Rare clouds that give off bright, multi-colored light like an aurora were recently spotted at multiple locations in the Arctic. But what causes them?
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The dark skies in the Arctic Circle recently shone with ethereal multi-colored light. But this jaw-dropping spectacle was not caused by auroras. Instead, the iridescent rainbows were caused by clouds of tiny ice crystals floating higher in the atmosphere than is normally possible.
The clouds, known as polar stratospheric clouds (PSC), only form when the lower stratosphere reaches temperatures below minus 114 degrees Fahrenheit (minus 81 degrees Celsius). Normally, clouds do not form in the stratosphere because it is too dry, but at these extremely low temperatures “widely-spaced water molecules begin to coalesce into tiny ice crystals” that form into clouds, Spaceweather.com(opens in new tab) reported. This means PSCs can form much higher up than normal clouds, between 9.3 and 15.5 miles (15 to 25 kilometers) above the ground.
There are two types of PSCs: Type I, which are made from a mix of ice crystals and nitric acid, which produces less spectacular colors and may be linked to the formation of ozone holes; and Type II, which are composed of pure ice crystals and produce more vivid colors. The ones that recently formed over the Arctic were Type II.
Type II PSCs are often referred to as nacreous clouds because their iridescent hues can sometimes resemble nacre, also known as mother of pearl, which is produced in the shells of some mollusks. However, they are much rarer than Type I clouds.
Type II clouds typically occur no more than two or three times a year in the Arctic, normally during the colder winter months, according to Spacewaether.com. However, experts believe that both types of PSCs could occur more often in the future as climate change creates more extreme weather, which could have a knock-on impact on the ozone layer if more Type I clouds can form, according to NASA(opens in new tab).
Due to their intense colors, nacreous clouds are often confused with the northern lights, or aurora borealis, in the Arctic. These more common phenomena occur when highly energetic particles emitted by the sun travel down the magnetic field lines of Earth’s magnetosphere.
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