Valopilvet

Valopilvet, also known as nacreous clouds or polar stratospheric clouds, are rare, colorful clouds that form in the very high atmosphere, typically in the polar regions during winter. They are observed at altitudes between 15 to 25 kilometers (9 to 16 miles), far above the altitude of normal weather clouds.

These clouds are composed of tiny ice crystals or supercooled water droplets, which are thought to be coated with nitric acid or sulfuric acid. Their distinctive iridescent colors, often appearing in shades of pink, green, and blue, are a result of diffraction and interference of sunlight by these small particles. The colors are most vivid at twilight, when the sun is just below the horizon, illuminating the clouds from below.

The formation of valopilvet is dependent on extremely cold temperatures, often below -78 degrees Celsius (-108 degrees Fahrenheit). These frigid conditions are found in the polar stratosphere during winter. While visually stunning, these clouds also play a role in atmospheric chemistry. Their formation is linked to the depletion of the ozone layer, as the surfaces of the ice crystals can act as sites for chemical reactions that convert stable forms of chlorine into more reactive forms, which then destroy ozone.

Valopilvet are most commonly seen in the Arctic and Antarctic regions during their respective winter months. Their rarity and spectacular appearance make them a sought-after sight for meteorologists and skywatchers alike.