Cumulonimbus arcus praecipitatio pannus (Cb arc pra pan)

Summer thunderstorms! Here’s what a severe storm looks like after a hot and muggy day in central Virginia in August of 2019. Looks pretty dark, pretty ominous. Definitely not a cloud you want to mess with.

Summary: The prominent feature pictured is a shelf cloud attached to a severe thunderstorm. There are also some scud clouds sitting beneath the cloud’s base, and heavy rain can be seen above the horizon. This cloud is classified as cumulonimbus arcus praecipitatio pannus (Cb arc pra pan).

Cloud Type. We’re in luck. Of the ten different types of clouds you can choose from, identifying cumulonimbus clouds come pretty easy when you come across a stormy scene like this. Though you can’t see it in the photograph, there was a significant amount of lightning. The National Weather Service issued a severe thunderstorm warning.

Although you can’t see its impressive vertical height from this picture, storm clouds like this can grow to over 16km (10 miles) in height under the right conditions. This cloud as identified as a cumulonimbus cloud.

Cloud Species. Cumulonimbus clouds have two species associated with it: calvus and capillatus. The calvus species describes a cumulonimbus cloud with a bald, cumuliform, cauliflower-like top. The capillatus species describes a cumulonimbus cloud with a hairy and unkempt top.

If we were looking at the cloud from miles away, it would be easier see the upper portion of the storm. In this case, our view of the top of the cloud is obscured. When classifying a cloud, you don’t need to classify a species if it isn’t appropriate. Instead of taking a guess, we won’t apply a species to this cloud’s classification.

Cloud Varieties. Guess what? Cumulonimbus clouds don’t have any associated cloud varieties. You’re off the hook!

Supplementary Features. Of the eleven cloud features, cumulonimbus clouds are associated with eight of them. They are arcus, cauda, incus, mamma, murus, praecipitatio, tuba, and virga.

This example includes two of the eight, with the most prominent feature being the shelf cloud (cloud feature arcus) attached to its base. The shelf cloud is the low-hanging leading edge of the storm; they’re formed when rain-cooled air collides with warm air in front of the storm and pushes it up and out of its way.

Many times (but not always), heavy rain is followed by the shelf cloud. On the right side of the image under the cloud, you can make out a heavy rain curtain. The cloud feature praecipitatio would be an appropriate classification in this case as well.

The cloud feature cauda (tail cloud) doesn’t apply, we can’t see the top of the cloud so it’s not possible to tell if the cloud has an anvil cloud feature (incus), and we don’t see any mammatus cloud present (cloud feature mamma). It’s difficult to find a storm with a wall cloud (cloud feature murus) and a shelf cloud on the same storm, so no wall cloud here. There’s no funnel cloud present either (cloud feature tuba) and finally, no virga (evaporating precipitation) either.

Cloud Accessories & Other Clouds. Cumulonimbus clouds have four associated accessory clouds and one other cloud associated with the cloud type. In this example, the pannus cloud accessory applies. Pannus refers to the ragged scud clouds that sit below the cloud’s base just before the rain.

The cloud accessories flumen (beaver tail cloud), pileus (cap cloud, generally found above a cumulonimbus calvus cloud), and velum (altostratus-like cloud veil) are absent and don’t apply here. Finally, on the rarest of occasions, cumulonimbus clouds can be caused by fire and given the classification flammagenitus (better known as pyrocumulus clouds.

In this lesson, we determined the cloud classification starting from the cloud type and working our way down through the other subtypes. We’ve arrived at our official classification: Cumulonimbus arcus praecipitatio pannus (Cb arc pra pan).