Nacreous clouds, or ice polar stratospheric clouds, are a rare type of cloud that forms in very cold conditions and at high altitudes.
They are known for their iridescent colors, which are created when sunlight interacts with the tiny ice crystals they are made of, according to the World Meteorological Organization (WMO). The name “nacreous” comes from “nacre,” another name for mother-of-pearl, which refers to their iridescent appearance. They are sometimes called mother-of-pearl clouds.
Nacreous clouds are found during the winter in high-latitude areas, including the Arctic, Scotland, Scandinavia and Alaska. They sometimes form over other areas of Northern Europe but are the most common in Antarctica. Some nacreous clouds are wave clouds, which often form near mountain ranges.
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In some places, nacreous clouds are so brilliantly colorful and unusual that they are sometimes confused for the aurora borealis, or northern lights, which are colorful light displays caused by the interaction between electrically charged particles and Earth’s atmosphere and are unrelated to nacreous clouds.
How do nacreous clouds form?
For nacreous clouds to form, the air must be colder than about minus 121 degrees Fahrenheit (minus 85 degrees Celsius), according to the WMO. Unlike most clouds, which form in the first layer of Earth’s atmosphere, nacreous clouds form in the stratosphere, Earth’s second atmospheric layer. Under these frigid conditions, all moisture in the air has been transformed into supercooled liquid and ice crystals, according to The Conversation. The spherical ice crystals that make up the clouds are only 10 micrometers in diameter — thinner than the typical width of a human hair, according to Thin Metal Parts.
Nacreous clouds are also called Type II polar stratospheric clouds (PSCs), which are clouds that form in the winter polar stratosphere. However, whereas nacreous clouds are composed only of ice crystals, another type, called Type I PSCs, can form in slightly warmer conditions and are made of mainly supercooled droplets of water, nitric acid and sulfuric acid, according to the Australian Antarctic Program. Type I PSCs have less spectacular colors, though they can form among areas of nacreous clouds.
Related: What are different types of clouds, and how do they form?
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