See these rare high-altitude clouds light up the Arctic sky
Sunday, December 20, 2015, 5:46 PM - As the winter darkness arrives for Arctic regions, beautiful natural phenomena tends to brighten up the sky.
Photographer Truls Tiller managed to capture strange iridescent features that formed over the northern Norwegian town of Tromso on December 16.
“Here the sun is gone for now, but this beautiful view makes the winter darkness nice to be in as well,” Tiller told SpaceWeather.com.
The stunning photos below were taken around 10:30 a.m.
The rare clouds are called polar stratospheric clouds (PSCs). They form at very high altitudes, between 15 and 25 km (about 50,000 to 80,000 feet) at very cold temperatures around minus 78oC.
Typical grey-white clouds form in the troposphere at five to 10 km in altitude, according to Discovery News.
Called "Mother of Pearl" by Scandinavians due to their iridescent appearance, PSCs are composed of mixtures of water and nitric acid. If the angle of the sun is right below the horizon, they can create a dazzling light show.
They trap outgoing longwave radiation (OLR) and emit some of it back towards the Earth's surface, so they hinder atmospheric cooling. While they may seem beautiful, PSCs have been linked to the formation of holes in the ozone due to their chemical composition.
Watch more: How Rudolph guides Santa through the clouds and snow on Christmas Eve