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.

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."[2]

## Theoretical clouds

An emission nebula is shown. Credit: NASA, ESA and Jesœs Maz Apellÿniz (Instituto de astrofsica de Andaluca, Spain). Acknowledgement: Davide De Martin (ESA/Hubble).

Def. a "visible mass of

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

Def. a "cloud in outer space"[4] "consisting of gas or dust (e.g. a"[5] "cloud formed after a star exploded [explodes][6]"[7]) is called a nebula.

## Reds

This region of sky includes glowing red clouds of mostly hydrogen gas. Credit: ESO.

"[T]he extended red emission (ERE) [is] observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. ... silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE."[8]

"The broad, 60 < FWHM < 100 nm, featureless luminescence band known as extended red emission (ERE) is seen in such diverse dusty astrophysical environments as reflection nebulae17, planetary nebulae3, HII regions (Orion)12, a Nova11, Galactic cirrus14, a dark nebula7, Galaxies8,6 and the diffuse interstellar medium (ISM)4. The band is confined between 540-950 nm, but the wavelength of peak emission varies from environment to environment, even within a given object. ... the wavelength of peak emission is longer and the efficiency of the luminescence is lower, the harder and denser the illuminating radiation field is13. These general characteristics of ERE constrain the photoluminescence (PL) band and efficiency for laboratory analysis of dust analog materials."[9]

In interstellar astronomy, visible spectra can appear redder due to scattering processes in a phenomenon referred to as interstellar reddening[10] — similarly Rayleigh scattering causes the atmospheric reddening of the Sun seen in the sunrise or sunset and causes the rest of the sky to have a blue color. This phenomenon is distinct from redshifting because the spectroscopic lines are not shifted to other wavelengths in reddened objects and there is an additional dimming and distortion associated with the phenomenon due to photons being scattered in and out of the line-of-sight.

"The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud. This region of sky includes glowing red clouds of mostly hydrogen gas, blue regions where starlight is being reflected from tiny particles of dust and also dark regions where the dust is thick and opaque."[11]

"The blue section of the photo — representing a "reflection nebula" — shows light from the newly formed stars in the cosmic nursery being reflected in all directions by the particles of dust made of iron, carbon, silicon and other elements in the interstellar cloud."[12]

## Infrareds

The Trifid Nebula is a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius. Credit: NASA/JPL-Caltech/J. Rho (SSC/Caltech).

"The glowing Trifid Nebula [in the image at right] is revealed in an infrared view from NASA's Spitzer Space Telescope. The Trifid Nebula is a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius."[13]

"The false-color Spitzer image reveals a different side of the Trifid Nebula. Where dark lanes of dust are visible trisecting the nebula in a visible-light picture, bright regions of star-forming activity are seen in the Spitzer picture. All together, Spitzer uncovered 30 massive embryonic stars and 120 smaller newborn stars throughout the Trifid Nebula, in both its dark lanes and luminous clouds. These stars are visible in the Spitzer image, mainly as yellow or red spots. Embryonic stars are developing stars about to burst into existence."[13]

"Ten of the 30 massive embryos discovered by Spitzer were found in four dark cores, or stellar "incubators," where stars are born. Astronomers using data from the Institute of Radioastronomy millimeter telescope in Spain had previously identified these cores but thought they were not quite ripe for stars. Spitzer's highly sensitive infrared eyes were able to penetrate all four cores to reveal rapidly growing embryos."[13]

"Astronomers can actually count the individual embryos tucked inside the cores by looking closely at this Spitzer image taken by its infrared array camera (IRAC). This instrument has the highest spatial resolution of Spitzer's imaging cameras. The embryos are thought to have been triggered by a massive "type O" star, which can be seen as a white spot at the center of the nebula. Type O stars are the most massive stars, ending their brief lives in explosive supernovas. The small newborn stars probably arose at the same time as the O star, and from the same original cloud of gas and dust."[13]

"This Spitzer mosaic image uses data from IRAC showing light of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange) and 8.0 microns (red)."[13]

Interstellar dust can be studied by infrared spectrometry, in part because the dust is an astronomical infrared source and other infrared sources are behind the diffuse clouds of dust.[14]

Far-infrared astronomy deals with objects visible in far-infrared radiation (extending from 30 µm towards submillimeter wavelengths around 450 µm).

