Strong winds raised dust from the Sahara Desert and sculpted paisley-like patterns in the clouds in mid-May 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired this stunning true-color image of dust and vortices swirling in the atmosphere off of western Africa on May 12.
Theodore von Kármán, a Hungarian-American physicist, was the first to describe the physical processes that create long chains of spiral eddies like the ones shown trailing in the wake caused by wind striking the Canary Islands. Known as von Kármán vortices, the patterns can form nearly anywhere that fluid flow is disturbed by an object. In this case, the unique flow occurs as winds rush past the tall peaks on the islands. As winds are diverted around these high areas, the disturbance in the flow propagates downstream in the form of vortices that alternate their direction of rotation.
On this particular day, wind also carried dust from the Sahara Desert offshore, tinting the air south of the Canary Islands a camel-colored tan. Some of the dust appears to be caught in the vortices, swirling along with the cloud. The slice of silver seen at the top of the image and crossing into the vortices is not dust. This is an optical phenomenon known as “sunglint”. It occurs when sunlight reflects off the surface of the ocean at the same angle that the satellite sensor is viewing the surface. The result is a mirror-like specular reflection off the water and back to the sensor, which, on smooth water gleams with a silver tint.