Electric Blue Sunsets
 
Aug 16, 2016:  Northern summer is underway. It's time for picnics, hot dogs,
dips in the pool... and, oh yes, electric blue sunsets.
 
Just below the Arctic Circle, evening skies are filling with pale blue
ripples. They appear just after sunset or just before sunrise and are called
noctilucent clouds (NLCs). When viewed from space, the same atmospheric
phenomenon is referred to as polar mesospheric clouds (PMCs).
 
https://www.youtube.com/watch?v=EAVJrLRBRPY
 
Cora Randall, chair of the Department of Atmospheric and Oceanic Sciences at
the University of Colorado says, "The 2016 season for noctilucent clouds began
on May 24th when NASA's Aeronomy of Ice in the Mesosphere satellite (AIM)
spotted a puff of electric blue around the Arctic Circle."
 
NLCs are Earth's highest clouds. Seeded by meteoroids, they float at the edge
of space more than 50 miles above the planet's surface. The clouds are very
cold and filled with tiny ice crystals. Those tiny crystals do a good job of
scattering blue light from the rising or setting sun-hence their electric blue
color.
 
Noctilucent clouds appear during summer because, ironically, that is the only
time the upper atmosphere at high latitudes is cold enough to crystalize
molecules of water around specks of meteor dust. The role of meteor dust in
forming NLCs is one of many discoveries made by AIM, which NASA launched in
2007 to study the mysterious clouds.
 
James Russell, principal investigator for AIM at Hampton University says,
"These clouds continue to reveal intimate details about how the atmosphere
works. Each season yields new information."
 
The first recorded observations of noctilucent clouds were made in the 19th
century after the eruption of super-volcano Krakatoa. At the time, people
thought the clouds were caused by the eruption, but long after Krakatoa's ash
settled, the clouds remained. In those days, NLCs were a polar phenomenon
confined mainly to the Arctic. In recent years, they have intensified and
spread with sightings as far south as Colorado and Kansas.
 
The reason may be climate change on the edge of space.  A recent study by AIM
science team member Mark Hervig and colleagues published March 2016 in the
Journal of Geophysical Research confirms that the mesosphere, the atmospheric
layer where NLCs form, has gotten colder and moister in recent decades. Both
trends promote the formation of NLCs.
 
Their results are consistent with a simple model linking the clouds to two
greenhouse gases. First, carbon dioxide promotes NLCs by making the mesosphere
colder. While increasing carbon dioxide warms the surface of the Earth, those
same molecules cool the upper atmosphere - a yin-yang relationship long known
to climate scientists. Second, methane promotes the clouds by adding moisture
to the mesosphere because methane oxidizes into water as it rises in the
atmosphere.
 
Noctilucent clouds appear to be a telltale sign of important greenhouse
gases.  And that, says Russell, is a great reason to study them. "They may be
at the edge of space, but they're telling us something very important about
our own planet."
 
For more news from the edge of space, stay tuned to science.nasa.gov.
 
 
Regards,
 
Roger

--- DB 3.99 + W10 (1607)
 * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)