Cumulus and Altocumulus – A Family Portrait

This sky spotted over Boca Raton, Florida, US by Andi and Ray Popkin (Member 63,500) is like a family portrait of convection clouds. Two of the ten main cloud types are present: Cumulus and Altocumulus. These clouds, as their names suggest, are related. They’re both solid-looking clouds. Cumulus is Latin for a ‘stack’ or ‘heap’, and it’s where we get the term ‘accumulation’. They both form as warm, moist air rises in columns through colder air above. And though they look very different in size when compared side by side like this, the large-looking clumps of Cumulus are in fact about the same size as the small-looking clumps of Altocumulus.

It’s all about the distance. Cumulus clouds look bigger because they’re much closer by – in this case, probably at altitudes of around 1 km (3,000 ft). The Altocumulus are much higher, likely up around 7 km (23,000 ft). Were you looking at them from up in an aircraft, they’d appear big instead and the Cumulus way below would look tiny.

Where these two cousins of the cloud world differ is in the arrangement of their elements, or clumps. Though we can see only a few of the nearby Cumulus, they’re likely arranged in a far less orderly way than the neat pattern of Altocumulus above. This difference is to do with the winds. Low winds swirl around buildings and terrain, and the turbulent flow means the Cumulus-forming columns of air, which are known as thermals, tend to be arranged rather chaotically. The smooth, unimpeded airflows many kilometres above mean their Altocumulus-forming equivalents can arrange themselves into much more orderly patterns.

The resulting cloudlets (as clumps of Altocumulus are known) in Andi and Ray’s image have lined up into parallel bands, some going this way and others going that. This pattern is the undulatus variety of Altocumulus. It is traditionally known as a ‘mackerel sky’ as it resembles the banded markings on those fish (the cloudlets themselves being the fish scales). This pattern of Altocumulus can appear ahead of a weather front that brings a chance of precipitation.

OK, everyone. Stay still, look at the camera, and say ‘convection clouds’!

Altocumulus stratiformis undulatus and Cumulus spotted over Boca Raton, Florida, US by Andi and Ray Popkin (Member 63,500). View this image in the photo gallery.