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Noctilucent cloud

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Polar mesospheric cloud
Noctilucent clouds over Lake Saimaa.
AbbreviationNLC/PMC
Altitude75,000 to 85,000 m
(250,000 to 280,000 ft)
ClassificationOther
PrecipitationNo

Noctilucent clouds, also known as polar mesospheric clouds, are cloud-like phenomena in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly 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 are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (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 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 their discovery the occurrence of noctilucent clouds has been increasing in frequency, brightness and extent. It is theorised that this increase is connected to climate change.

Formation

Noctilucent clouds are composed of tiny crystals of water ice 40 to 100 nanometers in diameter[1] and exist at a height of about 76 to 85 kilometers (47 to 53 mi),[2] higher than any other clouds in Earth's atmosphere.[3] Much like the more familiar lower altitude clouds, the noctilucent clouds are formed from water collecting on the surface of dust particles.[4] 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 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.[5]

The exhaust from Space Shuttles, which is almost entirely water vapour, has been found to generate individual clouds. About half of the vapor is released into the thermosphere, usually at altitudes of 103 to 114 kilometers (64 to 71 mi).[6]

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.[7] 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.[5]

As the mesosphere contains very little moisture, approximately one hundred millionth that of air from the Sahara desert,[8] and is extremely thin, the ice crystals can only form at temperatures below about −120 °C (−184 °F).[5] This means that noctilucent clouds form predominantly during summer when, counterintuitively, the mesosphere is coldest.[9] Noctilucent clouds form mostly near the polar regions[4], because the mesosphere is coldest there.[10] Clouds in the southern hemisphere are about 1km higher up than those in the northern hemisphere.[4]

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.[11]

Noctilucent clouds are known to exhibit high radar reflectivity[9], in a frequency range of 50MHz to 1.3GHz.[12] This behaviour is not well understood but Caltech's Prof. 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.[9]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 vapor can rapidly be deposited onto an ice surface.[9]

Discovery and investigation

Noctilucent clouds were first observed in 1885, two years after the volcanic eruption of Krakatoa.[4] 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.[10] 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.[8] The theory that the clouds were composed of volcanic dust was finally disproved by Malzev in 1926.[13]

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"[14], 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.[15] Systematic photographic observations of the clouds were organized in 1887 by Jesse, Foerster and Dr. Stolze and, after that year, continuous observations were carried out at the Berlin Observatory.[16] During this research the height of the clouds was first determined, via triangulation.[17] The project was discontinued in 1896.

In the decades after Otto Jesse's death in 1901, there were few new insights into the nature noctilucent clouds. Wegener's conjecture that they were composed of water ice was later shown to be correct.[18] 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.[19]

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.[20] A later satellite, the Solar Mesosphere Explorer, mapped the distribution of the clouds between 1981 and 1986 with its ultraviolet spectrometer.[21] The first physical confirmation that water ice is indeed the primary component of noctilucent clouds came from the HALOE instrument on the UARS satellite in 2001.[22] In 2001 the Swedish Odin satellite performed spectral analyses on the clouds, and produced daily global maps that revealed large patterns in their distribution.[23]

On April 25, 2007, the AIM satellite (Aeronomy of Ice in the Mesosphere) was launched.[24] It is the first satellite dedicated to studying noctilucent clouds,[25] and made its first observations on May 25, 2007.[26] 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 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 only be observed when the Sun is below the horizon.[27]

Observation

Noctilucent clouds are generally colourless or pale blue[28] although occasionally other colours including red and green occur.[29] The characteristic blue colour comes from absorption by ozone in the path of the sunlight illuminating the noctilucent cloud.[30] They can appear as featureless bands[28] but frequently show distinctive patterns such as streaks, wave-like undulations and whirls.[31] They are considered a "beautiful natural phenomenon".[32]Noctilucent clouds can be confused with cirrus clouds, but appear sharper under magnification.[28] Those caused by rocket exhausts tend to show colours other than silver or blue[33] because of iridescence caused by the similar sizes of water droplets produced.[34]

Noctilucent clouds photographed by the crew of the ISS.

Noctilucent clouds can be seen by observers at a latitude of 50 to 60 degrees.[35] They seldom occur at lower latitudes (although there have been sightings as far south as Utah and Italy)[28] and, closer to the poles, it does not get dark enough for the clouds to become visible.[36] 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.[28] They are very faint and tenuous, and can 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.[36] They are best seen when the sun is between 6 and 16 degrees below the horizon.[37] Although noctilucent clouds occur in both hemispheres, 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.[38][10]

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.[39]

They can be studied from the ground, from space, and directly by sounding rocket. Also, some also noctilucent clouds are made of smaller crystals, 30nm 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.[40] Atmospheric scientist Gary Thomas of 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 20th century. The connection remains controversial however.[1]

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.[38] A competing theory is that larger methane emissions from intensive farming activities produce more water vapour in the upper atmosphere-[10] 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.[41]

See also

Wikimedia Commons has media related to Noctilucent clouds.

