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Noctilucent cloud

Noctilucent cloud

Overview
Night clouds or Noctilucent clouds are tenuous cloud-like
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds
Polar mesospheric clouds
Polar mesospheric clouds are a diffuse scattering layer of water ice crystals near the summer polar mesopause.-Description:Observed from the ground, this phenomenon is known as noctilucent clouds...

in the upper atmosphere, visible in a deep twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

. They are made of crystals of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

. The name means roughly night shining in Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

. They are most commonly observed in the summer months at latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s between 50° and 70° north and south of the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

.

They are the highest cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s in the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, located in the mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

 at altitudes of around 76 to 85 km (47.2 to 52.8 mi).
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Encyclopedia
Night clouds or Noctilucent clouds are tenuous cloud-like
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds
Polar mesospheric clouds
Polar mesospheric clouds are a diffuse scattering layer of water ice crystals near the summer polar mesopause.-Description:Observed from the ground, this phenomenon is known as noctilucent clouds...

in the upper atmosphere, visible in a deep twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

. They are made of crystals of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

. The name means roughly night shining in Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

. They are most commonly observed in the summer months at latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s between 50° and 70° north and south of the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

.

They are the highest cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s in the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, located in the mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

 at altitudes of around 76 to 85 km (47.2 to 52.8 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
Earth's shadow
The Earth's shadow or Earth shadow are names for the shadow that the Earth itself casts on its atmosphere. This shadow is often visible from the surface of the Earth, as a dark band in the sky near the horizon...

. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

; there is no evidence that they were observed 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. Since 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
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

.

Formation


Night clouds or noctilucent clouds are composed of tiny crystals of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 up to 100 nanometers in diameter and exist at a height of about 76 to 85 km (47.2 to 52.8 mi), 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.

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
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

, although particulates from volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es and dust from the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 are also possibilities. The moisture could be lifted through gaps in the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...

, as well as forming from the reaction of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 with hydroxyl radical
Hydroxyl radical
The hydroxyl radical, •OH, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides or, in...

s in the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

.

The exhaust from Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

s, which is almost entirely water vapour, has been found to generate minuscule individual clouds. About half of the vapour is released into the thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...

, usually at altitudes of 103 to 114 km (64 to 70.8 mi).

This exhaust can be transported to the Arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...

 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
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

 contains very little moisture, approximately one hundred millionth that of air from the Sahara desert, and is extremely thin, the ice crystals can only form at temperatures below about -120 C. This means that noctilucent clouds form predominantly during summer when, counterintuitively, the mesosphere is coldest. 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
Solar cycle
The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...

 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
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 reflectivity, in a frequency range of 50 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 to 1.3 GHz. This behavior is not well understood but Caltech
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...

's Prof. Paul Bellan has proposed a possible explanation: that the ice grains become coated with a thin metal film composed of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, which makes the cloud far more reflective to radar, although this explanation remains controversial. Sodium and iron atoms are stripped from incoming micrometeor
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s 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
1883 eruption of Krakatoa
The 1883 eruption of Krakatoa began in May 1883 and culminated with the destruction of Krakatoa on 27 August 1883. Minor seismic activity continued to be reported until February 1884, though reports after October 1883 were later dismissed by Rogier Verbeek's investigation.-Early phase:In the years...

. It remains unclear whether their appearance had anything to do with the volcano, 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. The theory that the clouds were composed of volcanic dust was finally disproved by Malzev in 1926.
In the years following their discovery the clouds were studied extensively by Otto Jesse of Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, 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
Wilhelm Julius Foerster
Wilhelm Julius Foerster was a German astronomer, father of the pacifist and ethicist Friedrich Wilhelm Foerster. His name can also be written Förster, but is usually written "Foerster" even in most German sources where 'ö' is otherwise used in the text.A native of Grünberg, Silesia, he worked as...

 and Dr. Stolze and, after that year, continuous observations were carried out at the Berlin Observatory
Berlin Observatory
The Berlin Observatory is a series of observatories and related organizations in and around the city of Berlin in Germany, starting from the 18th century...

. During this research the height of the clouds was first determined, via triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

. 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
Orbiting Geophysical Observatory
Orbiting Geophysical Observatory refers to the six satellites launched by the United States that were in use from September 1964 to 1972, designed to study the Earth's magnetosphere. The satellites successfully studied the interactions between the Earth and the Sun, despite a number of technical...

-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
Solar Mesosphere Explorer
The Solar Mesosphere Explorer was an United States unmanned spacecraft to investigate the processes that create and destroy ozone in Earth's upper atmosphere. The mesosphere is a layer of the atmosphere extending from the top of the stratosphere to an altitude of about...

, mapped the distribution of the clouds between 1981 and 1986 with its ultraviolet spectrometer. The clouds were detected with a lidar
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...

 in 1995 at Utah State University
Utah State University
Utah State University is a public university located in Logan, Utah. It is a land-grant and space-grant institution and is accredited by the Northwest Commission on Colleges and Universities....

, even when they were not visible with 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
Upper Atmosphere Research Satellite
The Upper Atmosphere Research Satellite was a NASA-operated orbital observatory whose mission was to study the Earth’s atmosphere, particularly the protective ozone layer. The satellite was deployed from Space Shuttle Discovery during the STS-48 mission on September 15, 1991...

 in 2001.

