From December 19 to 21, residents of the northern regions of Norway, Sweden, Finland, England, Ireland and Alaska could see clouds that gave the sky a color similar to an oil slick.
The US National Oceanic and Atmospheric Administration (NOAA) reported that it is a fairly rare phenomenon that occurs when clouds form at extremely low temperatures (below minus 85 degrees Celsius).
The water molecules begin to combine into tiny ice crystals, which later gather into clouds.
Source
portaltele