These weird cloud patterns are from Kármán’s vortex attributable to the air that wraps across the excessive islands
This is a picture of some of the islands that make up the nation of Cape Verde. While most of this group of ten islands are flat, some are very large: Fogo, Santa Antão, and São Nicolau. These three stand well above their compatriots, and Fogo reaches a height of 2,829 meters.
The three tall volcanic islands sometimes interact with the wind to create von Kármán vortices, also called Kármán vortex streets.
The eddies become visible when clouds form, which is a near-laboratory example of the von Kármán effect. When the wind blows past the peaks, the current alternates around the sides of the peak. This oscillation creates alternating eddies around the object.
A phenomenon called vortex shedding is responsible for the effect. Vortex shedding occurs only at certain wind speeds, and only when the body the wind is passing is not streamlined like the tips in the picture.
A topographic map of Cape Verde.
The von Kármán vortex blades are named after the mathematician, physicist and aerospace engineer Theodore von Kármán. He worked in the aerospace industry and was responsible for making important advances in understanding supersonic and hypersonic flows. He was Jewish and originally came from Austria-Hungary. He then worked at a university in Germany. Concerned about the rise of National Socialism in Germany, he fled Europe to the USA. He ended up at the California Institute of Technology (Caltech).
Although these features had previously been observed, von Kármán was the first to describe them mathematically. As he said, “I found that only the antisymmetric arrangement can be stable and only for a certain ratio of the distance between the rows and the distance between two consecutive vertebrae of each row.”
This image shows Kármán clouds that form near the Norwegian island of Jan Mayen in the Greenland Sea in the North Atlantic. They form when the wind blows past the Beerenberg Volcano, a snow-capped peak on the eastern end of the island that rises 2.2 kilometers above the sea surface. Photo credit: Jeff Schmaltz, LANCE / EOSDIS MODIS Fast response.
The images of these cool cloud shapes were taken with NASA’s Terra satellite and the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument. Terra is the flagship of NASA’s Earth observation system. They were captured on December 20, 2020.
Satellites have captured other examples of Kármán vortex streets. They discovered them in the Greenland Sea, the Indian Ocean and elsewhere.
This MODIS image is from NASA’s Operational Land Imager on its Landsat 8 satellite. It shows Mawson Peak on the Australian territory of Heard Island. The wind sometimes flows past the 2745-meter-high summit and creates vortex beds from Kármán. Photo credit: Jeff Schmaltz, LANCE / EOSDIS Rapid Response
Kármán vortex streets do not only form around mountain peaks. You can learn about electrical and communication cables, car antennas, and buildings, chimneys and bridges. The vibration is undesirable, however, and engineers have found ways to disrupt the flow of air and prevent its formation. Sometimes they put fins or ribbons on structures to break up the oscillating pattern.
These chimneys are tied with ribbons to prevent eddies from forming. Photo credit: Von Greditdesu – Using my Canon IXUS 105, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18741199
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