Posted by Hervé
(here)
In favor of cometary/space dust:
Noctilucent clouds appear out of season and far to the south - cometary dust from the Russian meteor blast?
Sott.net
Mon, 25 Feb 2013 06:28 CST
From 19-21 February 2013, noctilucent clouds were observed in the UK, Denmark, Norway and the Netherlands. Since these clouds are usually only seen in summer, it is suspected that they may be the result of comet dust deposited in the upper atmosphere by the Chelyabinsk/Chebarkul meteor or comet fragment explosion over Southern Russia on 15 February.
The following images were submitted to
spaceweather.com
© Terry Parker, Image taken by Terry Parker on Feb. 20, 2013 from above Birmingham, UK. 'I am an airline pilot in the UK and I occasionally see noctilucent clouds. Usually during the summer at about midnight looking North. But yesterday (20 Feb 13) I was very surprised to see them looking south towards France and so close to sunrise.'
© Tom Axelsen, Photo by Tom Axelsen, Copenhagen area, Denmark
****************************************
Wikipedia:
Noctilucent cloud
From Wikipedia, the free encyclopedia
Night clouds or
noctilucent clouds are tenuous
cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called
polar mesospheric clouds in the upper
atmosphere, visible in a deep
twilight. They are made of crystals of
water ice.
Noctilucent roughly means
night shining in
Latin. They are most commonly observed in the summer months at
latitudes between 50° and 70° north and south of the
equator. They can only be observed when the Sun is below the horizon.
They are the highest
clouds in the
Earth's atmosphere, located in the
mesosphere at altitudes of around 76 to 85
kilometres (47 to 53
mi). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the
Earth's shadow. Noctilucent clouds are not fully understood and are a recently-discovered
meteorological phenomenon; there is no record of their observation before 1885.
Formation
Night clouds or noctilucent clouds are composed of tiny crystals of
water ice up to 100
nm in diameter
[1] and exist at a height of about 76 to 85
km (47 to 53
mi),
[2] higher than any other clouds in Earth's atmosphere.
[3] Clouds in the Earth's lower atmosphere form when water collects on particles, but mesospheric clouds may form directly from water vapour
[4] in addition to forming on dust particles.
[5]
The sources of both the dust and the water vapour in the upper atmosphere are not known with certainty. The dust is believed to come from
micrometeors, although particulates from
volcanoes and dust from the
troposphere are also possibilities. The moisture could be lifted through gaps in the
tropopause, as well as forming from the reaction of
methane with
hydroxyl radicals in the
stratosphere.
[6]
The exhaust from
Space Shuttles, which is almost entirely water vapour after the detachment of the
Solid Rocket Booster at a height of about 46 km, has been found to generate minuscule individual clouds. About half of the vapour is released into the
thermosphere, usually at altitudes of 103 to 114 km (64 to 71 mi).
[7]
This exhaust can be transported to the
Arctic region in little over a day, although the exact mechanism of this very high-speed transport is unknown. As the water migrates northward, it falls from the thermosphere down into the colder mesosphere, which occupies the region of the atmosphere just below.
[8] Although this mechanism is the cause of individual noctilucent clouds, it is not thought to be a major contributor to the phenomenon as a whole.
[6]
As the
mesosphere contains very little moisture, approximately one hundred millionth that of air from the
Sahara desert,
[9] and is extremely thin, the ice crystals can only form at temperatures below about −120 °C (−184 °F).
[6] This means that noctilucent clouds form predominantly during summer when, counterintuitively, the mesosphere is coldest,
[10] therefore they can't be observed (even if they are present) inside the
Polar circles because the Sun is never low enough under the horizon at this season at these latitudes.
[11] Noctilucent clouds form mostly near the polar regions,
[5] because the mesosphere is coldest there.
[11] Clouds in the southern hemisphere are about 1 km (0.62 mi) higher than those in the northern hemisphere.
[5]
Ultraviolet radiation from the Sun breaks water molecules apart, reducing the amount of water available to form noctilucent clouds. The radiation is known to vary cyclically with the
solar cycle and satellites have been tracking the decrease in brightness of the clouds with the increase of ultraviolet radiation for the last two solar cycles. It has been found that changes in the clouds follow changes in the intensity of ultraviolet rays by about a year, but the reason for this long lag is not yet known.
[12]
Noctilucent clouds are known to exhibit high
radar reflectivity,
[10] in a frequency range of 50
MHz to 1.3 GHz.
[13] This behaviour is not well understood but a
Caltech professor, Paul Bellan, has proposed a possible explanation: that the ice grains become coated with a thin metal film composed of
sodium and
iron, which makes the cloud far more reflective to radar,
[10] although this explanation remains controversial.
[14] Sodium and iron atoms are stripped from incoming
micrometeors and settle into a layer just above the altitude of noctilucent clouds, and measurements have shown that these elements are severely depleted when the clouds are present. Other experiments have demonstrated that, at the extremely cold temperatures of a noctilucent cloud, sodium vapour can rapidly be deposited onto an ice surface.
[15]
Research published in the journal
Geophysical Research Letters in June 2009 suggests that noctilucent clouds observed following the
Tunguska Event are evidence that the impact was caused by a comet.
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