Notes

  1. ^ a b c d e f Phillips, Tony (August 25, 2008). "Strange Clouds at the Edge of Space". NASA.
  2. ^ a b Hsu, Jeremy (2008-09-03). "Strange clouds spotted at the edge of Earth's atmosphere". USAtoday. {{cite web}}: Check date values in: |date= (help)
  3. ^ Simons, Paul (2008-05-12). "Mysterious noctilucent clouds span the heavens". TimesOnline. Retrieved 2008-10-06. {{cite news}}: Check date values in: |date= (help)
  4. ^ a b c d Chang, Kenneth (2007-07-24). "First Mission to Explore Those Wisps in the Night Sky". New York Times. Retrieved 2008-10-05. {{cite news}}: Check date values in: |date= (help)
  5. ^ a b c About NLCs, Polar Mesospheric Clouds, from Atmospheric optics
  6. ^ "Study Finds Space Shuttle Exhaust Creates Night-Shining Clouds" (Press release). Naval Research Laboratories. 2003-03-06. Retrieved 2008-10-19.
  7. ^ "STUDY FINDS SPACE SHUTTLE EXHAUST CREATES NIGHT-SHINING CLOUDS". NASA. 2003-06-03. Retrieved 2008-10-05. {{cite web}}: Check date values in: |date= (help)
  8. ^ a b Phillips, Tony (2003-02-19). "Strange Clouds". NASA. Retrieved 2008-10-05. {{cite web}}: Check date values in: |date= (help)
  9. ^ a b c d "Caltech Scientist Proposes Explanation for Puzzling Property of Night-Shining Clouds at the Edge of Space" (Press release). Caltech. 09-25-2008. Retrieved 2008-10-19. {{cite press release}}: Check date values in: |date= (help)
  10. ^ a b c d "Noctilucent clouds". Australian Antarctic Division.
  11. ^ Cole, Stephen (2007-03-14). "AIM at the Edge of Space". NASA. {{cite web}}: Check date values in: |date= (help)
  12. ^ "Project Studies Night Clouds, Radar Echoes". ECE News. Virginia Tech: 3. Fall 2003. Retrieved 2008-10-19.
  13. ^ Bergman, Jennifer (2004-08-17). "History of Observation of Noctilucent Clouds". Retrieved 2008-10-06. {{cite web}}: Check date values in: |date= (help)
  14. ^ Schröder, Wilfried. "On the Diurnal Variation of Noctilucent Clouds". German Commission on History of Geophysics and Cosmical Physics. Retrieved 2008-10-06.
  15. ^ Schröder (2001), p.2457
  16. ^ Schröder (2001), p.2459
  17. ^ Schröder (2001), p.2460
  18. ^ Keesee, Bob. "Noctilucent Clouds". University of Albany. Retrieved 2008-10-19.
  19. ^ Schröder (2001), p.2464
  20. ^ Gadsden (1995), p.18.
  21. ^ Gadsden (1995), p.18.
  22. ^ Hervig, Mark; Thompson, Robert E.; McHugh, Martin; Gordley, Larry L.; Russel, James M.; Summers, Michael E. (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. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  23. ^ 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: 1921. Bibcode:2004cosp...35.1921K. Retrieved 2008-10-16.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  24. ^ "Launch of AIM Aboard a Pegasus XL Rocket". NASA. Retrieved 2008-10-19.
  25. ^ NASA/Goddard Space Flight Center Scientific Visualization Studio. "The First Season of Noctilucent Clouds from AIM". NASA. Retrieved 2008-10-19.
  26. ^ O'Carroll, Cynthia (2007-06-28). "NASA Satellite Captures First View of 'Night-Shining Clouds". {{cite web}}: Check date values in: |date= (help)
  27. ^ SPACE.com staff (2006-08-28). "Mars Clouds Higher Than Any On Earth". SPACE.com. Retrieved 2008-10-19.
  28. ^ a b c d e Cowley, Les. "Noctilucent Clouds, NLCs". Atmospheric Optics. Retrieved 2008-10-18.
  29. ^ Gadsden (1995), p.13.
  30. ^ Gadsen, M. (1975). "Observations of the colour and polarization of noctilucent clouds". Annales de Geophysique. 31: 507–516. Bibcode:1975AnG....31..507G. {{cite journal}}: Unknown parameter |month= ignored (help)
  31. ^ Gadsden (1995), pp.8-10.
  32. ^ Gadsden (1995), p.9.
  33. ^ Gadsden (1995), p.13.
  34. ^ "Rocket Trails". Atmospheric Optics. Retrieved 2008-10-19.
  35. ^ Gadsden (1995), p.8.
  36. ^ a b Giles, Bill. "Nacreous and Noctilucent Clouds". BBC Weather. Retrieved 2008-10-05.
  37. ^ Gadsden (1995), p.11.
  38. ^ a b A. Klekociuk, R. Morris, J. French (2008). "First Antarctic ground-satellite view of ice aerosol clouds at the edge of space". Australian Antarctic Division. Retrieved 2008-10-19.{{cite web}}: CS1 maint: multiple names: authors list (link)
  39. ^ Gadsden (1995), pp.9-10.
  40. ^ Thomas, GE (2001). "Noctilucent clouds as possible indicators of global change in the mesosphere". Advances in Space Research. 28: 939–946. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  41. ^ Tracey K. Tromp, Run-Lie Shia, Mark Allen, John M. Eiler, Y. L. Yung (June 2003). "Potential Environmental Impact of a Hydrogen Economy on the Stratosphere". Science Magazine. 300 (5626): 1740–1742. doi:10.1126/science.1085169. Retrieved 2008-10-19.{{cite journal}}: CS1 maint: multiple names: authors list (link)

References

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