What are Actinoform Clouds?

Elizabeth Borneman


Most of us could look up in the sky on any given day and recognize at least some of the kinds of clouds we see above us.

Cirrus clouds are wispy, cumulonimbus clouds are dark and towering, and cumulus clouds are the white puffy clouds we drew as kids.

One type of cloud that we might not be able to identify from the ground is called an actinoform cloud. It wasn’t until the first weather satellite, NASA’s TIROS-5, launched in 1962 that researchers were able to observe this unique cloud formation on satellite imagery.

Discovering Actinoform Clouds with Satellite Data

Thanks to satellites, actinoform clouds were discovered to form mainly over open ocean. Actinoform clouds are very large, making them difficult to distinguish from our viewpoint on Earth’s surface.

Far above us, satellites like NASA’s Terra satellite can take pictures of these massive formations in order to understand how they are different from other kinds of clouds. 

Satellite image of actinoform clouds off the western coast of Australia.  Image: NASA, Aqua satellite, January 29, 2020.
Satellite image of actinoform clouds off the western coast of Australia. The radiating arms of the clouds are called actiniae. Image: NASA, Aqua satellite, January 29, 2020.

Actinoform clouds are named after their ray-like appearance- the word ‘actino’ is based on the Greek word for ‘ray.’ These clouds can look different from one another but maintain the same general structure; the rays coming out of the clouds are called actiniae.

These clouds often have a radial structure or look like a wheel with spokes. An individual actinoform cloud can be 180 miles long, and these clouds can link together like a chain in formations many hundreds of miles long.

These clouds are much more common than researchers realized, in addition to being more complex than originally assumed.

Geography of Actinoform Clouds

Actinoform Clouds tend to form amid low stratocumulus cloud systems that are frequently encountered over open ocean. Actinoform cloud formations can persist for up to 72 hours. Drizzling rain can frequently accompany actinoform clouds.

Satellite image of cloud formations.
This NOAA-20 satellite image shows two actinoform clouds to the west of Alejandro Selkirk and Robinson Crusoe islands.

Where Were Actinoform Clouds First Observed?

Actinoform clouds were first observed off of the coast of Hawaii and have since been photographed off of Peru and Australia as well as the West Coast of the United States.  Actinoform clouds tend to be found along large Western landmasses in stable air over the ocean.

How Actinoform Clouds Are Created

How these clouds actually form is still being studied.

The first actinoform clouds were photographed by NASA’s TIROS V satellite in 1962, and improvements in satellite technologies in the decades since have only increased our access to data about these atmospheric oddities.

Satellite image from October 7, 1962 showing actinoform clouds over the ocean west of Peru.
On October 7, 1962, NASA’s Television Infrared Observation Satellite V (TIROS V) captured actinoform clouds over the ocean west of Peru. The numbered arrows depict a line of actinoform patterns at distinct developmental phases. While numbers 4 and 5 practically blend into one and 1 is disorganized, the normal pattern is readily seen in numbers 2 and 3. From: “Picture of the Month” image from Monthly Weather Review, 1963 (Vol. 91, p. 2).

Actinoform clouds are often found over the open ocean, but can still impact weather patterns on land.

These data have allowed researchers to find that actinoform clouds are associated with dreary, drizzly weather, like the weather patterns that hit Southern California, resulting in names like May Gray and June Gloom.

Actinoform clouds are found in the lower atmosphere in similar territory as marine stratus or stratocumulus clouds, and tend to last up to 72 hours. 

With the use of satellite technologies, researchers are hoping to get to the bottom of how actinoform clouds are formed and what makes them different from other clouds in the sky. 

Clouds and Climate Change

Clouds play an important role in regulating the climate here on Earth. All types of clouds are essential for temperature regulation as they reflect solar radiation from the sun back into space.

Additional research into actinoform clouds may continue to provide greater insights into weather patterns, atmospheric conditions, and the importance of these types of clouds in regulating the environment on Earth in changing times


NASA Earth Observatory. Evolving Views of Clouds. Accessed May 30, 2021. Retrieved from https://earthobservatory.nasa.gov/images/78562/evolving-views-of-clouds

Patel, Kasha. NASA Earth Observatory. Cloud Rosettes in the Sky. Accessed May 30, 2021. Retrieved from https://earthobservatory.nasa.gov/images/146244/cloud-rosettes-in-the-sky

NOAA. “Wagon Wheel” Clouds Drift Towards Hawaii. 2 July 2020. Accessed May 30, 2021. Retrieved from https://www.nesdis.noaa.gov/news/wagon-wheel-clouds-drift-toward-hawaii


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About the author
Elizabeth Borneman
My name is Elizabeth Borneman and I am a freelance writer, reader, and coffee drinker. I live on a small island in Alaska, which gives me plenty of time to fish, hike, kayak, and be inspired by nature. I enjoy writing about the natural world and find lots of ways to flex my creative muscles on the beach, in the forest, or down at the local coffee shop.

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