Noctilucent cloud

Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. They consist of ice crystals and are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between 50° and 70° north and south of the Equator. They are visible only during local summer months and when the Sun is below the observer's horizon, but while the clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds.[1]

Noctilucent clouds
Noctilucent clouds over Kuresoo bog,
Viljandimaa, Estonia
AbbreviationNLC/PMC
Altitude76,000 to 85,000 m
(250,000 to 280,000 ft)
ClassificationOther
Precipitation cloud?No

They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 km (47 to 53 mi). They are too faint to be seen in daylight, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in Earth's shadow.

General
Noctilucent clouds over Uppsala, Sweden
Noctilucent clouds over Varbla, Estonia, 13 July 2016
Noctilucent clouds over Landskrona, Sweden, 21 June 2019

Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon. No confirmed record of their observation exists before 1885, although they may have been observed a few decades earlier by Thomas Romney Robinson in Armagh.[2] Doubts now surround Robinson's out-of-season records, following observations, from several points around high northern latitudes, of NLC-like phenomena following the Chelyabinsk superbolide entry in February 2013 (outside the NLC season) that were in fact stratospheric dust reflections visible after sunset.

Noctilucent clouds can form only under very restricted conditions during local summer; 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

Noctilucent clouds are composed of tiny crystals of water ice up to 100 nm in diameter[3] and exist at a height of about 76 to 85 km (47 to 53 mi),[4] higher than any other clouds in Earth's atmosphere.[5] Clouds in the Earth's lower atmosphere form when water collects on particles, but mesospheric clouds may form directly from water vapour[6] in addition to forming on dust particles.[7]

Data from the Aeronomy of Ice in the Mesosphere satellite suggests that noctilucent clouds require water vapour, dust, and very cold temperatures to form.[8] 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.[9]

The exhaust from Space Shuttles, in use between 1981 and 2011, 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 (64 to 71 mi).[10] In August 2014, a SpaceX Falcon 9 also caused noctilucent clouds over Orlando, FL after a launch.[11]

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.[12] 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.[9]

As the mesosphere contains very little moisture, approximately one hundred millionth that of air from the Sahara,[13] and is extremely thin, the ice crystals can form only at temperatures below about −120 °C (−184 °F).[9] This means that noctilucent clouds form predominantly during summer when, counterintuitively, the mesosphere is coldest as a result of seasonally varying vertical winds, leading to cold summertime conditions in the upper mesosphere (upwelling and adiabatic cooling) and wintertime heating (downwelling and adiabatic heating). 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.[14] Noctilucent clouds form mostly near the polar regions,[7] because the mesosphere is coldest there.[14] Clouds in the southern hemisphere are about 1 km (0.62 mi) higher than those in the northern hemisphere.[7]

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.[15]

Noctilucent clouds are known to exhibit high radar reflectivity,[16] in a frequency range of 50 MHz to 1.3 GHz.[17] 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,[16] although this explanation remains controversial.[18] 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.[19]

Discovery and investigation

Noctilucent clouds are first known to have been observed in 1885, two years after the 1883 eruption of Krakatoa.[7][20] It remains unclear whether their appearance had anything to do with the volcanic 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.[14] 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.[13] Finally, the theory that the clouds were composed of volcanic dust was disproved by Malzev in 1926.[20] 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",[21] which means "night-shining cloud".[3] 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.[22] 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.[23] During this research, the height of the clouds was first determined, via triangulation.[24] 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.[25] 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.[26]

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.[27] A later satellite, the Solar Mesosphere Explorer, mapped the distribution of the clouds between 1981 and 1986 with its ultraviolet spectrometer.[27] The clouds were detected with a lidar in 1995 at Utah State University, even when they were not visible to the naked eye.[28] 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.[29]

In 2001, the Swedish Odin satellite performed spectral analyses on the clouds, and produced daily global maps that revealed large patterns in their distribution.[30]

On April 25, 2007, the AIM satellite (Aeronomy of Ice in the Mesosphere) was launched.[31] It is the first satellite dedicated to studying noctilucent clouds,[32] and made its first observations on May 25, 2007.[33] Images taken by the satellite show shapes in the clouds that are similar to shapes in tropospheric clouds, hinting at similarities in their dynamics.[3]

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 be observed only when the Sun is below the horizon.[34]

Research published in the journal Geophysical Research Letters in June 2009 suggests that noctilucent clouds observed following the Tunguska Event of 1908 are evidence that the impact was caused by a comet.[35][36]

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.[37] The rocket's exhaust plume was observed and reported to news organizations in the United States from New Jersey to Massachusetts.[38]

Observation

Noctilucent clouds are generally colourless or pale blue,[39] although occasionally other colours including red and green occur.[40] The characteristic blue colour comes from absorption by ozone in the path of the sunlight illuminating the noctilucent cloud.[41] They can appear as featureless bands,[39] but frequently show distinctive patterns such as streaks, wave-like undulations, and whirls.[42] They are considered a "beautiful natural phenomenon".[43] Noctilucent clouds may be confused with cirrus clouds, but appear sharper under magnification.[39] Those caused by rocket exhausts tend to show colours other than silver or blue,[40] because of iridescence caused by the uniform size of the water droplets produced.[44]

