Cumulonimbus calvus praecipitatio (Cb cal pra)
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Cumulonimbus calvus praecipitatio (Cb cal pra)

Here’s a picture taken in central Virginia in August of 2019 during a thunderstorm at sunset. The storm wasn’t severe, but there was a bit of lightning and some rain that fell. You can see a rain shaft in the middle of the image to the right of the barn.

Summary: This is an example of a thunderstorm with a bald upper portion producing rain, classified as cumulonimbus calvus praecipitatio (Cb cal pra).

Cloud Type. As explained in the intro paragraph, there was some lightning associated with the storm. It’s not the most threatening thunderstorm, but usually lightning is a really good sign that the cloud has developed into a cumulonimbus cloud.

This cloud has a great deal of vertical growth, a dark base, lightning, and precipitation. It’s a fair bet that we’re looking at a cumulonimbus cloud.

Cloud Species. Cumulonimbus clouds are only associated with two of the fiften cloud species: calvus and capillatus. Both of these species are unique to cumulonimbus clouds.

The calvus species describes a cumulonimbus cloud with a bald, cauliflower-like top. The capillatus species describes a cumulonimbus cloud with a fibrous upper portion. Here, our view of the top of the cloud can be seen. You can even see its shadow coming off the top from the setting sun, casting on a higher layer of what looks to be cirrus and cirrocumulus clouds.

The top of the cloud is relatively cumuliform, meaning it’s still puffy and cauliflower-like. It would be a fair assessment to classify this as the cloud species calvus.

Cloud Varieties. Hey now! Cumulonimbus clouds don’t have any associated cloud varieties. Moving on…

Supplementary Features. Cumulonimbus clouds are associated with eight of the eleven supplementary features: arcus, cauda, incus, mamma, murus, praecipitatio, tuba, and virga.

Going down the list of all eight, the only visible feature that can be made out is the rain shaft. The cloud feature praecipitatio would be an appropriate classification.

Cloud Accessories & Other Clouds. Finally, cumulonimbus clouds have four associated accessory clouds and one other cloud associated with the cloud type: flumen, pannus, pileus, velum, and flammagenitus. But as far as we can see, none of these apply.

After working our way through the cloud types and subtypes, we can put all the pieces together and determine this cloud’s classification: Cumulonimbus calvus praecipitatio (Cb cal pra).

A graphical illustration of a cumulonimbus cloud

Cumulonimbus (Cb)


A graphical illustration of a cumulonimbus calvus cloud

Calvus (cal)

Without cirriform, hairless

A graphical illustration of a cumulonimbus praecipitatio cloud

Praecipitatio (pra)

Precipitation reaching the surface

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