COSMIC SECRETS
The Enigmas on Earth
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Noctilucent Clouds
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Credit K. Sassen
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Noctilucent clouds (NLC) are clouds that literally shine
at night. We who live in Alaska are lucky to have the chance to see them,
because they only occur in high latitudes where the summer upper atmosphere
gets unusually cold. They form at incredible heights, at about 80 km (50
miles) above the Earths surface, which is ten times higher than the usual
wispy cirrus clouds we regularly see. The place they form is called the
mesopause, a very frigid layer where what little water vapor that is up
there can freeze, maybe on meteoritic dust, to form tiny ice crystals.
These pictures of NLC were taken at 230 in the morning on 5 August 2004,
from near Cleary Summit to get a good view of the northern horizon. If
you want to see a rare noctilucent cloud, you need to keep in mind that
in Fairbanks there is only a brief period when you can see them, and that
is mainly in August when the mesopause is cold enough! You can only see
NLC when the sun is below the horizon and the sky is turning dark. Only
then can you see the dim sunlight that is reflected off the tiny ice crystals.
If you look carefully at the pictures, you can see some stars shining!
So, look to the northern horizon a few hours after sunset and you may be
lucky enough to see the elusive NLC.
SOURCE: Atmospheric
Sciences |
Noctilucent Clouds Seen Around Fairbanks in August
2005
Photos and text courtesy of Patrick Cobb
Sept 2005 - May2006
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Credit Patrick Cobb
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Looking north on a late summer evening from Fairbanks,
it is sometimes possible to observe noctilucent clouds hovering above the
horizon. While typical clouds form in the troposphere below 12km, noctilucents
develop in the upper mesosphere at an astonishing 83km. Like cirrus clouds,
noctilucents are composed of tiny ice crystals, but the processes that
initiate ice deposition at such high altitudes is still unclear. Some theorize
that their formation is tied to interstellar dust fluxes in the upper atmosphere,
while others believe that gravity waves and atmospheric circulation patterns
at lower altitudes provide the most reasonable explanation for their existence.
Water vapor mixing ratios at this altitude are on the order of 3ppm (4K
to 10K times drier than the lower troposphere) so extremely cold temperatures
(~150K) are needed in order to achieve supersaturation in this dry environment.
Credit Patrick Cobb
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People first took notice of noctilucents over Europe during
the summer of 1884. Coincidentally, this was only a few months after the
famous Krakatoa eruption in Indonesia. The massive eruption spewed millions
of tons of dust and ash into the upper atmosphere, resulting in a 1.2C
drop in average global temperatures and triggering an artist's feast of
colorful sunsets for many years after the eruption. Many believed that
the Krakatoa eruption was directly responsible for the subsequent noctilucent
sightings, but this theory has since been proven false.
Because they are so optically thin, noctilucents will
only appear under particular illumination conditions. Only when the sun
drops to between 6-12 degrees below the horizon can the clouds become visible
from the ground. At these angles, the sky is almost completely dark to
the viewer, but at 85km, the clouds sit in the path of direct sunlight,
making them look ethereal, like bright apparitions of fine cobweb fabric:
hence the name noctilucent‚ or night shining.
Credit Patrick Cobb
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Sightings of noctilucents have been increasing in recent
years, leading some to speculate that anthroprogenic pollution, mainly
the increase in atmospheric methane, are contributing to their formation.
The photodissociation of methane, a powerful greenhouse gas, by UV light
provides an important source of water vapor in the upper atmosphere. Since
methane concentrations have been steadily rising as a result of human activities,
some connect global change with the recent increase in noctilucent sightings.
Unfortunately the mesosphere is beyond the range of
weather balloons, yet it is too low and too dense for satellites to pass.
As a result, sampling this part of the atmosphere directly poses a considerable
challenge. However, ground based systems like LIDAR instruments offer researchers
the opportunity to study noctilucent clouds remotely. By measuring the
intensity and the polarization of backscattered light, scientists are able
to infer many of their physical properties like cloud thickness, optical
depth, and particle size and shape.
SOURCE: Atmospheric
Sciences |
Noctilucent Clouds
Denmark, July 2004
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Noctilucent clouds - Denmark, July 2004 taken by Ole Nielsen.
To view the whole series visit Ole's
Astronomy Site |
Noctilucent Clouds
Norway
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Credit Geir Oye, Ørsta, Norway.
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Mesospheric noctilucent clouds form above Earth's poles
in the summertime. Their presence increased during the 20th century, and
lately, they've been seen farther and farther from the poles. |
Noctilucent Clouds
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Credit Hugo Patten
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The string theorist Jonathan Shock gave me a link to the
Cloud
Appreciation Society. That's where I got this photo of a nacreous cloud,
taken by Hugo Patten:
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Credit Lee Montgomerie
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The even more elusive noctilucent clouds live in the mesopause
- the layer where the mesosphere ends, about 85 kilometers up. Sometimes
they're electric blue! They're hard to explain, because the mesosphere
is very dry and cold. Here's a photo of a noctilucent cloud taken by Lee
Montgomerie:
SOURCE: John
Baez Diary |
Noctilucent Clouds
July 26, 1999
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Credit & Copyright: Pekka Parviainen (NCWG/U.
Colorado)
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Explanation: Sometimes it's night on the ground
but day in the air. As the Earth rotates to eclipse the Sun, sunset rises
up from the ground. Therefore, at sunset on the ground, sunlight still
shines on clouds above. Under usual circumstances, a pretty sunset might
be visible, but unusual noctilucent clouds float so high up they can be
seen well after dark. Pictured above, a network of noctilucent clouds casts
a colorful but eerie glow visible above the dark. Although noctilucent
clouds are thought to be composed of small ice-coated particles, much remains
unknown about them.
SOURCE: NASA APOD |
Noctilucent Clouds
NASA
Gallery
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SOURCE: NASA
Gallery |
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