Noctilucent clouds may be seen by observers at a latitude of 50° to 65°.[45] They seldom occur at lower latitudes (although there have been sightings as far south as Paris, Utah, Italy, Turkey and Spain),[39][46][47][48] and closer to the poles it does not get dark enough for the clouds to become visible.[49] 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.[39] 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.[49] They are best seen when the Sun is between 6° and 16° below the horizon.[50] 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.[14][51]

These clouds 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.[3]

Forms

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.[52] As a result of more recent research, the World Meteorological Organization now recognizes four major forms that can be subdivided. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus or poorly defined cirrus.[53] Type II bands are long streaks that often occur in groups arranged roughly parallel to each other. They are usually more widely spaced than the bands or elements seen with cirrocumulus clouds.[54] Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus.[55] Type IV whirls are partial or, more rarely, complete rings of cloud with dark centres.[56]

See also
  • Aeronomy
  • Aeronomy of Ice in the Mesosphere
  • Iridescent cloud
  • Polar mesospheric cloud
  • Polar stratospheric cloud
  • Space jellyfish

Notes
  1. "Climate Change Is Responsible for These Rare High-Latitude Clouds". Smithsonian. 2018.
  2. Robinson made a series of interesting observations between 1849 and 1852, and two of his entries in May 1850 may describe noctilucent clouds. On the 1st of May he notes ‘strange luminous clouds in NW, not auroral'. This does sound very much like NLCs even though early May falls outside the typical NLC 'window'; however it is still possible as NLCs can form at Armagh's latitude within this period.
  3. Phillips, Tony (August 25, 2008). "Strange Clouds at the Edge of Space". NASA. Archived from the original on February 1, 2010.
  4. Hsu, Jeremy (2008-09-03). "Strange clouds spotted at the edge of Earth's atmosphere". USAtoday.
  5. Simons, Paul (2008-05-12). "Mysterious noctilucent clouds span the heavens". TimesOnline. Retrieved 2008-10-06.
  6. Murray, B.J.; Jensen, E.J. (2000). "Homogeneous nucleation of amorphous solid water particles in the upper mesosphere". Journal of Atmospheric and Solar-Terrestrial Physics. 72 (1): 51–61. Bibcode:2010JASTP..72...51M. doi:10.1016/j.jastp.2009.10.007.
  7. Chang, Kenneth (2007-07-24). "First Mission to Explore Those Wisps in the Night Sky". New York Times. Retrieved 2008-10-05.
  8. "Appearance of night-shining clouds has increased". Science Daily. April 11, 2014. Retrieved May 7, 2014.
  9. About NLCs, Polar Mesospheric Clouds, from Atmospheric optics
  10. "Study Finds Space Shuttle Exhaust Creates Night-Shining Clouds" (Press release). Naval Research Laboratories. 2003-03-06. Archived from the original on 2008-09-17. Retrieved 2008-10-19.
  11. https://twitter.com/SpaceX/status/498935052235857921 11 Aug 2014 SpaceX Falcon 9 caused spectacular noctilucent clouds
  12. "STUDY FINDS SPACE SHUTTLE EXHAUST CREATES NIGHT-SHINING CLOUDS". NASA. 2003-06-03. Retrieved 2008-10-05.
  13. Phillips, Tony (2003-02-19). "Strange Clouds". NASA. Archived from the original on 2008-10-12. Retrieved 2008-10-05.
  14. "Noctilucent clouds". Australian Antarctic Division.
  15. Cole, Stephen (2007-03-14). "AIM at the Edge of Space". NASA.
  16. "Caltech Scientist Proposes Explanation for Puzzling Property of Night-Shining Clouds at the Edge of Space" (Press release). Caltech. 2008-09-25. Archived from the original on 2008-09-29. Retrieved 2008-10-19.
  17. "Project Studies Night Clouds, Radar Echoes". ECE News: 3. Fall 2003. Retrieved 2008-10-19.
  18. Rapp, M.; Lubken, F.J. (2009). "Comment on 'Ice iron/sodium film as cause for high noctilucent cloud radar reflectivity' by P. M. Bellan". Geophys. Res. Lett. 114 (D11): D11204. Bibcode:2009JGRD..11411204R. doi:10.1029/2008JD011323.
  19. Murray, B.J.; Plane, J.M.C. (2005). "Uptake of Fe, Na and K atoms on low-temperature ice: implications for metal atom scavenging in the vicinity of polar mesospheric clouds". Phys. Chem. Chem. Phys. 7 (23): 3970–3979. Bibcode:2005PCCP....7.3970M. doi:10.1039/b508846a. PMID 19810327.
