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An Early Start for Noctilucent Clouds - Presented by Science@NASA
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.'
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 previous years.
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,
meteoroids play an important role in the formation of NLCs,'
explains Hampton University Professor James Russell,
the principal investigator of AIM.
Throughout the year
meteors disintegrate in the upper atmosphere,
leaving behind a haze of smokey debris.
During summer,
when the upper atmosphere is ironically coldest,
water molecules waft up from the Earth below
and form ice crystals around the 'meteor smoke.'
This is what makes noctilucent clouds.
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.'
This year researchers get a head start.
Noctilucent clouds are back,
earlier than ever.
For more news from the edge of space,
visit science.nasa.gov.