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.
Noctilucent clouds can form only under very restrictive conditions;
their occurrence can be used as a sensitive guide to changes in the
upper atmosphere. They are a relatively recent classification. The
occurrence of noctilucent clouds appears to be increasing in frequency,
brightness and extent. It is theorized that this increase is connected
to
climate change.
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]
Discovery and investigation
Noctilucent clouds are first known to have been observed in 1885, two years after the
1883 eruption of Krakatoa.
[5][16]
It remains unclear whether their appearance had anything to do with the
volcano eruption, or whether their discovery was due to more people
observing the spectacular sunsets caused by the volcanic debris in the
atmosphere. Studies have shown that noctilucent clouds are not caused
solely by volcanic activity, although dust and water vapour could be
injected into the upper atmosphere by eruptions and contribute to their
formation.
[11]
Scientists at the time assumed the clouds were another manifestation of
volcanic ash, but after the ash had settled out of the atmosphere, the
noctilucent clouds persisted.
[9] Finally, the theory that the clouds were composed of volcanic dust was disproved by Malzev in 1926.
[16] In the years following their discovery the clouds were studied extensively by
Otto Jesse of
Germany, who was the first to photograph them, in 1887, and seems to have been the one to coin the term "noctilucent cloud",
[17] which means "night-shining cloud".
[1]
His notes provide evidence that noctilucent clouds first appeared in
1885. He had been doing detailed observations of the unusual sunsets
caused by the Krakatoa eruption the previous year and firmly believed
that, if the clouds had been visible then, he would undoubtedly have
noticed them.
[18] Systematic photographic observations of the clouds were organized in 1887 by Jesse,
Foerster, and Stolze and, after that year, continuous observations were carried out at the
Berlin Observatory.
[19] During this research the height of the clouds was first determined, via
triangulation.
[20] The project was discontinued in 1896.
In the decades after Otto Jesse's death in 1901, there were few new insights into the nature of noctilucent clouds.
Wegener's conjecture, that they were composed of water ice, was later shown to be correct.
[21]
Study was limited to ground-based observations and scientists had very
little knowledge of the mesosphere until the 1960s, when direct rocket
measurements began. These showed for the first time that the occurrence
of the clouds coincided with very low temperatures in the mesosphere.
[22]
Noctilucent clouds were first detected from space by an instrument on the
OGO-6
satellite in 1972. The OGO-6 observations of a bright scattering layer
over the polar caps were identified as poleward extensions of these
clouds.
[23] A later satellite, the
Solar Mesosphere Explorer, mapped the distribution of the clouds between 1981 and 1986 with its ultraviolet spectrometer.
[23] The clouds were detected with a
lidar in 1995 at
Utah State University, even when they were not visible with the naked eye.
[24]
The first physical confirmation that water ice is indeed the primary
component of noctilucent clouds came from the HALOE instrument on the
Upper Atmosphere Research Satellite in 2001.
[25]
In 2001 the
Swedish Odin satellite performed spectral analyses on the clouds, and produced daily global maps that revealed large patterns in their distribution.
[26]
On April 25, 2007, the AIM satellite (
Aeronomy of Ice in the Mesosphere) was launched.
[27] It is the first satellite dedicated to studying noctilucent clouds,
[28] and made its first observations on May 25, 2007.
[29]
Images taken by the satellite show shapes in the clouds that are
similar to shapes in tropospheric clouds, hinting at similarities in
their dynamics.
[1]
On August 28, 2006, scientists with the
Mars Express mission announced that they found clouds of
carbon dioxide crystals over
Mars
that extended up to 100 km (62 mi) above the surface of the planet.
They are the highest clouds discovered over the surface of a planet.
Like noctilucent clouds on Earth, they can only be observed when the Sun
is below the horizon.
[30]
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.
