On most days, clouds hover over at least part of the Caspian Sea, the planet’s largest inland body of water. But a cloud that drifted across the Caspian on May 28, 2022, looked more peculiar than most.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this view on the morning of May 28. The lone cloud had well-defined edges resembling something from a cartoon, making it stand out from the more typical diffuse and dispersed cloud cover.
According to Bastiaan van Diedenhoven, an atmospheric scientist at SRON Netherlands Institute for Space Research, the cloud is a small stratocumulus. The cloud type is puffy like a cumulus cloud; cumulus is the Latin word for “heap” or “pile.” But unlike a cumulus cloud, the “heaps” in a stratocumulus cloud are clumped together, forming a widespread horizontal cloud layer. (Stratus is from a Latin verb that means “to spread out,” or “cover with a layer.”) The stratocumulus pictured here formed a layer spanning about 100 kilometers (60 miles) across.
Stratocumulus clouds form at low altitudes, generally between 600 and 2,000 meters (2,000 and 7,000 feet). This cloud was probably hovering at an altitude of about 1,500 meters (5,000 feet).
In the late morning (shown above), the cloud was poised over the central Caspian. By the afternoon it had drifted toward the northwest and hugged the coast of Makhachkala, Russia, along a low-lying plain near the foothills of the Caucasus Mountains. According to van Diedenhoven, the cloud could have formed when warm, dry air—possibly from the Balkans—encountered colder, moist air over the Caspian. It then drifted across the sea and dissipated when it reached land.
The scenario also explains the cloud’s well-defined edges. “Sharp edges are often formed when dry, warm air coming from land collides with colder moist air over the ocean, and the cloud forms at that boundary,” van Diedenhoven said. “You often see this off the west coast of Africa, but at much larger scales.”
NASA Earth Observatory image by Joshua Stevens, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Story by Kathryn Hansen.