Huge, cold clouds of gas and dust in our own galaxy, as well as in nearby galaxies, glow in far-infrared light. This is due to thermal radiation of interstellar dust contained in molecular clouds.

The monochromatic flux density radiated by a greybody at frequency ${\displaystyle \nu }$ through solid angle ${\displaystyle \Omega }$ is given by ${\displaystyle F_{\nu }=B_{\nu }(T)Q_{\nu }\Omega }$ where ${\displaystyle B_{\nu }(T)}$ is the Planck function for a blackbody at temperature T and emissivity ${\displaystyle Q_{\nu }}$.

For a uniform medium of optical depth ${\displaystyle \tau _{\nu }}$ radiative transfer means that the radiation will be reduced by a factor ${\displaystyle e^{-\tau _{\nu }}}$. The optical depth is often approximated by the ratio of the emitting frequency to the frequency where ${\displaystyle \tau =1}$ all raised to an exponent β.

For cold dust clouds in the interstellar medium β is approximately two. Therefore Q becomes,

${\displaystyle Q_{\nu }=1-e^{-\tau _{\nu }}=1-e^{-\tau _{0}(\nu /\nu _{0})^{\beta }}}$. (${\displaystyle \tau _{0}=1}$, ${\displaystyle \nu _{0}}$ is the frequency where ${\displaystyle \tau _{0}=1}$).

## Submillimeters

Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, the Herschel Space Observatory, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. The Atacama Large Millimeter Array, under construction, will operate in the submillimeter range. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

"[T]he detection of absorption by interstellar hydrogen fluoride (HF) [in the submillimeter band occurs] along the sight line to the submillimeter continuum sources W49N and W51."[15]

"HF is the dominant reservoir of fluorine wherever the interstellar H2/atomic H ratio exceeds ~ 1; the unusual behavior of fluorine is explained by its unique thermochemistry, F being the only atom in the periodic table that can react exothermically with H2 to form a hydride."[15]

The observations "toward W49N and W51 [occurred] on 2010 March 22 ... The observations were carried out at three different local oscillator (LO) tunings in order to securely identify the HF line toward both sight lines. The dual beam switch mode (DBS) was used with a reference position located 3' on either side of the source position along an East-West axis. We centered the telescope beam at α =19h10m13.2s, δ = 09°06'12.0" for W49N and α = 19h23m43.9s, δ = 14°30'30.5" for W51 (J2000.0). The total on-source integration time amounts to 222s on each source using the Wide Band Spectrometer (WBS) that offers a spectral resolution of 1.1 MHz (~0.3 km s-1 at 1232 GHz)."[15]

"[T]he first detection of chloronium, H2Cl+, in the interstellar medium, [occurred on March 1 and March 23, 2010,] using the HIFI instrument aboard the Herschel Space Observatory. The 212 − 101 lines of ortho-H235Cl+ and ortho-H237Cl+ are detected in absorption towards NGC 6334I, and the 111 − 000 transition of para-H235Cl+ is detected in absorption towards NGC 6334I and Sgr B2(S)."[16]

"The [microwave] detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[17] "H2CO is the first organic polyatomic molecule ever detected in the interstellar medium".[17]

"[A] number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.[18][19]

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."[20]

“[R]adio astronomy ... has resulted in the detection of over a hundred interstellar species, including radicals and ions, and organic (i.e. carbon-based) compounds, such as alcohols, acids, aldehydes, and ketones. One of the most abundant interstellar molecules, and among the easiest to detect with radio waves (due to its strong electric dipole moment), is CO (carbon monoxide). In fact, CO is such a common interstellar molecule that it is used to map out molecular regions.[21] The radio observation of perhaps greatest human interest is the claim of interstellar glycine,[22] the simplest amino acid, but with considerable accompanying controversy.[23] One of the reasons why this detection [is] controversial is that although radio (and some other methods like rotational spectroscopy) are good for the identification of simple species with large dipole moments, they are less sensitive to more complex molecules, even something relatively small like amino acids.

