Cloud iridescence

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Iridescent mid altitude clouds Highly iridising altocumulus.jpg
Iridescent mid altitude clouds
Iridescent polar stratospheric cloud at sunset over Aberdeen, Scotland Iridescent cloud at sunset.jpg
Iridescent polar stratospheric cloud at sunset over Aberdeen, Scotland
Cloud iridescence, seen above the clouds covered with grey clouds, Pondicherry, India Rainbow cloud 1.jpg
Cloud iridescence, seen above the clouds covered with grey clouds, Pondicherry, India

Cloud iridescence or irisation is a colorful optical phenomenon that occurs in a cloud and appears in the general proximity of the Sun or Moon. The colors resemble those seen in soap bubbles and oil on a water surface. It is a type of photometeor. This fairly common phenomenon is most often observed in altocumulus, [1] cirrocumulus, lenticular, [2] and cirrus clouds. [3] [4] [5] They sometimes appear as bands parallel to the edge of the clouds. Iridescence is also seen in the much rarer polar stratospheric clouds, also called nacreous clouds. [6]

Contents

The colors are usually pastel, but can be very vivid or mingled together, sometimes similar to mother-of-pearl. [7] When appearing near the Sun, the effect can be difficult to spot as it is drowned in the Sun's glare. This may be overcome by shielding the sunlight with one's hand or hiding it behind a tree or building. Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water.

Etymology

Irisations are named after the Greek goddess Iris, goddess of rainbows and messenger of Zeus and Hera to the mortals below. [8]

Mechanism

Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon. [9]

Irisation is caused by very uniform water droplets diffracting light (within 10 degrees from the Sun) and by first order interference effects [10] (beyond about 10 degrees from the Sun). It can extend up to 40 degrees from the Sun. [11]

If parts of clouds contain small water droplets or ice crystals of similar size, their cumulative effect is seen as colors. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, while newly forming clouds produce the brightest and most colorful iridescence. When the particles in a thin cloud are very similar in size over a large extent, the iridescence takes on the structured form of a corona, a bright circular disk around the Sun or Moon surrounded by one or more colored rings. [9] [12] [13]

See also

Related Research Articles

<span class="mw-page-title-main">Cirrus cloud</span> Genus of atmospheric cloud

Cirrus is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto rocky or metallic dust particles at high altitudes. Globally, they form anywhere between 4,000 and 20,000 meters above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.

<span class="mw-page-title-main">Cloud</span> Visible mass of liquid droplets or frozen crystals suspended in the atmosphere

In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.

<span class="mw-page-title-main">Cumulus cloud</span> Genus of clouds, low-level cloud

Cumulus clouds are clouds that have flat bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin cumulus, meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.

<span class="mw-page-title-main">Altostratus cloud</span> A type of middle-altitude cloud

Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.

<span class="mw-page-title-main">Cirrocumulus cloud</span> Genus of high-altitude clouds

Cirrocumulus is one of the three main genus types of high-altitude tropospheric clouds, the other two being cirrus and cirrostratus. They usually occur at an altitude of 5 to 12 km. Like lower-altitude cumuliform and stratocumuliform clouds, cirrocumulus signifies convection. Unlike other high-altitude tropospheric clouds like cirrus and cirrostratus, cirrocumulus includes a small amount of liquid water droplets, although these are in a supercooled state. Ice crystals are the predominant component, and typically, the ice crystals cause the supercooled water drops in the cloud to rapidly freeze, transforming the cirrocumulus into cirrostratus. This process can also produce precipitation in the form of a virga consisting of ice or snow. Thus, cirrocumulus clouds are usually short-lived. They usually only form as part of a short-lived transitional phase within an area of cirrus clouds and can also form briefly as a result of the breaking up of part of a cumulonimbus anvil.

<span class="mw-page-title-main">Halo (optical phenomenon)</span> Optical phenomenon of the sky

A halo is an optical phenomenon produced by light interacting with ice crystals suspended in the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spots in the sky. Many of these appear near the Sun or Moon, but others occur elsewhere or even in the opposite part of the sky. Among the best known halo types are the circular halo, light pillars, and sun dogs, but many others occur; some are fairly common while others are extremely rare.

<span class="mw-page-title-main">Iridescence</span> Optical property

Iridescence is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstructures or thin films. Examples of iridescence include soap bubbles, feathers, butterfly wings and seashell nacre, and minerals such as opal. Pearlescence is a related effect where some or most of the reflected light is white. The term pearlescent is used to describe certain paint finishes, usually in the automotive industry, which actually produce iridescent effects.

<span class="mw-page-title-main">Atmospheric diffraction</span>

Atmospheric diffraction is manifested in the following principal ways:

<span class="mw-page-title-main">Lenticular cloud</span> Cloud species

Lenticular clouds are stationary clouds that form mostly in the troposphere, typically in parallel alignment to the wind direction. They are often comparable in appearance to a lens or saucer. Nacreous clouds that form in the lower stratosphere sometimes have lenticular shapes.

<span class="mw-page-title-main">Cloud physics</span> Study of the physical processes in atmospheric clouds

Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.

<span class="mw-page-title-main">Noctilucent cloud</span> Cloud-like phenomena in the upper atmosphere of Earth

Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. When viewed from space, they are called polar mesospheric clouds (PMCs), detectable as a diffuse scattering layer of water ice crystals near the summer polar mesopause. They consist of ice crystals and from the ground are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between ±50° and ±70°. Too faint to be seen in daylight, they are visible only when the observer and the lower layers of the atmosphere are in Earth's shadow, but while these very high clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor through chemical reactions once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds.

