Winter in the Arctic Circle is an eerie time, with the Sun having long since dropped beneath the horizon, not to return for several months. However, the blackness of the icy northern sky also provides a perfect backdrop for some of the most spectacular and otherworldly natural phenomena known to man.
Of these, the most famous is of course the northlern lights – or aurora borealis – although a lesser-known yet equally jaw-dropping weather event has been igniting the skies above the Norwegian city of Tromsø over the past few days: polar stratoshperic clouds (PSCs).
As the name suggests, these are formed in the stratosphere, a layer of the Earth’s atmosphere that sits above the troposphere, where normal clouds are formed. Because PSCs occur at higher altitudes than these regular clouds, the air that surrounds them is particularly cold, and typically reaches around -85°C (-121°F).
Consequently, PSCs tend to be made up of tiny frozen particles, and are just high enough to receive some of the Sun’s rays. When this light hits these particles, it gets refracted, splitting up into the incredible array of colors seen in these images, which were taken by local photographer Truls Melbye Tiller.
However, there is a sting in the tail of these beautiful clouds, since they are thought to contribute to the destruction of the ozone layer. This is because they provide a surface for a number of harmful chemical reactions, resulting in the conversion of benign chlorine compounds into dangerous gases such as chlorine monoxide.
Amazingly, Tiller told Spaceweather.com that the pictures were snapped at 10.30 am – a fact which simply adds to the magnificence of these stunning yet alarmingly dangerous clouds.
All images in text show polar stratospheric clouds in northern Norway, photographed by Truls Melbye Tiller.