApril 2012
© Lionel Burch
Here, flattened mediocris have developed in a clear linear fashion, on a west wind blowing in from the Atlantic in a polar maritime air mass. Where the linear pattern extends over a large area downwind the resulting pattern is described as cloud streets. In these the air is rising along the lines of cloud and static or descending in the clear gaps between them
Flattened Cumulus mediocris, Ring of Brodgar, Mainland, Orkney
6thJune 2005
© Paul Swinhoe
In this example, a cold northerly wind was blowing on to the north coast of Lindisfarne over a relatively warm sea, a situation that usually results in convection and cumulus formation.
The growth of cloud, however, has been limited, possibly by a temperature inversion, but also at least partially by wind shear. This happens when one layer of air, usually higher up, is moving significantly faster than the air immediately below. As a result the upper parts of the clouds are being stretched out downwind giving them a tilted appearance.
Cumulus mediocris over the North Sea, Lindisfarne
22nd February 2001
© Paul Swinhoe
Not all mediocris has a flattened top. The term simply means of mediumvertical extent. In this case there was more vigorous convection, and some of the clouds in the distance could almost be classified as congestus implying much greater growth and a clearer cauliflowerform. The picture was taken during the morning, and as the day went on much larger cumulus congestus did eventually develop, giving heavy showers.
This was a day of polar maritime air on a WNW wind, following the passage of a cold front, an air mass which is often 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!
Cumulus mediocris over Westport Lake, Burslem, Stoke-on-Trent
June 2000
© Paul Swinhoe
This is a fully developed cumulus cloud showing good cauliflower form. It developed along a line of convection during a heat-wave brought by tropical continental air blowing from the SE.
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 top temperatures is well below zero, the cloud is still formed by minute drops of supercooled water.
Cumulus congestus often produces showers in its own right without developing any further. However, they tend to be short and sharp, producing relatively small amounts of rain unless they develop further into Cumulonimbus.
Cumulus congestus over Morridge, Staffordshire Peak District
26th July 2018
© Paul Swinhoe
Note that the edges of the cloud in this picture are particularly sharp. This indicates that the cloud is growing rapidly, and is a sure warning that heavy showers are on the way .
Here the combination of cloud and sun have produced strong upward crepuscular rays. These are usually most pronounced when the sun is low in the sky.
These clouds were a precursor to a heavy thunderstorm, which quickly developed in a hot humid SE flow in August 2004, the wettest month on record in North Staffordshire, with a total of over 230mm of rain at Wolstanton.
Cumulus congestus, Wolstanton, Staffordshire
23rd August 2004
© Paul Swinhoe
These towering clouds over the Welsh Borders were viewed from North Staffordshire, over 40 miles away. Again, their sharp edges indicate rapid growth, and to the right of the image the sun is lighting up part of an anvil from a Cumuluonimbus incus cloud. The sun has almost set and only the tops of the clouds are lit up.
The clouds developed in hot SE tropical continental air which brough violent thunderstorms to parts of the West Midlands and the Welsh Borders. The orange colour of the sky is indicative of the amount of dust and pollen that had risen into the atmosphere during the hot spell.
Cumulus congestus, Hanchurch Hills, Staffordshire
31st May 2018
© Paul Swinhoe
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.