## Solar coronal clouds

This is a coronagraph/polarimeter image of the solar corona on June 29, 1980, in H alpha light. Credit: NASA.

A coronal cloud is a cloud, or cloud-like, natural astronomical entity, composed of plasmas and usually associated with a star or other astronomical object where the temperature is such that X-rays are emitted. While small coronal clouds are above the photosphere of many different visual spectral type stars, others occupy parts of the interstellar medium (ISM), extending sometimes millions of kilometers into space, or thousands of light-years, depending on the size of the associated object such as a galaxy.

"Coronal clouds, type IIIg, form in space above a spot area and rain streamers upon it."[24]

"This energy [1032 to 1033 ergs] appears in the form of electromagnetic radiation over the entire spectrum from γ-rays to radio burst, in fast electrons and nuclei up to relativistic energies, in the creation of a hot coronal cloud, and in large-scale mass motions including the ejections of material from the Sun."[25]

"Coronal clouds are irregular objects suspended in the corona with matter streaming out of them into nearby active regions."[26]

## Venus

Venus in approximately true-color is a nearly uniform pale cream. Credit: NASA/Ricardo Nunes, http://www.astrosurf.com/nunes.
Imaged is the cloud structure in the the Venusian atmosphere in 1979, revealed by ultraviolet observations by Pioneer Venus Orbiter. Credit: .

In visual astronomy almost no variation or detail can be seen in the clouds. The surface is obscured by a thick blanket of clouds. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It has thick clouds of sulfur dioxide. There are lower and middle cloud layers. The thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.[27][28] These clouds reflect and scatter about 90% of the sunlight that falls on them back into space, and prevent visual observation of the Venusian surface. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well lit.

Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth days.[29] Venusian winds move at up to 60 times the speed of the planet's rotation, while Earth's fastest winds are only 10% to 20% rotation speed.[30]

## Earth

Clouds are shown along a jet stream over Canada. Credit: NASA.
Here at Réunion is an example that some of those white puffy objects in the sky may be quite close by. Credit: B.navez.
Cirrus clouds never seem to touch any mountain. Yet sunrise reveals they are closer to the ground than the Sun. Credit: Simon Eugster.
This image shows a late-summer rainstorm in Denmark. The nearly black color of the cloud's base indicates the foreground cloud is probably cumulonimbus. Credit: Malene Thyssen.

The image on the left shows two meteors, the clouds passing over land and the rain falling towards the ground from the clouds above as the water droplets either lose their static charge or reach too large a size to be held aloft either by the natural electric field of the Earth or by air currents, respectively. The water droplets are moving somewhat horizontally and also vertically.

## Nephology

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.[31]

Def. the "branch of meteorology that studies clouds"[32] is called nephology.

Forms and levels Stratiform
non-convective
Cirriform
mostly non-convective
Stratocumuliform
limited-convective
Cumuliform
free-convective
Cumulonimbiform
strong convective
Exosphere
Thermosphere
Mesosphere
(Extreme level)
Noctilucent clouds
(Polar mesospheric clouds)
Stratosphere
(Very high level)
Polar stratospheric clouds
Troposphere
(High-level)
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

Noctilucent cloud appears over Estonia. Credit: Martin Koitmäe.

Def. "very high-altitude[33] [shining or glowing at night;[34] nightshining[35]] clouds that reflect sunlight long after sunset"[36] are called noctilucent clouds.

Noctilucent clouds may occasionally take on more of a red or orange hue.[37]

They are not common or widespread enough to have a significant effect on climate.[38]

An increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.[39]

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.[40]

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.[38]

Smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud.[41]

Sightings are rare more than 45 degrees south of the north pole or north of the south pole.[37]

"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."[42]

## Ionospheres

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.[43] "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."[44]

"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)."[45]

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.[46]

"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)."[45]

"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."[45]

"[N]octilucent clouds are not an aspect of low and mid-laditude D region aeronomy."[45]

## Mars

This Hubble Space Telescope image shows a dust storm, just above center and lighter in contrast than the surface of Mars. Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA), J. Bell (Cornell University) and M. Wolff (Space Science Institute).