[31][32]
The
United States Naval Research Laboratory (NRL) and the
United States Department of Defense Space Test Program (STP) conducted the
Charged Aerosol Release Experiment (CARE) on September 19, 2009, using exhaust particles from a
Black Brant XII suborbital sounding rocket launched from
NASA's
Wallops Flight Facility
to create an artificial noctilucent cloud. The cloud was to be observed
over a period of weeks or months by ground instruments and the Spatial
Heterodyne IMager for MEsospheric Radicals (SHIMMER) instrument on the
NRL/STP STPSat-1 spacecraft.
[33] The rocket's exhaust plume was observed and reported to news organizations in the United States from
New Jersey to
Massachusetts.
[34]
Observation
Noctilucent clouds are generally colourless or pale blue,
[35] although occasionally other colours including red and green occur.
[36] The characteristic blue colour comes from absorption by
ozone in the path of the sunlight illuminating the noctilucent cloud.
[37] They can appear as featureless bands,
[35] but frequently show distinctive patterns such as streaks, wave-like undulations, and whirls.
[38] They are considered a "beautiful natural phenomenon".
[39] Noctilucent clouds may be confused with
cirrus clouds, but appear sharper under magnification.
[35] Those caused by rocket exhausts tend to show colours other than silver or blue,
[36] because of iridescence caused by the uniform size of the water droplets produced.
[40]
Noctilucent clouds photographed by the crew of the
ISS
Noctilucent clouds may be seen by observers at a
latitude of 50° to 65°.
[41] They seldom occur at lower latitudes (although there have been sightings as far south as
Utah,
Italy, and
Paris),
[35][42] and closer to the poles it does not get dark enough for the clouds to become visible.
[43]
They occur during summer, from mid-May to mid-August in the northern
hemisphere and between mid-November and mid-February in the southern
hemisphere.
[35] They are very faint and tenuous, and may only be observed in
twilight
around sunrise and sunset when the clouds of the lower atmosphere are
in shadow, but the noctilucent cloud is illuminated by the
Sun.
[43] They are best seen when the Sun is between 6° and 16° below the horizon.
[44] Although noctilucent clouds occur in both
hemispheres,
they have been observed thousands of times in the northern hemisphere,
but fewer than 100 times in the southern. Southern hemisphere
noctilucent clouds are fainter and occur less frequently; additionally
the southern hemisphere has a lower population and less land area from
which to make observations.
[11][45]
The clouds may show a large variety of different patterns and forms.
An identification scheme was developed by Fogle in 1970 that classified
five different forms. These classifications have since been modified and
subdivided.
[46]
They may be studied from the ground, from space, and directly by
sounding rocket.
Also, some noctilucent clouds are made of smaller crystals, 30 nm or
less, which are invisible to observers on the ground because they do not
scatter enough light.
[1]
Connection to climate change
There is evidence that the relatively recent appearance of noctilucent clouds, and their gradual increase, may be linked to
climate change.
[47]
Atmospheric scientist Gary Thomas of the
Laboratory for Atmospheric and Space Physics at the
University of Colorado has pointed out
[1] that the first sightings coincide with the
Industrial Revolution
and they have become more widespread and frequent throughout the
twentieth century. The connection remains controversial however.
[1]
Wilfried Schröder was the first to explain noctilucent clouds as
"indicators" for atmospheric processes (Gerlands Beiträge zur Geophysik,
1971, Meteorologische Rundschau 1968–1970).
Climate models predict that increased greenhouse gas emissions cause a
cooling of the mesosphere, which would lead to more frequent and
widespread occurrences of noctilucent clouds.
[45]
A competing theory is that larger methane emissions from intensive
farming activities produce more water vapour in the upper atmosphere.
[11] Methane concentrations have more than doubled in the past 100 years.
[2]
Tromp et al. suggest that a transition to a
hydrogen economy could increase the number of noctilucent clouds through increased emissions of free hydrogen.
[48]
http://en.wikipedia.org/wiki/Main_Page
/ˈneɪkriːəs/, from nacre, or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (49,000–82,000 ft). They are best observed during civil twilight when the sun is between 1 and 6 degrees below the horizon.[1] They are implicated in the formation of ozone holes[2]
The effects on ozone depletion arise because they support chemical
reactions that produce active chlorine which catalyzes ozone
destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction.[3]
