Editing Cloud (section)

==Luminance, reflectivity, and coloration==

The luminance or brightness of a cloud is determined by how light is reflected, scattered, and transmitted by the cloud's particles. Its brightness may also be affected by the presence of haze or photometeors such as halos and rainbows.<ref name="Luminance">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Luminance, International Cloud Atlas |url=https://cloudatlas.wmo.int/appearance-of-clouds-luminance.html |access-date=10 May 2017}}</ref> In the troposphere, dense, deep clouds exhibit a high reflectance (70–95%) throughout the [[visible spectrum]]. Tiny particles of water are densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color, especially when viewed from the top.<ref name="Steven Salter and John Latham">[http://www.21stcenturychallenges.org/60-seconds/increasing-cloud-reflectivity/ Increasing Cloud Reflectivity] {{webarchive|url=https://web.archive.org/web/20130511051600/http://www.21stcenturychallenges.org/60-seconds/increasing-cloud-reflectivity/ |date=11 May 2013 }}, Royal Geographical Society, 2010.</ref> Cloud droplets tend to [[scattering|scatter]] light efficiently, so that the intensity of the [[solar radiation]] decreases with depth into the gases. As a result, the [[cloud base]] can vary from a very light to very-dark-gray depending on the cloud's thickness and how much [[light]] is being reflected or transmitted back to the observer. High thin tropospheric clouds reflect less light because of the comparatively low concentration of constituent ice crystals or supercooled water droplets which results in a slightly off-white appearance. However, a thick dense ice-crystal cloud appears brilliant white with pronounced gray shading because of its greater reflectivity.<ref name="Luminance" />

As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets. If the droplets become too large and heavy to be kept aloft by the air circulation, they will fall from the cloud as [[rain]]. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, a percentage of the light that enters the cloud is not reflected back out but is absorbed, giving the cloud a darker look. A simple example of this is one's being able to see farther in heavy rain than in heavy fog. This process of [[Reflection (physics)|reflection]]/[[Absorption (electromagnetic radiation)|absorption]] is what causes the range of cloud color from white to black.<ref name="Bette Hileman">{{Cite journal |last=Hileman |first=B. |year=1995 |title=Clouds absorb more solar radiation than researchers previously thought |journal=Chemical & Engineering News |volume=73 |issue=7 |pages=33 |doi=10.1021/cen-v073n007.p033}}</ref>

Striking cloud colorations can be seen at any altitude, with the color of a cloud usually being the same as the incident light.<ref name="Coloration">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Coloration, International Cloud Atlas |url=https://cloudatlas.wmo.int/appearance-of-clouds-colour.html |access-date=13 May 2017}}</ref> During daytime when the sun is relatively high in the sky, tropospheric clouds generally appear bright white on top with varying shades of gray underneath. Thin clouds may look white or appear to have acquired the color of their [[Natural environment|environment]] or background. Red, orange, and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. When the Sun is just below the horizon, low-level clouds are gray, middle clouds appear rose-colored, and high clouds are white or off-white. Clouds at night are black or dark gray in a moonless sky, or whitish when illuminated by the Moon. They may also reflect the colors of large fires, city lights, or auroras that might be present.<ref name="Coloration" />

A cumulonimbus cloud that appears to have a greenish or bluish tint is a sign that it contains extremely high amounts of water; hail or rain which scatter light in a way that gives the cloud a blue color. A green colorization occurs mostly late in the day when the sun is comparatively low in the sky and the incident sunlight has a reddish tinge that appears green when illuminating a very tall bluish cloud. Supercell type storms are more likely to be characterized by this but any storm can appear this way. Coloration such as this does not directly indicate that it is a severe thunderstorm, it only confirms its potential. Since a green/blue tint signifies copious amounts of water, a strong updraft to support it, high winds from the storm raining out, and wet hail; all elements that improve the chance for it to become severe, can all be inferred from this. In addition, the stronger the updraft is, the more likely the storm is to undergo tornadogenesis and to produce large hail and high winds.<ref name="Curiosities">{{Cite web |year=2007 |editor-last=University of Wisconsin-Madison-News |title=Curiosities-Green sky before tornado |url=http://www.news.wisc.edu/14039 |access-date=17 January 2015}}</ref>

Yellowish clouds may be seen in the troposphere in the late spring through early fall months during [[forest fire]] season. The yellow color is due to the presence of pollutants in the smoke. Yellowish clouds are caused by the presence of nitrogen dioxide and are sometimes seen in urban areas with high air pollution levels.<ref name="Garrett Nagle">{{Cite book |last=Nagle, Garrett |title=Hazards |publisher=Nelson Thornes |year=1998 |isbn=978-0-17-490022-1 |page=101 |chapter=10. Cities and Air Pollution |chapter-url=https://books.google.com/books?id=qC1l1RdWpIwC&pg=PA101}}</ref>

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File:Sunrise In The Peak District.jpg|Stratocumulus stratiformis and small castellanus made orange by the sun rising
File:Irid clouds1.jpg|An occurrence of [[cloud iridescence]] with altocumulus volutus and cirrocumulus stratiformis
File:Red Color in Gray Clouds.JPG|Sunset reflecting shades of pink onto gray stratocumulus stratiformis translucidus (becoming perlucidus in the background)
File:Sharp View.JPG|Stratocumulus stratiformis perlucidus before sunset in [[Bangalore]], India
File:Regnbyge.jpg|Late-summer [[rainstorm]] in [[Denmark]]. Nearly black color of base indicates main cloud in foreground probably cumulonimbus.
File:Burning Yellow Sunset.jpg|Particles in the [[atmosphere]] and the [[sun]]'s angle enhance colors of stratocumulus cumulogenitus at evening [[twilight]].
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