Not all clouds are created equal. Some are puffy, others are grey, while some clouds are so erratic and capricious that the human mind starts to see things like unicorns, cows, bunnies, or a nation’s borders.
The various clouds are named based on their shape and how high up they hover in the troposphere. From the ethereal to the ominous, our planet’s skies are teeming with some very rare and strange clouds. The effects of rare phenomena produce unusually shaped cloud formations. Read on to learn about the six rarest cloud formations and the fascinating science behind them.
Lenticularis
Named for their smooth, lentil-like appearance, altocumulus lenticularis clouds are believed to be one of the most common explanations for UFO sightings. These spooky-looking clouds appear downwind of mountains and they hover in the sky, even in the wind. When moving air encounters an obstacle like a mountain, the air is forced to rise and pass over it. The air then spreads over the other side and gravity causes it to drop slightly before rising back up again. Lenticular clouds form when the airflow streaming over the mountain is both humid and stable. As it flows upwards and cools, the moisture in the air condenses to form clouds at the crests of the standing wave.
Nacreous clouds
Nacreous clouds are some of Earth’s highest and rarest clouds, but these iridescent specimens are both beautiful and lethal. They are a form of polar stratospheric cloud, that plays a central role in the chemical destruction of the ozone layer. Nacreous clouds accelerate the chemical reactions that convert benign chlorofluorocarbons (CFCs) into ozone-destroying chlorine. The nacreous clouds manifest close to the poles during the coldest of winters, when temperatures plummet below -83C.
Asperitas
Asperitas clouds are so rare and unusual, that they were not officially recognized until June 2015. They are the first new type of cloud to be identified in over half a century. Asperitas clouds consist of dark, chaotic waves that seem to swirl and tumble haphazardly across the sky. Scientists are still trying to figure out how these clouds are formed.
Kelvin-Helmholtz wave
Kelvin-Helmholtz wave is an extremely rare weather phenomenon where a cloud creates a dramatic wave pattern. These fleeting cartoon-like formations are the Holy Grail for many cloud-spotters. This distinctive cloud appears when there is a strong vertical layer of pressure between two airstreams in the atmosphere, causing winds to move quicker in higher levels than in lower levels.
Virga clouds
Virga clouds have a similar appearance to jellyfish and are most noticeable when illuminated by the sun at sunset, with a breeze curving their extended hanging tails. These clouds are formed when trails of rainfall disperse from the underside of a cloud but evaporate before reaching the ground.
Mammatus clouds
Mammatus clouds are pouch-like protrusions that hang from the undersides of clouds. With an array of lumps and bulges, the mammatus cloud is by far one of the most distinct and unusual cloud formations. Turbulence within cumulonimbus clouds causes mammatus clouds to appear in the sky.
Look out for these rare clouds, some may look stunning while others may appear apocalyptic and menacing.