At right is a Hubble Space Telescope image of a dust storm on Mars. The picture was snapped on October 28, 2005. The regional dust storm on Mars had "been growing and evolving over the past few weeks. The dust storm, which is nearly in the middle of the planet in this Hubble view is about 930 miles (1500 km) long measured diagonally, which is about the size of the states of Texas, Oklahoma, and New Mexico combined. No wonder amateur astronomers with even modest-sized telescopes have been able to keep an eye on this storm. The smallest resolvable features in the image (small craters and wind streaks) are the size of a large city, about 12 miles (20 km) across. The occurrence of the dust storm is in close proximity to the NASA Mars Exploration Rover Opportunity's landing site in Sinus Meridiani. Dust in the atmosphere could block some of the sunlight needed to keep the rover operating at full power. ... The large regional dust storm appears as the brighter, redder cloudy region in the middle of the planet's disk. This storm has been churning in the planet's equatorial regions for several weeks now, and it is likely responsible for the reddish, dusty haze and other dust clouds seen across this hemisphere of the planet in views from Hubble, ground based telescopes, and the NASA and ESA spacecraft studying Mars from orbit. Bluish water-ice clouds can also be seen along the limbs and in the north (winter) polar region at the top of the image."[47]

## Saturn

A global storm girdles Saturn in 2011. The head of the storm (bright area) passes the tail circling around the left limb. Credit: NASA/JPL-Caltech/SSI.
North polar hexagonal cloud feature, discovered by Voyager 1 and confirmed in 2006 by Cassini is shown. Credit: NASA / JPL-Caltech / Space Science Institute.
This is a closer view of the north polar vortex at the center of the hexagon. Credit: NASA / JPL-Caltech / Space Science Institute.

The upper clouds are composed of ammonia crystals.

In 1990, the Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that was not present during the Voyager encounters and in 1994, another, smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice.[48] Previous Great White Spots were observed in 1876, 1903, 1933 and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.[49]

Wind speeds on Saturn can reach 1,800 km/h (1,100 mph) ... Voyager data indicate peak easterly winds of 500 m/s (1800 km/h).[50]

Infrared imaging has shown that Saturn's south pole has a warm polar vortex, the only known example of such a phenomenon in the Solar System.[51] Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, believed to be the warmest spot on Saturn.[51]

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images.[52][53]

## Uranus

A 1998 false-colour near-infrared image of Uranus showing cloud bands, rings, and moons obtained by the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera. Credit: Hubble Space Telescope - NASA Marshall Space Flight Center.
Uranus's southern hemisphere in approximate natural colour (left) and in shorter wavelengths (right), shows its faint cloud bands and atmospheric "hood" as seen by Voyager 2. Credit: NASA.
The first dark spot is observed on Uranus. Image is obtained by the HST Advanced Camera for Surveys (ACS) in 2006. Credit: NASA, ESA, L. Sromovsky and P. Fry (University of Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute).
Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image). Credit: NASA, ESA, and M. Showalter (SETI Institute.
Zonal wind speeds are plotted as detected on Uranus. Shaded areas show the southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data. Credit: .

Uranus has a complex, layered cloud structure, with methane thought to make up the uppermost layer of clouds.[54] With a large telescope of 25 cm or wider, cloud patterns may be visible.[55] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[56][57] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[56]

In the 1990s, the number of the observed bright cloud features grew considerably partly because new high resolution imaging techniques became available.[58] Most were found in the northern hemisphere as it started to become visible.[58] An early explanation - that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them - was shown to be incorrect: the actual number of features has indeed increased considerably.[59][60] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[60] They appear to lie at a higher altitude.[60] The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud may have persisted since Voyager flyby.[58][57] Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.[58] For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature dubbed Uranus Dark Spot was imaged.[61] The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.[62]

For a short period from March to May 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.[60][63]

On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus's surface, giving astronomers more insight into the planet's atmospheric activity.[61]

The bright collar at −45° latitude is also connected with methane clouds.[64] Other changes in the southern polar region can be explained by changes in the lower cloud layers.[64]

The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).[58] The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.[58] At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s.[58][65] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.[58][66] Closer to the poles, the winds shift to a prograde direction, flowing with the planet's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.[58] Windspeeds at −40° latitude range from 150 to 200 m/s. Since the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.[58] In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.[58][65][67] ... Observations included record-breaking wind speeds of 229 m/s (824 km/h) and a persistent thunderstorm referred to as "Fourth of July fireworks".[57]

## Neptune

Combined colour and near-infrared image of Neptune, shows bands of methane in its atmosphere, and four of its moons, Proteus, Larissa, Galatea, and Despina. Credit: .
Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck. Credit: .
The Great Dark Spot (top), Scooter (middle white cloud),[68] and the Small Dark Spot (bottom), with contrast exaggerated. Credit: .
The Great Dark Spot is imaged by Voyager 2. Credit: .

At the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere possessed a Great Dark Spot. In 1989, the Great Dark Spot, an anti-cyclonic storm system spanned 13000×6600 km,[69] was discovered by NASA's Voyager 2 spacecraft. Some five years later, on 2 November 1994, the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere.[70]

The Scooter is another storm, a white cloud group farther south than the Great Dark Spot. Its nickname is due to the fact that when first detected in the months before the 1989 Voyager 2 encounter it moved faster than the Great Dark Spot.[71] Subsequent images revealed even faster clouds.

The Small Dark Spot is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It initially was completely dark, but as Voyager 2 approached the planet, a bright core developed and can be seen in most of the highest-resolution images.[72]

The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator or possibly through some other unknown mechanism.[73]

The upper-level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense.

High-altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high-altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck.[74]

Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.[75]

Neptune has the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph).[76]

On Neptune winds reach speeds of almost 600 m/s—nearly attaining supersonic flow.[76] More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward.[77] At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles.[78] Most of the winds on Neptune move in a direction opposite the planet's rotation.[71] The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes.[54] At 70° S latitude, a high-speed jet travels at a speed of 300 m/s.[54]

## Comets

Due to a need for accurate oscillator strengths and cross sections in studies of diffuse interstellar clouds and cometary atmospheres, emission lines in cometary spectra are being studied.[79]

## Interstellar clouds

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.[80]

The cyanide radical (called cyanogen) is used to measure the temperature of interstellar gas clouds.[81]

"Carbon monoxide is the second most abundant molecule, after H2, in interstellar clouds. In diffuse clouds, the amount of CO is mainly derived from measurements of absorption at UV wavelengths."[79]

## Hot ionized mediums

"Of interest is the hot ionized medium (HIM) consisting of a coronal cloud ejection from star surfaces at 106-107 K which emits X-rays. The ISM is turbulent and full of structure on all spatial scales. Stars are born deep inside large complexes of molecular clouds, typically a few parsecs in size. During their lives and deaths, stars interact physically with the ISM. Stellar winds from young clusters of stars (often with giant or supergiant HII regions surrounding them) and shock waves created by supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures are stellar wind bubbles and superbubbles of hot gas. The Sun is currently traveling through the Local Interstellar Cloud, a denser region in the low-density Local Bubble."[82]

## HI clouds

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."[83]

Galaxies "around us are hiding about a third more atomic hydrogen gas than previously calculated."[84]

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."[85]

“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.”[85]

“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.”[85]

SIMBAD contains some 6,010 entries of the astronomical object type 'HI' (H I region).

These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

The degree of ionization in an H I region is very small at around 10−4 (i.e. one particle in 10,000). The temperature of an H I region is about 100 K,[86] and it is usually considered as isothermal, except near an expanding H II region.[87]

For hydrogen, complete ionization "obviously reduces its cross section to zero, but ... the net effect of partial ionization of hydrogen on calculated absorption depends on whether or not observations of hydrogen [are] used to estimate the total gas. ... [A]t least 20 % of interstellar hydrogen at high galactic latitudes seems to be ionized".[88]

## HI shells

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."[89]

"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."[89]

"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."[89]

## HII clouds

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."[90]

"NGC 2174 lies about 6400 light-years away in the constellation of Orion (The Hunter)."[90]

"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."[90]

"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."[91]

## Molecular clouds

This image shows a colour composite of visible and near-infrared images of the dark cloud Barnard 68. Credit: ESO.
This cloud of gas and dust is being deleted. Credit: Hubble Heritage Team (STScI/AURA), N. Walborn (STScI) & R. Barbß (La Plata Obs.), NASA.

Def. a "large and relatively dense cloud of cold gas and dust in interstellar space from which new stars are formed"[92] 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.[93]

Barnard 68 is around 500 lyrs away in the constellation Ophiuchus.[93]

"At these wavelengths, the small cloud is completely opaque because of the obscuring effect of dust particles in its interior."[93]

"It was obtained with the 8.2-m VLT ANTU telescope and the multimode FORS1 instrument in March 1999."[93]

In the image at right is a molecular cloud of gas and dust that is being reduced. "Likely, within a few million years, the intense light from bright stars will have boiled it away completely. The cloud has broken off of part of the Carina Nebula, a star forming region about 8000 light years away. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This image spans about two light years and was taken by the orbiting Hubble Space Telescope in 1999."[94]

A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).

Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.[95]

Such clouds make up < 1% of the ISM, have temperatures of 10-20 K and high densities of 102 - 106 atoms/cm3. These clouds are astronomical radio and infrared sources with radio and infrared molecular emission and absorption lines.

## Globules

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."[96]

The "Thumbprint Nebula (TPN) in the Chamaeleon III region" is "a globule without any signs of star formation".[96]

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".[96]

## Cometary globules

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.[97]

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

## Circumstellar clouds

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."[98]

"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."[98]

"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. "[98]

"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."[98]

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

## High-velocity clouds

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.[99]

"The leading edge of this cloud [shown in the image on the right] is already interacting with gas from our Galaxy."[100]

"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."[101]

"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."[100]

"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."[100]

"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."[100]

## Giant molecular clouds

A vast assemblage of molecular gas with a mass of approximately 103–107 times the mass of the Sun[102] is called a giant molecular cloud (GMC). GMCs are ≈15–600 light-years in diameter (5–200 parsecs).[102] Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is 102–103 particles per cubic centimetre. Although the Sun is much denser than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps.[103]

The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are, unsurprisingly, called "dense molecular cores" and have densities in excess of 104–106 particles per cubic centimeter. Observationally molecular cores are traced with carbon monoxide and dense cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.[104]

GMCs are so large that "local" ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt.[105] The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring around the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.[106]

## Nebulas

This color picture was made by combining several exposures taken on the night of December 28th 1994 at the 0.9 m telescope of the Kitt Peak National Observatory. Credit: N.A.Sharp/NOAO/AURA/NSF.

"The Horsehead Nebula, a part of the optical nebula IC434 and also known as Barnard 33, was first recorded in 1888 on a photographic plate taken at the Harvard College Observatory. Its coincidental appearance as the profile of a horse's head and neck has led to its becoming one of the most familiar astronomical objects. It is, in fact, an extremely dense cloud projecting in front of the ionized gas that provides the pink glow so nicely revealed in this picture. We know this not only because the underside of the 'neck' is especially dark, but because it actually casts a shadow on the field to its east (below the 'muzzle')."[107]

## Dark nebulas

"The 111 → 110 rotational transition of formaldehyde (H2CO) [occurs] in absorption in the direction of four dark nebulae. The radiation ... being absorbed appears to be the isotropic microwave background".[108] One of the dark nebulae sampled, per SIMBAD is TGU H1211 P5.

## Supernova remnants

This is an image of NGC 2080, the Ghost Head Nebula. Credit: NASA, ESA and Mohammad Heydari-Malayeri (Observatoire de Paris, France).
The Crab Nebula is a remnant of an exploded star. This image shows the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. Each image is 6′ wide. Credit: .

