Smooth, fixed clouds are occasionally reported by the public as sightings of “unknown flying items.” But these clouds are not as mystical as they may initially seem.
On December 29, 2020, the Operational Land Imager (OLI) on Landsat 8 got these images of soft-edged clouds hovering over the Eisenhower Variety of Antarctica’s Transantarctic Mountains. The variety is bounded to the north by Priestley Glacier and to the south by Reeves Glacier, both of which feed into the Nansen Ice Rack on Terra Nova Bay.
The clouds have the trademarks of lenticular clouds that can form along the crests of mountain waves. Mountain waves form when fast moving wind is interrupted by a topographic barrier– in this case, the Eisenhower Range. Air is forced to stream up and over the mountains, causing waves of rising and falling air downwind of the variety. The increasing air cools and water vapor condenses into clouds. On the other hand, falling air results in evaporation.
Adding to their mystique, this cloud type appears to sit tight– sometimes for hours– defying the strong horizontal winds. In reality, the clouds are continuously building around the crest of the wave and then dissipating just beyond.
In the United States, lenticular clouds are especially common around the Rocky Mountains. They have been known to occur over Antarctic mountains, too, however there are very few witnesses besides satellites. The white-on-white color of clouds over ice make the Antarctic versions harder to discern, even in satellite images. This natural-color image has been enhanced with infrared light to separate the white clouds from the white snow and ice below. The clouds likewise tossed rounded shadows on the landscape.
Still, a couple of people have seen lenticular clouds in Antarctica firsthand. Researchers dealing with NASA‘s Operation Icebridge shot photos of the phenomenon near Mount Discovery in 2013 and over Penny Ice Cap in 2015.
NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey. Story by Kathryn Hansen with image interpretation by Bastiaan Van Diedenhoven (NASA GISS/Columbia) and Jan Lenaerts (CU Stone).