|All Sky Noctilucent Clouds - P-M Hed�n (photography site, more NLCs) saw this last week on the night of 24/25 June at Vallentuna Sweden. NLCs covered the sky and P-M assembled this 360° panorama to show the scene. ©P-M Hedén, shown with permission.
NLCs are Earth's highest clouds 50-53 miles up in the rarefied upper atmosphere where the pressure is around five millionths of that at sea level. The clouds of tiny ice crystals are a few miles below the mesopause, the atmosphere’s coldest point. The clouds need its exceptionally low temperatures (below –123C) for the crystals to form in the ultra dry conditions. Paradoxically, the mesosphere is at its coldest in summer - the season for seeing NLCs.