On the final day of 2019 we were blessed with the most gorgeous natural light show I have ever seen. These bizarre oil-stain-like shapes in the sky are caused by sunlight from the setting sun refracting through minute ice particles at 86,000ft. These seemingly magical clouds are called 'Polar Stratospheric Clouds' (PSCs) and occur when the stratosphere is very cold and moist air finds its way up there. These particular PSCs were caused by lee waves formed by the strong wind blowing over the Scandes mountains from the west. Lee waves are literal waves in the air caused by a strong wind blowing over a mountain, which is exactly what was happening on the 31st of December 2019 in mid-Scandinavia.
In this video, taken earlier on the same day, you can see just how windy it is and even see some blurry daytime PSCs in the sky behind:
Put simply, the peak of the lee wave was high enough to enter the -80°c stratosphere, so when the moisture-rich air reached the peak of the wave, it condensed and froze to form perfect ice crystals. They glow well after dark because of their altitude and the curvature of the earth as the sun can shine on them after it has set.
PSCs only tend to form in the winter, when the stratosphere becomes extremely cold (enough to form the perfect ice crystals needed for this effect) and only in the high latitudes. During the day, PSCs look like extremely out of focus clouds, which is a bizarre effect in itself. I have attached some photos of daytime PSCs above.
Environmental Issues around PSCs.
In the Antarctic, PSCs can form more readily due to the colder temperatures and don't need lee waves. Despite their beauty, PSCs have been linked to some negative environmental effects. They can cause heating in polar regions as they reflect long-wave infrared radiation back to earth and more significantly, they have been shown to be able to break down the Ozone layer. PSCs are fairly unique in forming in the stratosphere, at the same level as the ozone layer. Unfortunately, it seems that PSCs provide a surface for ozone to react with chlorine (and other reactants), breaking it down to basic oxygen, which doesn't have the same ability to protect us from incoming solar radiation. PSCs may also allow the chlorine to become reactive, accelerating this effect.
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