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 and Cumulus clouds can have a variety of forms and supplementary features:
Cumulonimbus at Wolstanton, February 1999
Cumulonimbus clouds can occur at any time of year in Wolstanton. Normally, they are 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.
In the example here, the only the tops of the cumulus clouds on the horizon have a fibrous appearance due to glaciation. Cumulonimbus clouds of this type are given the species name calvus; latin for bald. The clouds here do not appear to have particularly great vertical extent here because the tropopause was low due to the pressence of a polar martime airmass and it being winter.
Cumulonimbus capillatus incus over the Goyt Valley, Derbyshire, after the passage of a real April shower.
When glaciation becomes more extensive and much of the upper section of the cloud has a fibrous appearance, the cloud is given the species name capillatus; latin for long haired, reflecting the hairy streaks of ice crystal cloud at its top edges.
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 anvil under development
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 arcus - a poor example unfortunately!
Apologies for the photograph, but in an urban area and without a wide angle lens it is difficult to capture the full extent of cloud forms.
In this example only the lowest parts of the cumulonimbus cloud are shown. Almost at the base of thecloud is a sub-horizontal band forming an arc across the horizon, beyond which the sky is very dark. The arc is formedwhere cold air flowing out of the storm system undercuts warmer air flowing in. Its passage is usually marked bya squall which may involve winds of gale force. Very often, lightning is seen flickering behind the arc.
In June 1982 we suffered a an event like this during a particularly violent storm, which produced 25mm rain in 20 minutes and tore branches up to 30cm diameter off adjacent chestnut and beech trees. Several houses had their television aerials ripped from their rooves.
Virga trailing from the upper reaches of a Cumulonimbus calva cloud in August 2004, Wolstanton
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 has to be quite low, i.e. the air must be dry, in order for the falling precipitation to evaporate or sublimate. This is often the case in polar maritime and arctic airmasses.
Virga and praecipitation trailing below a cumulonimbus cloud, Sunderland
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.