Altocumulus clouds have a variety of forms and sizes, depending on the amount, strength and depth of convection, the effects of wave action and precipitation:
Although altocumulus clouds can occur at any time of year, certain types, e.g. castellanus, are far more common in the summer when the extra radiation from the sun is better able to cause convection.
These are mid-level white or grey clouds which occur as patches, rolls or rounded masses which can combine into a layer. Where they form discrete elements their apparent width is between 1° and 5°, very roughly the size of your thumb if held at arms length, assuming you are observing them at an angle of around 45o. If you were looking up vertically, they may be closer to three fingers width.
Where they form a sheet there are distinct and relatively regular patches of dark and light. In the lighter areas the sun or moon can be partly seen.
They occur where there is a thin layer of instability, i.e. where convection and subsidence occur but within a much restricted vertical range
Altocumulus at Harriseahead, Staffordshire, June 2011
© Lionel Burch
The form of the clouds can vary considerably. In this case there is no hint of a layer developing. The altocumulus clouds occur as discrete units clearly separated from each other.
There is also a hint of more vertical development and less horizontal spreading. If the vertical development was a little greater, they could be classed as Altocumulus castellanus, a type further described lower down this page.
Altocumulus at Harriseahead, Staffordshire July 2017
© Lionel Burch
Where convection is very limited flattened clusters of altocumulus develop. These are given the suffix stratiformis to indicate their stratiform nature.
Those in this image could also have the additional suffix lacunosus due to the relatively regular, very roughly circular shaped holes that appear between the cloud cells.
Altocumulus stratiformis lacunosus during later afternoon on a summer's day at Wolstanton
© Paul Swinhoe
Where the cloud elements are arranged with regular roughly round holes, they can give the appearance of a network or honeycomb, and are given the additional suffix lacunosus.
Altocumulus lacunosus at Wolstanton, Staffordshire, July 2018
© Paul Swinhoe
Where there is greater convection in the mid-troposphere the vertical extent of altocumulus can be greater, sometimes forming in lines, or streets, giving the appearance of turrets on a castle. Regardless of whether lines develop these clouds are given the suffix castellanus.
In this case there is a clear line of convection.
Altocumulus castellanus at Wolstanton, Staffordshire
© Paul Swinhoe
Very often, however, lines do not occur, or they may be difficult to see because you are viewing the clouds from directly underneath. This is the case with this photograph which was taken on the same summer's day
The dark scuds of cloud beneath the altocumulus castellanus are Stratus fractus pannus
Growth of this type is often indicative of instability and showery weather.
Altocumulus castellanus at Wolstanton, Staffordshire
© Paul Swinhoe
This photo was taken on the same day as the castellanus clouds above. The convection in the castellanus clouds had diminished but they had started to produce trailing virga - streaks of precipitation (amost certainly snow) falling towards the surface, but evaporating at the same time. When this happens, the suffix floccus applied to the cloud.
Floccus clouds also indicate instability and the likelyhood of shower and storm development nearby. On this occasion although it was around 1930 UTC the temperature was still around 30oC and a major thunderstorm cell developed over the Cheshire Plain about 20 kilometres to the northwest of the site. The storm caused lightning damage and flooding from NE Cheshire, through Manchester and ultimately into North Yorkshire.
Altocumulus floccus on a summer's evening at Wolstanton
© Paul Swinhoe
Altocumulus lenticularis are lens shaped wave clouds which form in areas of undulating air movement. Where the air is rising and cooling, when the dew point is reached, the cloud develops. Where the air is falling and warming, the cloud evaporates, usually leaving a clear area. As long as a wave remains in one position, its associated clouds will stay in same place, despite the fact that the winds at that altitude may be very strong, often over 50 MPH.
Such underlating, or waving, air movements usally occur in the lee of high hills or mountains. In this case, the wind was from the west and not far to the west is the main north-south highest ridge of the North Pennines. Because of the North Pennines, wave clouds are a common feature of the skies of North East England.
Where prominent lines of high ground are absent lenticular clouds are far less common, but can still occur on occasions where a layer of the atmosphere is subject to turbulent flow. The most extreme cases produce one of the rarest clouds, Asperitas. We have no photos of that but see this one on the WMO's Cloud Atlas website.
Altocumulus lenticularis over the Durham Dales, June 2018
© Paul Swinhoe
Altocumulus duplicatus occurs where there are layers of altocumulus at different levels in the troposphere. In this case, several Altocumulus lenticularis clouds are stacked on top of each other in the centre of the photograph. These are wave clouds that have formed in the lee of the highest parts of Snowdonia
Altocumulus lenticularis duplicatus at Deganwy, North Wales
September 2016
© Lionel Burch
Altocumulus undulatus occurs when there is wave motion in the air flow at the altitude of the clouds, as well as limited convection within a shallow layer.
Where numerous parallel bands occur, it creates an effect commonly known as mackerel sky, because of its similarity to the patterns on the flanks of mackerel.
Altocumulus undulatus at Harriseahead, Staffordshire, August 2013
© Lionel Burch
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.