The pictures above and below show mother-of-pearl or (nacreous clouds), photographed from Kiruna. Clouds like these are fairly common in Kiruna in January and February. They form at stratospheric altitudes (15-30 km) in local temperature minima caused by atmospheric \"lee\" waves. The waves form when strong winds blow from the Atlantic across the Scandinavian mountain chain, towards Kiruna.
Nacreous clouds are one type of polar stratospheric cloud (PSC). PSCs are an important factor in the breakdown of stratospheric ozone - ozone destruction through reaction with active chlorine compounds proceeds much faster on the surface of cloud particles than it would if the clouds were not there.
During winter these clouds and the lee waves are the subject of intensive study using, amongst other instruments, the radar and lidars at ALOMAR on the west side of the mountain chain and the lidar and ESRAD MST radar in Kiruna on the east side.