Altocumulus lenticularis duplicatus (Ac len du)

Here’s a photograph of a classic UFO-shaped cloud found near the foothills of Boulder, CO in February 2017. If you live near mountains, perhaps this isn’t too uncommon of a sight, but definitely an odd cloud to those that don’t live in close proximation to mountains.

Summary: This is an example of what’s known as a mid-altitude stacked lenticular cloud, classified as altocumulus lenticularis duplicatus (Ac len du).

Cloud Species. This is a really good example of a cloud where a feature, accessory, or species is more recognizable than the cloud type. To classify this cloud, we’ll start with the cloud species and work our way backwards to determine its final classification.

The distinct UFO-shaped cloud is commonly known as a lenticular cloud. They’re called lenticular clouds because of their smooth shape often resembles a lentil seed. Because they usually form as a result of a large object disrupting air currents, they’re more often observed near mountainous regions.

While these clouds are often referred to as lenticular clouds, speaking from a cloud classification standpoint, these are of the lenticularis cloud species.

Cloud Type. Because we know these clouds are lenticular clouds, it helps us narrow down the cloud type. Only three types of clouds can be of the lenticularis cloud species: cirrocumulus, altocumulus, and stratocumulus.

Cirrocumulus lenticularis clouds don’t take on this kind of UFO-shape, but more of a very thin and wavy appearance. And deciding between stratocumulus and altocumulus, consider the cloud height. Because this cloud isn’t very close to the ground, it makes it a good candidate for an altocumulus cloud.

Cloud Varieties. Altocumulus clouds are associated with seven cloud varieties: duplicatus, lacunosus, opacus, perlucidus, radiatus, translucidus, and undulatus. But as a single standalone cloud, you’re not going to find a lot of variety in this cloud.

It’s not the most extreme example, but the cloud pictured is a form of what are known as stacked lenticular clouds, where the clouds take on a stacked dinner plate appearance. When clouds are multilayered like this one pictured, you can apply the duplicatus cloud variety.

Supplementary Features. Of the eleven cloud features, the five features that apply to altocumulus clouds are asperitas (chaotic, wavy underneath), cavum (fallstreak hole), fluctus (Kelvin-Helmholtz waves), mamma (mammatus clouds), and virga (evaporating precipitation strips). None of these five features apply here.

Cloud Accessories & Other Clouds. That’s it… class dismissed! Altocumulus clouds don’t have any accessory clouds or other clouds associated with it.

In this example, the cloud classification was determined from recognizing the cloud species first, and working backwards to figuring out the cloud type. We’ve arrived at our official classification: Altocumulus lenticularis duplicatus (Ac len du).