When moist air flows over mountains, the water vapour it carries can condense into clouds over or beyond the summit. A friend of Chris Adams (Member 58,590) captured these streaks and curls over Mount Jungfrau when they were on holiday in Switzerland. They wondered what formation this was. The clouds look to be thin examples of Altocumulus lenticularis, the more typically disc-like form of the mid-level Altocumulus cloud. And these seem to have within them arched features with no official classification that we’ve come to call ‘supercilium’. The name is the Latin for an ‘eyebrow’.

Because of their unofficial classification status, supercilium cloud features are understudied. They always seem to appear in the churning, overturning, turbulent wake as winds flow over abrupt mountain peaks like Jungfrau, but we don’t know a lot about how they form. In other words, Chris, we wonder what formations they are too.