Faldt over dette paa Wikipedia... det er da smukt!

Noctilucent clouds are the highest clouds in Earth's atmosphere, being bright polar mesospheric clouds illuminated by the Arctic sun from below the horizon, between latitudes of 50° and 70°. They are composed of water ice crystals up to 100 nm across, forming most frequently in the northern summer at altitudes of 76 to 85 km (47 to 53 mi). The clouds form directly from water vapor and sometimes around dust: water may be produced from the reaction of CH4 and OH-, while the dust is believed to originate from micrometeors and possibly volcanic particulate matter entering the mesosphere. The clouds are detectable by radar and the observed increase in these clouds serves as a possible indicator of climate change.