We could see some very nice mother of pearl clouds over Oslo yesterday afternoon. These clouds are best seen when the sun is below the horizon, shining on the ice-crystals that make up the cloud. It’s actually a sign of stratospheric mountain wave conditions, where moist air from the trophosphere is pushed up into the cold polar stratosphere. The wave and clouds are extremely high, from 20 to 30 km high. (A jetliner cruises at 10km)

Steve Fosset have formed the Perlan project to soar these waves in a pressurized sailplane, the project goal is to reach 100000 feet.

Picture from met.no, link to norwegian met.no article.