This huge expanse of Stratocumulus cloud over the Indian Ocean was photographed by the MODIS camera aboard NASA’s Aqua satellite. The area in the image extends about 430 miles (700 km) from one side to the other. Within the cloud layer appear radial patterns known as actinoform clouds, a name derived from the Greek word for ‘ray’. These cloud patterns were only discovered in 1962 when they showed up in early satellite photographs. No one had ever noticed actinoform clouds before due to their huge size, which can be as great as 180 miles (300 km) across. You just can’t see a large enough expanse of cloud beneath a low formation like this marine Stratocumulus to be able to see the huge patterns.

63 years on from first spotting them, we still know very little about actinoform clouds. We know they form in Stratocumulus clouds over oceans, with all the world’s oceans producing them. The star-like forms are sometimes arranged in lines, and other times they’re in random arrangements like these ones. They seem to be associated with the onset of light drizzle. Other than that, the huge spoked patterns of actinoform clouds remain one of the great mysteries of our low atmosphere.

NASA Earth Observatory image by Joshua Stevens, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview.