Radiation astronomy/Clouds

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This image shows a cumulus cloud above Lechtaler Alps, Austria. Credit: Glg.
Cumulus clouds in fair weather are white. Credit: Michael Jastremski.

Def. a "large white puffy cloud"[1] is called a cumulus cloud.

Def. "[a] visible mass of

  1. water droplets suspended in the air ...
  2. dust,
  3. steam ...
  4. smoke ...
  5. a group or swarm"[2] is called a cloud.

Cumulus clouds look white because the water droplets reflect and scatter the sunlight without absorbing other colors.

"On any given day, about half of Earth is covered by clouds, which reflect more sunlight than land and water. Clouds keep Earth cool by reflecting sunlight, but they can also serve as blankets to trap warmth."[3]

Interstellar clouds[edit]

Def. an increase in the hydrogen density (nH) of the interstellar medium from ~ 0.01 H cm-3 to ≳ 0.1 H cm-3 is called an interstellar cloud.[4]

HI clouds[edit]

An HI cloud apparently is near or directly in front of the quasar 3C 147. Credit: Hubble Legacy Archive.
The bright areas of this image of the LMC are where the most atomic hydrogen gas is found. Credit: S. Kim et al. / CSIRO.

Def. an interstellar cloud composed primarily of neutral atomic hydrogen is called an HI cloud, H I cloud, or HI region.

"Although there is a possibility that we are seeing the edge of a larger feature, we may be seeing a cloud of higher density superposed on a slowly varying background. If one assumes that to be the case, one finds that the H I cloud has a column density 1020 atoms cm-2 at maximum (assuming an arbitrary kinetic temperature of 50 K and a half-width of 2 km s-1). Although one cannot determine the distance to the absorbing cloud, one can estimate a reasonable upper limit. The quasar 3C 247 [in the image on the right] lies at galactic latitude 100; the assumption of a hydrogen layer extending 100 pc above the plane leads to a maximum probable distance of 600 pc. The linear diameter of the cloud (if the angular diameter is taken to be 0.1") is then at most 3 x 10-4 pc, or 70 AU! The neutral hydrogen density is 105 atoms cm-3; the mass, 3 x 10-7 M."[5]

Galaxies "around us are hiding about a third more atomic hydrogen gas than previously calculated."[6]

The neutral atomic hydrogen "gas is distributed very differently from how it was in the past, with much less in the galaxies’ outer suburbs than billions of years ago."[7]

“This means that it’s much harder for galaxies to pull the gas in and form new stars. It’s why stars are forming 20 times more slowly now than in the past.”[7]

“Even though there’s more atomic hydrogen than we thought, it’s not a big enough percentage to solve the Dark Matter problem. If what we are missing had the weight of a large kangaroo, what we have found would have the weight of a small echidna.”[7]

HI shells[edit]

The image shows an HI shell surrounding the magnetar 1E 1048.1-5937. Credit: B. M. Gaensler, N. M. McClure-Griffiths, S. Oey, M. Haverkorn, J. Dickey, and A. Green.

"The Southern Galactic Plane Survey (SGPS; see the 2002 Annual Report), which combines 21-cm HI observations from Parkes and the Compact Array, is now complete. The SGPS provides a wonderful resource for understanding populations such as magnetars in the context of their environment. Examination of SGPS data around the position of the well-known magnetar 1E 1048.1­5937 reveals a striking cavity in HI, designated as GSH 288.3-0.5-28, that is almost centred on the position of the neutron star. The SGPS data imply that GSH 288.3-0.5-28 is at a distance of approximately 2.7 kpc, and is expanding at a velocity of approximately 7.5 kilometres per second into gas of density ~17 atoms cm-3."[8]

"Shells like GSH 288.3-0.5-28 are common, and represent wind-blown bubbles powered by massive stars expanding into the interstellar medium. The size and expansion speed of GSH 288.3-0.5-28 then imply that the bubble is several million years old, and has been blown by a wind of mechanical luminosity ~4 x 1034 ergs per second, corresponding to a single star of initial mass 30 to 40 solar masses."[8]

"Usually in such cases, the central star is obvious, in the form of a bright O star, supergiant or WR star at the shell's centre. However, even though this field lies in the rich Carina OB1 region, there are no known stars of the appropriate position, distance or luminosity to argue for an association with GSH 288.3-0.5-28. This raises the intriguing possibility that GSH 288.3-0.5-28 was blown by the massive star whose collapse formed 1E 1048.1-5937. The central location of the magnetar within the HI shell suggests that the supernova occurred quite recently. The corresponding blast waves would impact the walls of the HI shell approximately 3000 years after core collapse, producing significant X-ray and radio emission. The lack of such emission requires the neutron star to be very young, consistent with the small ages expected for active magnetars. A common distance of around three kpc is suggested by the properties of both objects."[8]

HII clouds[edit]

This shows NGC 3603, Giant HII cloud and its Core cluster HD97950. Credit: Robert Gendler, NASA/ESA Hubble Space Telescope.
The NASA/ESA Hubble Space Telescope has imaged a violent stellar nursery called NGC 2174. Credit: ESA/Hubble & NASA.

In the upper image on the right, the reddish region is a giant HII cloud.

Def. an interstellar cloud in which the primary constituent is monatomic hydrogen undergoing ionization and emission is called an HII cloud.

"The nebula [in the second image down on the right] is mostly composed of hydrogen gas, which is ionised by the ultraviolet radiation emitted by the hot stars, leading to the nebula’s alternative title as an HII region. This picture shows only part of the nebula, where dark dust clouds are strikingly silhouetted against the glowing gas."[9]

"NGC 2174 lies about 6400 light-years away in the constellation of Orion (The Hunter)."[9]

"This picture was created from images from the Wide Field Planetary Camera 2 on Hubble. Images through four different filters were combined to make the view shown here. Images through a filter isolating the glow from ionised oxygen (F502N) were coloured blue and images through a filter showing glowing hydrogen (F656N) are green. Glowing ionised sulphur (F673N) and the view through a near-infrared filter (F814W) are both coloured red. The total exposure times per filter were 2600 s, 2600 s, 2600 s and 1000 s respectively and the field of view is about 1.8 arcminutes across."[9]

"The Maryland-Green Bank hydrogen-line survey maps reveal this feature [the emission nebula surrounding NGC 2175] as part of a large neutral hydrogen cloud in the galactic plane that is situated at the edge of the association Gem.I. It is most unlikely that such a large neutral hydrogen cloud would be connected with the emission nebula surrounding NGC 2175. Indeed, in a medium with a mean density of hydrogen atoms of 20 cm-3, the Strömgren radius of an HII region around an O6-type star would be more than 16 pc.* However, if a distance of 2 kpc is accepted, the linear radius of the full extent of the continuum source is less than 10 pc. Thus the ionized nebula is density bounded rather than ionization bounded, its small size implying that it is not part of a large neutral hydrogen cloud which would be ionized by radiation from the O6-type star."[10]

Molecular clouds[edit]

This image shows a colour composite of visible and near-infrared images of the dark cloud Barnard 68. Credit: ESO.

Def. a "large and relatively dense cloud of cold gas and dust in interstellar space from which new stars are formed"[11] is called a molecular cloud.

The image on the right is a composite of visible (B 440 nm and V 557 nm) and near-infrared (768 nm) of the dark cloud (absorption cloud) Barnard 68.[12]

Barnard 68 is around 500 lyrs away in the constellation Ophiuchus.[12]

"At these wavelengths, the small cloud is completely opaque because of the obscuring effect of dust particles in its interior."[12]

"It was obtained with the 8.2-m VLT ANTU telescope and the multimode FORS1 instrument in March 1999."[12]


This image of the Snake Nebula contains globules. Credit: Friendlystar.

Def. a small, isolated round dark cloud is called a globule.

"By comparing the properties of globules with and without star formation one can study the processes that lead to star formation in molecular clouds."[13]

The "Thumbprint Nebula (TPN) in the Chamaeleon III region" is "a globule without any signs of star formation".[13]

The "globule DC 303.8-14.2 (Hartley et al. 1986) [is] located in the eastern part of the Chamaeleon II dark cloud complex" and is "a star forming globule".[13]

Cometary globules[edit]

The flower-like image is of cometary globule CG4. Credit: T.A. Rector/University of Alaska Anchorage, T. Abbott and NOAO/AURA/NSF.

Def. "a dense dust cloud with a faint luminous tail" is called a cometary globule.[14]

The image on the right shows a flower-like cometary globule.

Circumstellar clouds[edit]

Astronomers use polarized light to map the hypergiant star VY Canis Majoris. Credit: NASA, ESA, and R. Humphreys (University of Minnesota).
This is a visible light image of VY Canis Majoris. Credit: NASA, ESA, and N. Smith (University of Arizona).

Def. an interstellar-like cloud apparently surrounding or in orbit around a star is called a circumstellar cloud.

"VY Canis Majoris [a red hypergiant star is] an irregular pulsating variable [that] lies about 5,000 light-years away in the constellation Canis Major."[15]

"Although VY Can is about half a million times as luminous as the Sun, much of its visible light is absorbed by a large, asymmetric cloud of dust particles that has been ejected from the star in various outbursts over the past 1,000 years or so. The infrared emission from this dust cloud makes VY Can one of the brightest objects in the sky at wavelengths of 5–20 microns."[15]

"In 2007, a team of astronomers using the 10-meter radio dish on Mount Graham, in Arizona, found that VY Can's extended circumstellar cloud is a prolific molecule-making factory. Among the radio emissions identified were those of hydrogen cyanide (HCN), silicon monoxide (SiO), sodium chloride (NaCl) and a molecule, phosphorus nitride (PN), in which a phosphorus atom and a nitrogen atom are bound together. Phosphorus-bearing molecules are of particular interest to astrobiologists because phosphorus is relatively rare in the universe, yet it is a key ingredient in molecules that are central to life as we know it, including the nuclei acids DNA and RNA and the energy-storage molecule, ATP. "[15]

"Material ejected by the star is visible in this 2004 image [on the top right] captured by the Hubble Space Telescope's Advanced Camera for Surveys, using polarizing filters."[15]

For comparison, the second image down on the right is captured using visuals.

High-velocity clouds[edit]

Smith's Cloud is a hydrogen gas, high-velocity cloud on the outskirts of the Milky Way Galaxy. Credit: Bill Saxton, NRAO/AUI/NSF.

Def. any cloud having a velocity "inconsistent with simple Galactic rotation models that generally fit the stars and gas in the Milky Way disk" is called a high-velocity cloud.[16]

"The leading edge of this cloud [shown in the image on the right] is already interacting with gas from our Galaxy."[17]

"The cloud, called Smith's Cloud, after the astronomer who discovered it in 1963, contains enough hydrogen to make a million stars like the Sun. Eleven thousand light-years long and 2,500 light-years wide, it is only 8,000 light-years from our Galaxy's disk. It is careening toward our Galaxy at more than 150 miles per second, aimed to strike the Milky Way's disk at an angle of about 45 degrees."[18]

"This is most likely a gas cloud left over from the formation of the Milky Way or gas stripped from a neighbor galaxy. When it hits, it could set off a tremendous burst of star formation. Many of those stars will be very massive, rushing through their lives quickly and exploding as supernovae. Over a few million years, it'll look like a celestial New Year's celebration, with huge firecrackers going off in that region of the Galaxy."[17]

"If you could see this cloud with your eyes, it would be a very impressive sight in the night sky. From tip to tail it would cover almost as much sky as the Orion constellation. But as far as we know it is made entirely of gas -- no one has found a single star in it."[17]

"Its shape, somewhat similar to that of a comet, indicates that it's already hitting gas in our Galaxy's outskirts. It is also feeling a tidal force from the gravity of the Milky Way and may be in the process of being torn apart. Our Galaxy will get a rain of gas from this cloud, then in about 20 to 40 million years, the cloud's core will smash into the Milky Way's plane."[17]

Outflow clouds[edit]

The image shows three quasars A, B and C, each of which also has outflow clouds. Credit: Halton Arp.

Def. an interstellar-like or intergalactic-like cloud appearing to outflow from a quasar is called an outflow cloud.

The image on the right labels three quasars that have outflow clouds associated with them. The other objects labeled are nearby stars.


Relationship exits between the atmosphere and ionosphere. Credit: Bhamer.{{free media}}
Diagram of Earth's atmosphere is adapted from NASA document. Credit: Minesweeper.{{free media}}
Ionospheric layers are the E layer and F layer are present at night, during the day, a D layer forms and the E and F layers become much stronger, often during the day the F layer will differentiate into F1 and F2 layers. Credit: Naval Postgraduate School.{{free media}}

From 1972 to 1975 NASA launched the AEROS and AEROS B satellites to study the F region.[19] "The Es layer (sporadic E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, rarely up to 225 MHz."[20]

"The total time for transport of metal ions from the equatorial E region to the higher latitudes (within ± 30" magnetic latitude) of the F region must not exceed about 12 hours if the entire "circulation" process is to occur during the time the fountain effect is operative. This requirement seems unnecessary in that the "reverse fountain effect" which occurs when the daytime eastward E field reverses to the west is weaker than the daytime fountain (WOODMAN et al., 1977) thus leading to an apparent daily net positive flux of metal ions into the equatorial F region from the equatorial E region. Some evidence for this "pulsed" source of metal ions is found in the observed "clouds" of Mg+ reported by MENDE et al., (1985) and possibly by KUMAR and HANSON (1980)."[21]

During solar proton events, ionization can reach unusually high levels in the D-region over high and polar latitudes, known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region.[22]

"Dust quite probably plsys a major role in noctilucent cloud formation (TURCO et al., 1982) and possibly modifies D region ion chemistry (eg. PARTHASARATHY, 1976)."[21]

"Dust has long been considered important to the formation of noctiluent clouds at high latitudes. TURCO et al., (1982) extensively treats the problem of noctilucent cloud formation including effects of ion attachment to dust or ice particles. PARTHASARATHY (1976) has considered dust a direct "sink" for D region ionization."[21]

"[N]octilucent clouds are not an aspect of low and mid-laditude D region aeronomy."[21]


Cumuliform cloudscape is over Swifts Creek, Victoria, Australia. Credit: Fir0002.

In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, ice crystals, or other particulates suspended in the atmosphere of a planetary body.[23]

Def. the "branch of meteorology that studies clouds"[24] is called nephology.

Forms and levels Stratiform
mostly non-convective
strong convective
(Extreme level)
Noctilucent clouds
(Polar mesospheric clouds)
(Very high level)
Polar stratospheric clouds
Cirrostratus clouds Cirrus clouds Cirrocumulus clouds
(Mid-level) Altostratus clouds Altocumulus clouds
(Low-level) Stratus clouds Stratocumulus clouds Cumulus humilis
Multi-level/vertical Nimbostratus clouds Cumulus mediocris
Towering vertical Cumulus congestus Cumulonimbus clouds
Surface-level Fog

Noctilucent clouds[edit]

Noctilucent cloud appears over Estonia. Credit: Martin Koitmäe.

Def. "very high-altitude[25] [shining or glowing at night;[26] nightshining[27]] clouds that reflect sunlight long after sunset"[28] are called noctilucent clouds.

Noctilucent clouds may occasionally take on more of a red or orange hue.[29]

They are not common or widespread enough to have a significant effect on climate.[30]

An increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.[31]

Noctilucent clouds are the highest in the atmosphere and form near the top of the mesosphere at about ten times the altitude of tropospheric high clouds.[32]

Convective lift in the mesosphere is strong enough during the polar summer to cause adiabatic cooling of small amount of water vapour to the point of saturation which tends to produce the coldest temperatures in the entire atmosphere just below the mesopause resulting in the best environment for the formation of polar mesospheric clouds.[30]

Smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud.[33]

Sightings are rare more than 45 degrees south of the north pole or north of the south pole.[29]

"The mesopause occurs, by definition, at the top of the mesosphere and at the bottom of the thermosphere. Noctilucent clouds appear always in the vicinity of the mesopause."[34]

See also[edit]


  1. cumulus. San Francisco, California: Wikimedia Foundation, Inc. February 8, 2013. Retrieved 2013-02-17.
  2. cloud. San Francisco, California: Wikimedia Foundation, Inc. February 13, 2013. Retrieved 2013-02-18.
  3. Baffled Scientists Say Less Sunlight Reaching Earth. LiveScience. 2006-01-24. Retrieved 2011-08-19.
  4. Alfred Vidal-Madjar, Claudine Laurent, and Paul Bruston (15 July 1978). "Is the solar system entering a nearby interstellar cloud". The Astrophysical Journal 223 (07): 589-600. doi:10.1086/156294. http://adsabs.harvard.edu/abs/1978ApJ...223..589V. Retrieved 2015-09-30. 
  5. N. H. Dieter and W. J. Welch and J. D. Romney (1 June 1976). "A very small interstellar neutral hydrogen cloud observed with VLBI techniques". The Astrophysical Journal 206 (06): L113-5. doi:10.1086/182145. http://adsabs.harvard.edu/abs/1976ApJ...206L.113D. Retrieved 2015-10-05. 
  6. Anne's Astronomy News (31 May 2012). There’s More Star-Stuff Out There But It’s Not Dark Matter. .com: BeforeItsNews. Retrieved 2015-10-05.
  7. 7.0 7.1 7.2 Robert Braun (31 May 2012). There’s More Star-Stuff Out There But It’s Not Dark Matter. .com: BeforeItsNews. Retrieved 2015-10-05.
  8. 8.0 8.1 8.2 B. M. Gaensler (2004). A wind bubble around a magnetar. Australia Telescope National Facility. Retrieved 2015-10-06.
  9. 9.0 9.1 9.2 potw1106a (7 February 2011). Fiery young stars wreak havoc in stellar nursery. Baltimore, Maryland: Space Telescope. Retrieved 2015-10-06.
  10. H. M. Tovmassian and E. T. Shahbazian (June 1973). "Hydrogen Content of Young Stellar Clusters II.* Clusters NGC 2175, 2264, and 2362". Australian Journal of Physics 26 (6): 837-42. doi:10.1071/PH730837. http://www.publish.csiro.au/?act=view_file&file_id=PH730837.pdf. Retrieved 2015-10-06. 
  11. SemperBlotto (20 April 2006). molecular cloud. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2015-09-30.
  12. 12.0 12.1 12.2 12.3 eso0102 (10 January 2001). How to Become a Star. European Southern Observatory. Retrieved 2015-09-30.
  13. 13.0 13.1 13.2 K. Lehtinen (January 1997). "Spectroscopic evidence of mass infall towards an embedded infrared source in the globule DC 303.8-14.2". Astronomy and Astrophysics 317 (01): L5-9. http://adsabs.harvard.edu/full/1997A%26A...317L...5L. Retrieved 2015-09-30. 
  14. P. W. J. L. Brand, T. G. Hawarden, A. J. Longmore, P. M. Williams and J. A. R. Caldwell (1983). "Cometary Globule 1". Monthly Notices of the Royal Astronomical Society 203 (1): 215-22. doi:10.1093/mnras/203.1.215. http://mnras.oxfordjournals.org/content/203/1/215.short. Retrieved 2015-09-30. 
  15. 15.0 15.1 15.2 15.3 David Darling (2007). VY Canis Majoris. Encyclopedia of Science. Retrieved 7 October 2015.
  16. Hugo van Woerden, Ulrich J. Schwarz, Reynier F. Peletier, Bart P. Wakker and Peter M. W. Kalberla (8 July 1999). "A confirmed location in the Galactic halo for the high-velocity cloud 'chain A'". Nature 400 (6740): 138-41. http://www.nature.com/nature/journal/v400/n6740/abs/400138a0.html. Retrieved 2015-10-03. 
  17. 17.0 17.1 17.2 17.3 Felix J. Lockman (11 January 2008). Massive Gas Cloud Speeding Toward Collision With Milky Way. National Radio Astronomy Observatory (NRAO). Retrieved 2015-10-03.
  18. Dave Finley (11 January 2008). Massive Gas Cloud Speeding Toward Collision With Milky Way. National Radio Astronomy Observatory (NRAO). Retrieved 2015-10-03.
  19. Yenne, Bill (1985). The Encyclopedia of US Spacecraft. Exeter Books (A Bison Book), New York. ISBN 978-0-671-07580-4. p. 12 AEROS
  20. Reddi (7 February 2004). Ionosphere. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 7 February 2019.
  21. 21.0 21.1 21.2 21.3 J. D. Mathews (1988). Some aspects of metallic ion chemistry and dynamics in the mesosphere and thermosphere (PDF). NASA. pp. 228–254. Retrieved 7 February 2019.
  22. Rose, D.C.; Ziauddin, Syed (June 1962). "The polar cap absorption effect". Space Science Reviews 1 (1): 115. doi:10.1007/BF00174638. 
  23. "Weather Terms". National Weather Service. Retrieved 21 June 2013.
  24. Widsith (17 June 2006). nephology. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 5 February 2019.
  25. WikiPedant (22 August 2008). noctilucent. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 6 February 2019.
  26. Eean (28 November 2004). noctilucent. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 6 February 2019.
  27. DerekWinters (20 September 2015). noctilucent. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 6 February 2019.
  28. SemperBlotto (6 July 2007). noctilucent. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 6 February 2019.
  29. 29.0 29.1 World Meteorological Organization, ed. (2017). "Upper atmospheric clouds, International Cloud Atlas". Retrieved 31 July 2017.
  30. 30.0 30.1 Turco, R. P.; Toon, O. B.; Whitten, R. C.; Keesee, R. G.; Hollenbach, D. (1982). "Noctilucent clouds: Simulation studies of their genesis, properties and global influences". Planetary and Space Science 30 (11): 1147–1181. doi:10.1016/0032-0633(82)90126-X. 
  31. Project Possum, ed. (2017). "About Noctiluent Clouds". Retrieved 6 April 2018.
  32. Michael Gadsden; Pekka Parviainen (September 2006). Observing Noctilucent Clouds (PDF). International Association of Geomagnetism & Aeronomy. p. 9. Retrieved 31 January 2011.
  33. Fox, Karen C. (2013). "NASA Sounding Rocket Observes the Seeds of Noctilucent Clouds". Retrieved 1 October 2013.
  34. Michael Gadsden and Wilfried Schröder (1989). Noctilucent Clouds, In: Noctilucent Clouds. 18. Berlin: Springer. pp. 1-12. doi:10.1007/978-3-642-48626-5_1. ISBN 978-3-642-48628-9. https://link.springer.com/chapter/10.1007/978-3-642-48626-5_1. Retrieved 7 February 2019. 

External links[edit]