In 2001 the Swedish
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 Odin
Odin (satellite)
Odin is a Swedish satellite working in two disciplines; astrophysics and aeronomy, and it was named after god Odin. Within the field of astrophysics, Odin is used in the study of star formation...

 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 (Aeronomy of Ice in the Mesosphere
Aeronomy of Ice in the Mesosphere
The Aeronomy of Ice in the Mesosphere is a satellite to conduct a 26-month study of noctilucent clouds . It is the ninetieth Explorer program mission and is part of the NASA-funded Small Explorer program...

) 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
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...

 mission announced that they found clouds of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 crystals over Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 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 only be observed 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
Tunguska event
The Tunguska event, or Tunguska blast or Tunguska explosion, was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 7:14 a.m...

 are evidence that the impact was caused by a comet.

The United States Naval Research Laboratory
United States Naval Research Laboratory
The United States Naval Research Laboratory is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a program of scientific research and development. NRL opened in 1923 at the instigation of Thomas Edison...

 (NRL) and the United States Department of Defense
United States Department of Defense
The United States Department of Defense is the U.S...

 Space Test Program
Space Test Program
The Space Test Program is the primary provider of spaceflight for the United States Department of Defense space science and technology community. STP is conducted by the Space Development and Test Wing of the United States Air Force...

 (STP) conducted the Charged Aerosol Release Experiment
Charged Aerosol Release Experiment
The Charged Aerosol Release Experiment also known as CARE, is a project run by NASA which will use a rocket to release of dust in the upper atmosphere to form a dusty plasma in space. NASA plans to trigger cloud formation around the rocket's exhaust particles...

 (CARE) on September 19, 2009, using exhaust particles from a Black Brant
Black Brant (rocket)
The Black Brant is a Canadian-designed sounding rocket built by Bristol Aerospace in Winnipeg, Manitoba. Over 800 Black Brants of various versions have been launched since they were first produced in 1961, and the type remains one of the most popular sounding rockets ever built...

 XII suborbital sounding rocket launched from NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Wallops Flight Facility
Wallops Flight Facility
Wallops Flight Facility , located on the Eastern Shore of Virginia, is operated by NASA’s Goddard Space Flight Center, primarily as a rocket launch site to support science and exploration missions for NASA and other U.S. government agencies...

 to create an artificial noctilucent cloud. The cloud will 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. The rocket's exhaust plume was observed and reported to news organizations in the United States from New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

 to Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

.

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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 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 can be confused with cirrus cloud
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

s, 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 can be seen by observers at a latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 of 50 to 65 degrees. They seldom occur at lower latitudes (although there have been sightings as far south as Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...

, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

, and Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

) 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 can only be observed in twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

 around sunrise and sunset when the clouds of the lower atmosphere are in shadow but the noctilucent cloud is illuminated by the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. They are best seen when the sun is between 6 and 16 degrees below the horizon. Although noctilucent clouds occur in both hemispheres
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, they have been observed thousands of times in the northern hemisphere but less 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 can 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 can be studied from the ground, from space, and directly by sounding rocket
Sounding rocket
A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The origin of the term comes from nautical vocabulary, where to sound is to throw a weighted line from a ship into...

. 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
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

.

Atmospheric scientist Gary Thomas of the Laboratory for Atmospheric and Space Physics
Laboratory for Atmospheric and Space Physics
The Laboratory for Atmospheric and Space Physics is a research organization at the University of Colorado at Boulder. LASP is a research institute with over 100 research scientists ranging in fields from solar influences, to Earth's and other planetary atmospherics processes, space weather, space...

 at the University of Colorado
University of Colorado at Boulder
The University of Colorado Boulder is a public research university located in Boulder, Colorado...

 has pointed out that the first sightings coincide with the Industrial Revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

 and they have become more widespread and frequent throughout the 20th century. The connection remains controversial however. Wilfried Schröder was the first who explained 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. 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. suggest that a transition to a hydrogen economy
Hydrogen economy
The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors Technical Center....

 could increase the number of noctilucent clouds through increased emissions of free hydrogen.

See also



  • Iridescent Cloud
  • Polar Mesospheric Clouds
    Polar mesospheric clouds
    Polar mesospheric clouds are a diffuse scattering layer of water ice crystals near the summer polar mesopause.-Description:Observed from the ground, this phenomenon is known as noctilucent clouds...

  • Polar stratospheric cloud
    Polar stratospheric cloud
    Polar stratospheric clouds or PSCs, also known as nacreous clouds , are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters...

    s
  • Aeronomy
    Aeronomy
    Aeronomy is the science of the upper region of the atmosphere, where dissociation and ionization are important. The term aeronomy was introduced by Sydney Chapman, and the above definition stems from 1960. Today the term also includes the science of the corresponding regions of the atmospheres of...

  • Aeronomy of Ice in the Mesosphere
    Aeronomy of Ice in the Mesosphere
    The Aeronomy of Ice in the Mesosphere is a satellite to conduct a 26-month study of noctilucent clouds . It is the ninetieth Explorer program mission and is part of the NASA-funded Small Explorer program...


External links