Nacreous clouds shine in the twilight sky ~ Luleå, Sweden December 2006. Imaged by Åke Svensson. 15 - 25 km (9 -16 mile) high in the stratosphere, well above our ordinary clouds, nacreous clouds catch sunlight and blaze with iridescent colours long after the sun has gone at ground level. Their small ice crystals need exceptionally low temperatures, below -85C, to form. Usually seen only in winter at high latitudes, some say they are becoming more frequent but a factor could be that more and more of us look up to the skies.   ©Åke Svensson, shown with permission.