"The supernova SN1987A in the Large Magellanic Cloud (LMC) was discovered on February 23, 1987, and its progenitor is a blue supergiant (Sk -69 202) with luminosity of 2-5 x 1038 erg/s.[109] The 847 keV and 1238 keV gamma-ray lines from 56Co decay have been detected.[109]

At right is a Hubble Space Telescope image of the Ghost Head Nebula. "This nebula is one of a chain of star-forming regions lying south of the 30 Doradus nebula in the Large Magellanic Cloud. The red and blue light comes from regions of hydrogen gas heated by nearby stars. The green light comes from glowing oxygen, illuminated by the energy of a stellar wind. The white center shows a core of hot, massive stars."[110]

On July 21, 1964, the Crab Nebula supernova remnant was discovered to be a hard X-ray (15 – 60 keV) source by a scintillation counter flown on a balloon launched from Palestine, Texas, USA. This was likely the first balloon-based detection of X-rays from a discrete cosmic X-ray source.[111]

"The high-energy focusing telescope (HEFT) is a balloon-borne experiment to image astrophysical sources in the hard X-ray (20–100 keV) band.[112] Its maiden flight took place in May 2005 from Fort Sumner, New Mexico, USA. The angular resolution of HEFT is ~1.5'. Rather than using a grazing-angle X-ray telescope, HEFT makes use of a novel tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. HEFT has an energy resolution of 1.0 keV full width at half maximum at 60 keV. HEFT was launched for a 25-hour balloon flight in May 2005. The instrument performed within specification and observed Tau X-1, the Crab Nebula."[82]

## Large Magellanic Clouds

For coronal cloud observations of the Large Magellanic Cloud, "[b]ackground spectra were obtained from observations of the Lockman Hole."[113]

## Outflow clouds

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.

## Satellites

The Submillimeter Wave Astronomy Satellite (SWAS) [is in] low Earth orbit ... to make targeted observations of giant molecular clouds and dark cloud cores. The focus of SWAS is five spectral lines: water (H2O), isotopic water (H218O), isotopic carbon monoxide (13CO), molecular oxygen (O2), and neutral carbon (C I).

## Spectroscopy

By comparing astronomical observations with laboratory measurements, astrochemists can infer the elemental abundances, chemical composition, and temperatures of stars and interstellar clouds. This is possible because ions, atoms, and molecules have characteristic spectra: that is, the absorption and emission of certain wavelengths (colors) of light, often not visible to the human eye. However, these measurements have limitations, with various types of radiation (radio, infrared, visible, ultraviolet etc.) able to detect only certain types of species, depending on the chemical properties of the molecules. Interstellar formaldehyde was the first polyatomic organic molecule detected in the interstellar medium.

## Spacecraft

Clouds of material are along the paths of the Voyager 1 and Voyager 2 spacecraft through interstellar space. Credit: NASA, ESA, and Z. Levay (STScI).

The Voyager 1 spacecraft is a 722 kg (1,592 lb) space probe launched by NASA on September 5, 1977 to study the outer Solar System and interstellar medium.

The Cosmic Ray System (CRS) determines the origin and acceleration process, life history, and dynamic contribution of interstellar cosmic rays, the nucleosynthesis of elements in cosmic-ray sources, the behavior of cosmic rays in the interplanetary medium, and the trapped planetary energetic-particle environment.

Measurements from the spacecraft revealed a steady rise since May in collisions with high energy particles (above 70 MeV), which are believed to be cosmic rays emanating from supernova explosions far beyond the Solar System, with a sharp increase in these collisions in late August. At the same time, in late August, there was a dramatic drop in collisions with low-energy particles, which are thought to originate from the Sun.[114]

"It's important for us to be aware of what kinds of objects are present beyond our solar system, since we are now beginning to think about potential interstellar space missions, such as Breakthrough Starshot."[115]

At "least two interstellar clouds [have been discovered] along Voyager 2's path, and one or two interstellar clouds along Voyager 1's path. They were also able to measure the density of electrons in the clouds along Voyager 2's path, and found that one had a greater electron density than the other."[116]

"We think the difference in electron density perhaps indicates a difference in composition of overall density of the clouds."[115]

A "broad range of elements [were detected]] in the interstellar medium, such as electrically charged ions of magnesium, iron, carbon and manganese [and] neutrally charged oxygen, nitrogen and hydrogen."[116]

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