explanation
Sometimes it's night on the ground but day in the air. As the Earth rotates to eclipse the Sun, sunset [ http://antwrp.gsfc.nasa.gov/apod/ap060712.html ] rises up from the ground. Therefore, at sunset on the ground, sunlight still shines on clouds [ http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/home.rxml ] above. Under usual circumstances, a pretty sunset [ http://antwrp.gsfc.nasa.gov/apod/ap980526.html ] might be visible, but unusual noctilucent clouds [ http://lasp.colorado.edu/noctilucent_clouds/ ] float so high up they can be seen well after dark. Pictured above [ http://www.clearskies.se/NLC%2016.htm ] last month, a network of noctilucent clouds [ http://science.nasa.gov/headlines/y2003/19feb_nlc.htm ] cast a colorful but eerie glow after dusk near Vallentuna [ http://en.wikipedia.org/wiki/Vallentuna ], Sweden [ http://en.wikipedia.org/wiki/Sweden ]. Although noctilucent clouds [ http://en.wikipedia.org/wiki/Noctilucent_clouds ] are thought to be composed of small ice-coated particles, much remains unknown [ http://www.nlcnet.co.uk/ ] about them. Recent evidence [ http://www.nrl.navy.mil/pressRelease.php?Y=2003&R=35-03r ] indicates that at least some noctilucent clouds [ http://www.gsfc.nasa.gov/topstory/2003/0522shuttleshine.html ] result from freezing water exhaust from Space Shuttle [ http://antwrp.gsfc.nasa.gov/apod/ap021023.html ]s.
Explanation
false