Pileus Clouds

We’re constantly amazed around here when it comes to the seemingly unending fodder that clouds provide to the project. 

Way back in Issue #49, we took an intro-level class on clouds. Before we even consider some of the rarer types of water vapor, the amount of variation in the everyday sky is incredible. Earlier in 2022, we studied a bizarre formation called arcus clouds, which appear like giant rolls or shelves. 

In August 2022, a gorgeous cloud anomaly appeared in China. This particular specimen was an example of a pileus cloud. Before we gaze at the cloud’s resplendence, let’s learn about basic pilei.

A pileus (outlined in red) on a cumulus cloud (blue) - photo by Dhaluza

Pictured above is a pileus on top of a cumulus cloud. Unlike most types of clouds, you’ll never see a pileus by itself. Pileus is Latin for “cap.” These clouds are the hats of the atmospheric world.

Pilei are relatively small, horizontal, lenticular (stationary and high) clouds. They tend to form above cumulus or cumulonimbus clouds. Often, the pileus is a short-lived formation, as the clouds beneath them rise to gobble them.

These cotton balls of the sky often signal inclement weather, as they are formed by strong updrafts at lower altitudes. When this upward movement interacts with moist air above it, it causes the wet air to cool to the dew point, which forms the cloud. If you see a cap on top of a cumulus cloud, it’s often a sign that a big, bad cumulonimbus (thunderstorm) cloud is in the making.

This cumulonimbus cloud has a cap, called a pileus - photo by Couch-scratching-cats
Photo by Sylke Boyd
The process of a pileus being munched by a cumulonimbus cloud - graphic by Couch-scratching-cats

This phenomenon is not, however, linked solely to other clouds.

Pilei can form through the same upward dynamics of other mechanisms, such as ash clouds from volcanoes and even mushroom clouds of nuclear blasts. According to the American Meteorological Society, “a smooth lenticular cap (pileus) or ‘scarf cloud’ formed just above the top” of the cloud created by tests in the Bikini Atoll.

The images and footage of these types of occurrences inspire awe:

Pileus over ash cloud from an eruption of Sarychev Peak - photo by NASA

The video above of the Able Test starts just before the formation of the pileus. Truly spectacular.

The impetus for the article, however, stemmed from a plain-old, natural cloud. The qualities of caps are perfect for diffraction. The droplets of a pileus cloud are small and similarly sized, attributes that just happen to produce the strongest iridescence via diffraction.

Translation: pileus clouds can produce vibrant rainbow light shows.

The Chinese cloud of August 2022 produced a tremendous bit of iridescence:

An Iridescent Pileus Cloud over China - image by Jiaqi Sun(孙嘉琪)

Fortunate were the eyes that viewed this glory! Though it seems an uncontrollable addition, an iridescent pileus cloud certainly joins the bucket list of natural phenomena we hope to one day experience.

Further Reading and Exploration

Pileus – International Cloud Atlas

Pileus Clouds – Atmospheric Optics

An Iridescent Pileus Cloud over China – NASA

Weather  and  The  Atomic  Bomb  Tests  at  Bikini – The Bulletin of the American Meteorological Society

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