<span class="mw-page-title-main">Polar stratospheric cloud</span> Clouds occurring in the stratosphere in high-latitude regions

Polar stratospheric clouds (PSCs) are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 m (49,000–82,000 ft). They are best observed during civil twilight, when the Sun is between 1 and 6 degrees below the horizon, as well as in winter and in more northerly latitudes. One main type of PSC is made up mostly of supercooled droplets of water and nitric acid and is implicated in the formation of ozone holes. The other main type consists only of ice crystals which are not harmful. This type of PSC is also referred to as nacreous.

<span class="mw-page-title-main">Pileus (meteorology)</span> Small, horizontal, lenticular cloud

A pileus, also called scarf cloud or cap cloud, is a small, horizontal, lenticular cloud appearing above a cumulus or cumulonimbus cloud. Pileus clouds are often short-lived, appearing for typically only a few minutes, with the main cloud beneath them rising through convection to absorb them. Furthermore, the clouds are typically formed by drier air with a higher lifting condensation level, which often prevents vertical growth and leads to the smooth horizontal cap shape that the cloud is named for.

<span class="mw-page-title-main">Corona (optical phenomenon)</span> Optical phenomenon of the sky

In meteorology, a corona is an optical phenomenon produced by the diffraction of sunlight or moonlight by individual small water droplets and sometimes tiny ice crystals of a cloud or on a foggy glass surface. In its full form, a corona consists of several concentric, pastel-colored rings around the celestial object and a central bright area called an aureole. The aureole is often the only visible part of the corona and has the appearance of a bluish-white disk which fades to reddish-brown towards the edge. The angular diameter of a corona depends on the sizes of the water droplets involved; smaller droplets produce larger coronae. For the same reason, the corona is the most pronounced when the size of the droplets is most uniform. Coronae differ from halos in that the latter are formed by refraction from comparatively large rather than small ice crystals. The diffraction pattern is called an Airy disk.

<span class="mw-page-title-main">Fog bow</span> Type of rainbow formed by fog droplets

A fog bow, sometimes called a white rainbow, is a similar phenomenon to a rainbow; however, as its name suggests, it appears as a bow in fog rather than rain. Because of the very small size of water droplets that cause fog—smaller than 0.05 millimeters (0.0020 in)—the fog bow has only very weak colors, with a red outer edge and bluish inner edge. The colors fade due to being smeared out by the diffraction effect of the smaller droplets.

<span class="mw-page-title-main">Circumhorizontal arc</span> Optical phenomenon

A circumhorizontal arc is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sunlight or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in actual cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band running parallel to the horizon, located far below the Sun or Moon. The distance between the arc and the Sun or Moon is twice as far as the common 22-degree halo. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as the Moon.

<span class="mw-page-title-main">Fallstreak hole</span> Large gap in cirrocumulus or altocumulus clouds

A fallstreak hole is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds. The holes are caused by supercooled water in the clouds suddenly evaporating or freezing, and may be triggered by passing aircraft.

<span class="mw-page-title-main">Atmospheric optics</span> Study of the optical characteristics of the atmosphere or products of atmospheric processes

Atmospheric optics is "the study of the optical characteristics of the atmosphere or products of atmospheric processes .... [including] temporal and spatial resolutions beyond those discernible with the naked eye". Meteorological optics is "that part of atmospheric optics concerned with the study of patterns observable with the naked eye". Nevertheless, the two terms are sometimes used interchangeably.

<span class="mw-page-title-main">Optical phenomenon</span> Observable events that result from the interaction of light and matter

Optical phenomena are any observable events that result from the interaction of light and matter.

References

  1. Gedzelman, Stanley David (1 June 1988). "In Praise of Altocumulus". Weatherwise. 41 (3): 143–149. doi:10.1080/00431672.1988.9930533.
  2. Answers.com – Sci-Tech Dictionary: irisation
  3. Nemiroff, R.; Bonnell, J., eds. (25 November 2007). "An Iridescent Cloud Over Colorado". Astronomy Picture of the Day . NASA.
  4. "Iridescent Clouds". Atmospheric Optics.
  5. Sassen, Kenneth (1 January 2003). "Cirrus cloud iridescence: a rare case study". Applied Optics. 42 (3): 486–491. Bibcode:2003ApOpt..42..486S. doi:10.1364/AO.42.000486. PMID   12570270.
  6. "Nacreous Clouds". Atmospheric Optics.
  7. PHOTOMETEORS, by Jesús Martínez-Frías Archived 2009-12-13 at the Wayback Machine
  8. The Cloudspotter's Guide By Gavin Pretor-Pinney, p. 233
  9. 1 2 "Cloud Iridescence | SKYbrary Aviation Safety". skybrary.aero. Retrieved 2022-08-19.
  10. Color and Light in Nature By David K. Lynch, William Charles Livingston, p. 133
  11. PHOTOMETEORS, by Jesús Martínez-Frías Archived 2009-12-13 at the Wayback Machine
  12. "Corona". Atmospheric Optics.
  13. Shaw, Joseph A.; Pust, Nathan (12 August 2011). "Icy wave-cloud lunar corona and cirrus iridescence". Applied Optics. 50 (28): F6. Bibcode:2011ApOpt..50F...6S. doi:10.1364/AO.50.0000F6. PMID   22016246.