If Cumulus congestus clouds (the caulifower shaped ones) continue to grow vertically, as their summits reach very high altitudes where the temperature is below -30 oC, the water droplets in the cloud begin to freeze, giving a fibrous appearance, in contrast to the sharp-edged cumulus formations at lower levels. Cumulonimbus has a distinct life-cycle, with calvus being the earliest stage, followed by capillatus and incus, which represents the mature cloud. At a later stage the cloud decays from the base upwards.
Cumulonimbus and Cumulus clouds can have a variety of forms and supplementary features, such as mamma, arcus, virga and praecipitatio :
Cumulonimbus clouds develop from large large Cumulus congestus. As they begin to form, some of the caulifower-like protuberances in their upper parts begin to disappear, turning into a more flattened whitish mass, sometimes with a hint of striations. At this point the cloud has not quite developed its cirriform fibrous top, but there may well be hints of it in parts of the cloud. This is evident in the closer of the two large clouds shown in the photograph.
Cumulonimbus clouds can occur at any time of year in the British Isles. They are usually associated with hot thundery weather in the summer, but they can equally well occur in the unstable polar maritime and artic air masses that follow the passage of cold fronts.
Cumulonimbus calvus at Harriseahead, Staffordshire
17thth September 2017
© Lionel Burch
Cumulonimbus capillatus represents the next stage of growth of the cloud. By this stage the tops of the cloud on the horizon have a fibrous appearance due to glaciation.
The Latin term capillatus means curls and is used because the fibrous upper parts of the cloud often appear as small curls during the early stage of their growth. In this example the cloud is at a slightly later stage and curls are difficult to detect.
Cumulonimbus capillatus at Harriseahead, Staffordshire
24thth April 2018
© Lionel Burch
When glaciation becomes more extensive much of the upper section of the cloud has a fibrous appearance, and very often the supplementary feature incus (anvil) is also present. The anvil is caused by convected air being forced outward once it reaches the tropopause. The flat top of such clouds usually marks the tropopause, but sometimes it represents a simple temperature inversion.
Cumulonimbus incus over the Goyt Valley, Derbyshire, after the passage of an April shower.
© Paul Swinhoe
Sometimes, the complete anvil can be difficult to pick out, especially where there are several clouds standing side by side and one partly obscures the extent of the other.
In this case, an unstable cold Polar Maritime or perhaps even Acrtic air mass was affecting the area, with NW winds blowing through the Cheshire Gap. The convection producing the Cumulonimbus developed as the cold air mass passed over the relatively warm Irish Sea, with pockets of warmer air near the sea rising rapidly up to the tropopause.
The image also shows clear striations in the anvil caused by air currents.
At this time of year, as these clouds move inland, the convection dies away because the underlying heat source is lost. It is the reason why wintry showers are often restricted to windward coasts.
Cumulonimbus incus over the Cheshire Plain, viewed from Harriseahead
15th January 2016
© Lionel Burch
The convection needed to sustain Cumulonimbus requires a heat source. When this dies away the lower parts of the cloud evaporate, because there is no more warm air rising from the surface and condensing to form cloud. All that is left is the anvil, and in time this too will disappear or flatten out to give temporary Cirrus spissatus cloud. The loss of heat occurs for two main reasons:
Dying anvil, Mow Cop, Staffordshire 27th December 2013
© Lionel Burch
Sometimes the underside of an anvil has pendant mamma (billows). This occurs when the cloud is so dense that the outward currents of air are not strong enough to fully support the weight of ice crystals and produce the normally fibrous streaky appearance in the cloud.
In this example, a heavy thunderstorm was developing during a heatwave in August 2004. The cloud was so high and the anvil so deep that it seemed to have its own internal lightning system. The storm that followed was unusual in that the the thunder continued without a break from beginning to end, with discharges every couple of seconds. Despite the frequency of electrical activity the thunder was never very loud as its origin was way up in the anvil, probably between 5 and 6km altitude.
Cumulonimbus anvil with mamma, Wolstanton, Staffordshire
11th August 2004
© Paul Swinhoe
Even with a wide angle lens it can be difficult to capture the full extent of cloud forms.
In this example the dark clouds in the foreground form an arch ahead of the main body of Cumulonimbus behind, from which heavy rain was falling. The arc is formed where cold air flowing out of the storm system undercuts warmer air flowing in. Its passage is usually marked by squall which may involve winds of gale force. Very often, lightning is seen flickering behind the arc.
In June 1982 Wolstanton in Staffordshire suffered an event like this during a particularly violent storm, which produced 25mm rain in 20 minutes and tore branches up to 30cm diameter from chestnut and beech trees. Several houses had their television aerials ripped from their rooves. Such extreme winds associated with thunderstorms are often referred to as blow-downs, for obvious reasons!
Cumulonimbus arcus, Silverdale, Staffordshire
14thJune 2016
© Paul Swinhoe
In North America, especially over the Great Plains, the size of Cumulonimbus and thunderstorm systems if often far greater than we experience in the British Isles. The shear scale, can make them even more difficult to capture fully on camera.
In this case, the view is along the length of an arcus cloud, and here there are dramatic mamma present in the ach cloud itself. The main body of the storm and its associated precipitation is over to the left.
Cumulonimbus arcus, Alberta, Canada
3 rd August 2016
© Lionel Burch
Virga are supplementary features often seen with cumulonimbus clouds. They are formed by precipitation falling from the cloud, but evaporating before it reaches the surface.
Virga are often described in english as fallstreaks, which is a good descriptive term. The latin word virga means stripes, amongst other things.
Near the base of the cloud is a broken layer of dark ragged pannus clouds. In latin,pannus means a ragged piece of cloth, a rather apt description.
For virga to form relative humidity of the air surrounding the cloud has to be quite low, i.e. the air must be dry enough to cause the falling precipitation to evaporate or sublimate. This is often the case in polar maritime and arctic airmasses, but in this case a heat-wave was underway and the air mass was of tropical continental origin.
Virga trailing from a Cumulonimbus calvus cloud, Wolstanton
23rd August 2004
© Paul Swinhoe
Praecipitatio occur where the precipitation actually reaches the surface. In the picture on the right, both virga and praecipitatio hang from the underside of a large cumulonimbus cloud which developed in an arctic airstream blowing down the coast of North East England in late December.
It soon resulted in a heavy shower of soft hail, sleet and snow, and winds picked up from force 4 to around 6 with gusts up to gale force at the height of the shower.
Virga and praecipitatio below cumulonimbus, Sunderland
December 1999
© Paul Swinhoe