  20. Bergman, Jennifer (2004-08-17). "History of Observation of Noctilucent Clouds". Archived from the original on 2009-06-28. Retrieved 2008-10-06.
  21. Schröder, Wilfried. "On the Diurnal Variation of Noctilucent Clouds". German Commission on History of Geophysics and Cosmical Physics. Retrieved 2008-10-06.
  22. Schröder (2001), p.2457
  23. Schröder (2001), p.2459
  24. Schröder (2001), p.2460
  25. Keesee, Bob. "Noctilucent Clouds". University of Albany. Retrieved 2008-10-19.
  26. Schröder (2001), p.2464
  27. Gadsden (1995), p.18.
  28. "Welcome". agu.org.
  29. Hervig, Mark; Thompson, Robert E.; McHugh, Martin; Gordley, Larry L.; Russel, James M.; Summers, Michael E. (March 2001). "First Confirmation that Water Ice is the Primary Component of Polar Mesospheric Clouds". Geophysical Research Letters. 28 (6): 971–974. Bibcode:2001GeoRL..28..971H. doi:10.1029/2000GL012104.
  30. Karlsson, B.; Gumbel, J.; Stegman, J.; Lautier, N.; Murtagh, D.P.; The Odin Team (2004). "Studies of Noctilucent Clouds by the Odin Satellite" (PDF). 35th COSPAR Scientific Assembly. 35: 1921. Bibcode:2004cosp...35.1921K. Retrieved 2008-10-16.
  31. "Launch of AIM Aboard a Pegasus XL Rocket". NASA. Retrieved 2008-10-19.
  32. NASA/Goddard Space Flight Center Scientific Visualization Studio. "The First Season of Noctilucent Clouds from AIM". NASA. Retrieved 2008-10-19.
  33. O'Carroll, Cynthia (2007-06-28). "NASA Satellite Captures First View of 'Night-Shining Clouds".
  34. SPACE.com staff (2006-08-28). "Mars Clouds Higher Than Any On Earth". SPACE.com. Retrieved 2008-10-19.
  35. Kelly, M.C.; C.E. Seyler; M.F. Larsen (2009-06-22). "Two-dimensional turbulence, space shuttle plume transport in the thermosphere, and a possible relation to the Great Siberian Impact Event". Geophysical Research Letters. 36 (14): L14103. Bibcode:2009GeoRL..3614103K. doi:10.1029/2009GL038362.
  36. Ju, Anne (2009-06-24). "A mystery solved: Space shuttle shows 1908 Tunguska explosion was caused by comet". Cornell Chronicle. Cornell University. Retrieved 2009-06-25.
  37. NASA (2009-09-19). "Night Time Artificial Cloud Study Using NASA Sounding Rocket". NASA.
  38. "Rocket launch prompts calls of strange lights in sky". Cable News Network (CNN). 2009-09-20.
  39. Cowley, Les. "Noctilucent Clouds, NLCs". Atmospheric Optics. Retrieved 2008-10-18.
  40. Gadsden (1995), p.13.
  41. Gadsen, M. (October–December 1975). "Observations of the colour and polarization of noctilucent clouds". Annales de Géophysique. 31: 507–516. Bibcode:1975AnG....31..507G.
  42. Gadsden (1995), pp.8–10.
  43. Gadsden (1995), p.9.
  44. "Rocket Trails". Atmospheric Optics. Archived from the original on 2008-08-04. Retrieved 2008-10-19.
  45. Gadsden (1995), p.8.
  46. Hultgren, K.; et al. (2011). "What caused the exceptional mid-latitudinal Noctilucent Cloud event in July 2009?". Journal of Atmospheric and Solar-Terrestrial Physics. 73 (14–15): 2125–2131. Bibcode:2011JASTP..73.2125H. doi:10.1016/j.jastp.2010.12.008.
  47. Tunç Tezel (13 Jul 2008). "NLC Surprise". The World At Night (TWAN). Retrieved 17 July 2014.
  48. Calar Alto Observatory (July 2012). "Noctilucent clouds from Calar Alto". Calar Alto Observatory. Archived from the original on 25 July 2014. Retrieved 17 July 2014.
  49. Giles, Bill (1933). "Nacreous and Noctilucent Clouds". Monthly Weather Review. BBC Weather. 61 (8): 228–229. Bibcode:1933MWRv...61..228H. doi:10.1175/1520-0493(1933)61<228:NANC>2.0.CO;2. Archived from the original on 2008-10-11. Retrieved 2008-10-05.
  50. Gadsden (1995), p.11.
  51. A. Klekociuk; R. Morris; J. French (2008). "First Antarctic ground-satellite view of ice aerosol clouds at the edge of space". Australian Antarctic Division. Archived from the original on 2012-02-25. Retrieved 2008-10-19.
  52. Gadsden (1995), pp.9–10.
  53. World Meteorological Organization, ed. (2017). "Type I Veils, International Cloud Atlas". Retrieved 18 July 2019.
  54. World Meteorological Organization, ed. (2017). "Type II Bands, International Cloud Atlas". Retrieved 18 July 2019.
  55. World Meteorological Organization, ed. (2017). "Type III Billows, International Cloud Atlas". Retrieved 18 July 2019.
  56. World Meteorological Organization, ed. (2017). "Type IV Whirls, International Cloud Atlas". Retrieved 18 July 2019.

References

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