Polar stratospheric clouds are back again around the Arctic Circle

    

The new outbreak of these colorful high-altitude clouds has started on December 26, 2015. Look at them floating above Porjus, Lappland, Sweden:

    

For the second time this month, sky watchers around the Arctic Circle are reporting an outbreak of colorful polar stratospheric clouds (PSCs). Also known as “nacreous” or “mother of pearl” clouds, icy PSCs form in the lower stratosphere when temperatures drop to around minus 85ºC.

    

High-altitude sunlight shining through tiny ice particles ~10µm across produce the characteristic bright iridescent colors by diffraction and interference. Once thought to be mere curiosities, some PSCs are now known to be associated with the destruction of ozone.

The best time to look for PSCs is just before sunrise or right after sunset, when the ground is dark but the upper atmosphere is still illuminated by sunlight.