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 be
observed only when the Sun is below the horizon.
They are the highest clouds in Earth's atmosphere, located in the mesosphere at
altitudes of around 76 to 85 kilometres. 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 restricted 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. Formation
Night clouds or noctilucent clouds are composed of tiny crystals of water ice
up to 100 nm in diameter and exist at a height of about 76 to 85 km, higher than
any other clouds in Earth's atmosphere. Clouds in the Earth's lower atmosphere
form when water collects on particles, but mesospheric clouds may form directly
from water vapour in addition to forming on dust particles.
Data from the Aeronomy of Ice in the Mesosphere satellite suggests that
noctilucent clouds require water vapour, dust, and very cold temperatures to
form. 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. The exhaust from Space Shuttles, which
was almost entirely water vapour after the detachment of the Solid Rocket
Booster at a height of about 46 km, was found to generate minuscule individual
clouds. About half of the vapour was released into the thermosphere, usually
at altitudes of 103 to 114 km. In August 2014, a SpaceX Falcon 9 caused
spectacular noctilucent clouds over Orlando, FL.
The 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.
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.
As the mesosphere contains very little moisture, approximately one hundred
millionth that of air from the Sahara desert, and is extremely thin, the ice
crystals can form only at temperatures below about −120 °C. This means that
noctilucent clouds form predominantly during summer when, counterintuitively,
the mesosphere is coldest, therefore they can't be observed inside the Polar
circles because the Sun is never low enough under the horizon at this season
at these latitudes. Noctilucent clouds form mostly near the polar regions,
because the mesosphere is coldest there. Clouds in the southern hemisphere are
about 1 km higher than those in the northern hemisphere.
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.
Noctilucent clouds are known to exhibit high radar reflectivity, in a frequency
range of 50 MHz to 1.3 GHz. This behaviour is not well understood but a
possible explanation is 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, although this explanation remains controversial. 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.
Discovery and investigation Noctilucent clouds are first known to
have been observed in 1885, two years after the 1883 eruption of Krakatoa. 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. 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. Finally, the theory that the clouds were composed of volcanic dust
was disproved by Malzev in 1926. 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", which means "night-shining cloud". 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. 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. During this research, the height of the clouds was
first determined, via triangulation. 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. 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. 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. A later satellite, the
Solar Mesosphere Explorer, mapped the distribution of the clouds between 1981
and 1986 with its ultraviolet spectrometer. The clouds were detected
with a lidar in 1995 at Utah State University, even when they were not
visible to the naked eye. 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.
In 2001, the Swedish Odin satellite performed spectral analyses on the
clouds, and produced daily global maps that revealed large patterns in their
distribution. On April 25, 2007, the AIM satellite was
launched. It is the first satellite dedicated to studying noctilucent
clouds, and made its first observations on May 25, 2007. Images taken by the
satellite show shapes in the clouds that are similar to shapes in tropospheric
clouds, hinting at similarities in their dynamics.
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
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 be
observed only when the Sun is below the horizon.
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.
The United States Naval Research Laboratory and the United States
Department of Defense Space Test Program conducted the Charged Aerosol Release
Experiment 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 instrument on the NRL/STP STPSat-1 spacecraft. The
rocket's exhaust plume was observed and reported to news organizations in the
United States from New Jersey to Massachusetts.
Observation Noctilucent clouds are generally
colourless or pale blue, although occasionally other colours including red
and green occur. The characteristic blue colour comes from absorption by ozone in
the path of the sunlight illuminating the noctilucent cloud. They can appear
as featureless bands, but frequently show distinctive patterns such as
streaks, wave-like undulations, and whirls. They are considered a "beautiful
natural phenomenon". Noctilucent clouds may be confused with cirrus clouds, but
appear sharper under magnification. Those caused by rocket exhausts tend to
show colours other than silver or blue, because of iridescence caused by the
uniform size of the water droplets produced.
Noctilucent clouds may be seen by observers at a latitude of 50° to 65°.
They seldom occur at lower latitudes, and closer to the poles it does not get
dark enough for the clouds to become visible. 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. They are very faint and tenuous, and may
be observed only 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. They are best seen when the Sun is between 6° and 16° below the horizon.
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.
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.
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. 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. The author of
this study, atmospheric scientist Gary Thomas of the Laboratory for Atmospheric
and Space Physics at the University of Colorado has pointed out that the first
sightings coincide with both Krakatoa and the nascent Industrial Revolution,
and they have become more widespread and frequent throughout the twentieth
century, including an uptick between 1964 and 1986. The connecting of global
warming and noctilucent clouds however, remains controversial. Gary Thomas may
have penned his paper after Wilfried Schröder, who might hold the distinction
of being the first to explain noctilucent clouds as "indicators" for
atmospheric processes. Most recently in 2012 Lonnie Cumberland's physics PhD
work supported viewing noctilucent clouds as a possible Miner's Canary for
climate change as her third conclusion as a sign of increasing the presence of
water in the high atmosphere. NASA scientists speculate that methane may be
driven higher into the mesophere where noctilucent clouds form by climate
change and through reactions end up producing water at such altitudes.
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. A competing theory is that larger methane emissions
from intensive farming activities produce more water vapour in the upper
atmosphere. Methane concentrations have more than doubled in the past 100 years.
Tromp et al. also controversially suggest that a transition to a hydrogen
economy would result in an increase in the free hydrogen concentration of the
atmosphere by 1 ppm, which would increase the number of noctilucent
clouds. See also
Aeronomy Aeronomy of ice in the mesosphere
Iridescent cloud Polar mesospheric cloud
Polar stratospheric cloud Notes
References External links
NLC time-lapse movies AIM satellite mission
BBC News Article - Mission to Target Highest Clouds
Noctilucent Cloud Observers' Homepage Solar Occultation for Ice Experiment
Southern Noctilucent Clouds observed at Punta Arenas, Chile
Astronomy Picture of the Day: Noctilucent Clouds Over Sweden
BBC Article - Spacecraft Chases Highest Clouds
CNN Article - Rocket launch prompts calls of strange lights in sky
BBC News - Audio slideshow: Noctilucent clouds
The Harbingers of Change Can Now Be Seen All Around the World! Mysterious
Noctilucent Clouds Brighten Up Night Skies
Noctilucent cloud in Belarus 'Space Clouds' Facebook page - public
engagement in NLC observing and science.