Noctilucent Clouds Get an Early Start   
       
   June 7, 2013:  Every summer, something strange and wonderful happens high   
   above the north pole.  Ice crystals begin to cling to the smoky remains of   
   meteors, forming electric-blue clouds with tendrils that ripple hypnotically   
   against the sunset sky.  Noctilucent clouds-a.k.a. "NLCs"--are a delight for   
   high-latitude sky watchers, and around the Arctic Circle their season of   
   visibility is always eagerly anticipated.   
       
   News flash: This year, NLCs are getting an early start. NASA's AIM spacecraft,   
   which is orbiting Earth on a mission to study noctilucent clouds, started   
   seeing them on May 13th.   
       
   "The 2013 season is remarkable because it started in the northern hemisphere a   
   week earlier than any other season that AIM has observed," reports Cora   
   Randall of the Laboratory for Atmospheric and Space Physics at the University   
   of Colorado. "This is quite possibly earlier than ever before."   
       
   http://www.youtube.com/watch?v=ohQzHz9gy6c   
       
   A new ScienceCast video explores the early start of the 2013 season for NLCs.   
   Play it!   
       
   The early start is extra-puzzling because of the solar cycle.  Researchers   
   have long known that NLCs tend to peak during solar minimum and bottom-out   
   during solar maximum-a fairly strong anti-correlation.  "If anything, we would   
   have expected a later start this year because the solar cycle is near its   
   maximum," Randall says. "So much for expectations."   
       
   For sky watchers, this means it's time to pay attention to the sunset sky,   
   where NLCs are most often seen.  An early start could herald brighter clouds   
   and wider visibility than ever before.   
       
   Noctilucent clouds were first noticed in the mid-19th century after the   
   eruption of super-volcano Krakatoa. Volcanic ash spread through the   
   atmosphere, painting vivid sunsets that mesmerized observers all around the   
   world.  That was when the NLCs appeared. At first people thought they must be   
   some side-effect of the volcano, but long after Krakatoa's ash settled the   
   noctilucent clouds remained.   
       
   "They've been with us ever since," says Randall.  "Not only that, they are   
   spreading."   
       
   When AIM was launched in 2007, the underlying cause of NLCs was still unknown.   
   Researchers knew they formed 83 km above Earth's surface where the atmosphere   
   meets the vacuum of space--but that's about all.  AIM quickly filled in the   
   gaps.   
       
   "It turns out that meteoroids play an important role in the formation of   
   NLCs," explains Hampton University Professor James Russell, the principal   
   investigator of AIM. "Specks of debris from disintegrating meteors act as   
   nucleating points where water molecules can gather and crystallize."   
       
   http://science.nasa.gov/media/medialibrary/2013/06/07/geometry.gif   
       
   This diagram shows why NLCs are best seen at sunset or sunrise. Click to view   
   the full-sized diagram. NLCs appear during summer because that is when water   
   molecules are wafted up from the lower atmosphere to mix with the "meteor   
   smoke." That is also the time when the upper atmosphere is ironically coldest.   
       
   Back in the 19th century, NLCs were confined to high latitudes.  You had to go   
   to Alaska or Scandinavia to see them.  In recent years, however, they have   
   been sighted as far south as Utah, Colorado, and Nebraska. Some researchers   
   believe that the spread of NLCs is a sign of climate change.   
       
   One of the greenhouse gases that has become more abundant in Earth's   
   atmosphere since the 19th century is methane.  "When methane makes its way   
   into the upper atmosphere, it is oxidized by a complex series of reactions to   
   form water vapor," says Russell. "This extra water vapor is then available to   
   grow ice crystals for NLCs."   
       
   The early start of the 2013 season appears to be caused by a change in   
   atmospheric "teleconnections."   
       
   "Half-a-world away from where the northern NLCs are forming, strong winds in   
   the southern stratosphere are altering global circulation patterns," explains   
   Randall. "This year more water vapor is being pushed into the high atmosphere   
   where NLCs love to form, and the air there is getting colder."   
       
   "All of this has come as an interesting surprise for us," notes Russell. "When   
   we launched AIM, our interest was in the clouds themselves.  But now NLCs are   
   teaching us about connections between different layers of the atmosphere that   
   operate over great distances. Our ability to study these connections will   
   surely lead to new understanding about how our atmosphere works."   
       
   Credits:   
       
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information:   
       
   AIM home page -- Hampton University   
       
   Meteor Smoke Makes Strange Clouds -- Science@NASA   
       
   Strange Clouds at the Edge of Space -- Science@NASA   
       
   NLC Photo Gallery -- spaceweather.com   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.94   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)