During midwinter, under the right conditions, the polar sky can be transformed into a colorful spectacle by shimmering clouds.
These rainbow clouds, also known as “nacre clouds” due to their resemblance to iridescent shells, are an incredibly rare atmospheric phenomenon.
Scientifically referred to as “polar stratospheric clouds,” they exist at much higher altitudes in the atmosphere compared to typical clouds. While most clouds are found below 10 km above the Earth’s surface, rainbow clouds form in the lower stratosphere, between 15 and 25 km in altitude.
In the usually dry stratosphere, temperatures dropping below -80°C can cause sparse water molecules to form small ice crystals that eventually gather to create a cloud. These tiny ice crystals scatter sunlight, producing a dazzling rainbow effect and a pearlescent appearance.
Due to their high altitudes, the curvature of the Earth allows sunlight from below the horizon to hit and reflect off the underside of these clouds, illuminating them at dawn and dusk.
Rainbow clouds formed from pure water crystals are the thickest and most vivid, while those formed from air pollutants like nitric acid can result in thin, less impressive clouds. The latter type of cloud is associated with the degradation of the Antarctic ozone layer.
To witness rainbow clouds, consider visiting Antarctica during winter or look up at the sky around sunrise or sunset in parts of Norway, Sweden, Finland, Alaska, or even Scotland. These breathtaking phenomena are a result of unique atmospheric conditions.
This article addresses the question posed by Ollie Peterson via email: “What causes rainbow clouds?”
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