Clouds throughout the homosphere::Cloud


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Clouds throughout the homosphere

Luminance and reflectivity

The luminance or brightness of a cloud in the homosphere (which includes the troposphere, stratosphere, and mesosphere) 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">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> In the troposphere, dense, deep clouds exhibit a high reflectance (70% to 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">Increasing Cloud Reflectivity, Royal Geographical Society, 2010.</ref> Cloud droplets tend to 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-grey 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 grey 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/absorption is what causes the range of cloud color from white to black.<ref name="Bette Hileman">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


An occurrence of altocumulus and cirrocumulus cloud iridescence
Sunset reflecting shades of pink onto grey stratocumulus clouds.
Effect of sunlight before sunset. Bangalore, India.

Striking cloud colorations can be seen at many altitudes in the homosphere. The color of a cloud is usually the same as the incident light.<ref name="Coloration">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

During daytime when the sun is relatively high in the sky, tropospheric clouds generally appear bright white on top with varying shades of grey underneath. Thin clouds may look white or appear to have acquired the color of their 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-etage clouds are gray, middle clouds appear rose-colored, and high-etage clouds are white or off-white. Clouds at night are black or dark grey 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/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">{{#invoke:citation/CS1|citation |CitationClass=web }}</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 caused by the presence of nitrogen dioxide are sometimes seen in urban areas with high air pollution levels.<ref name="Garrett Nagle">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Particles in the atmosphere and the sun's angle enhance cloud colors at evening twilight

In high latitude regions of the stratosphere, nacreous clouds occasionally found there during the polar winter tend to display quite striking displays of mother-of-pearl colorations.<ref name="PSC"/> This is due to the refraction and diffusion of the sun's rays through thin clouds with supercooled droplets that often contain compounds other than water. At still higher altitudes up in the mesosphere, noctilucent clouds made of ice crystals are sometimes seen in polar regions in the summer. They typically have a bluish or silvery white coloration that can resemble brightly illuminated cirrus. Noctilucent clouds may occasionally take on more of a red or orange hue.<ref name="Noctilucent"/>

Effects on climate and the atmosphere

Global cloud cover, averaged over the month of October 2009. NASA composite satellite image; larger image available here:<ref></ref>

The role of tropospheric clouds in regulating weather and climate remains a leading source of uncertainty in projections of global warming.<ref>Randall, D. et al. (2007) "Climate models and their evaluation" in S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. Averyt, M.Tignor, and H. Miller (eds.) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This uncertainty arises because of the delicate balance of processes related to clouds, spanning scales from millimeters to planetary. Hence, interactions between the large-scale (synoptic meteorology) and clouds becomes difficult to represent in global models. The complexity and diversity of clouds, as outlined above, adds to the problem. On the one hand, white-colored cloud tops promote cooling of Earth's surface by reflecting short-wave radiation from the sun. Most of the sunlight that reaches the ground is absorbed, warming the surface, which emits radiation upward at longer, infrared, wavelengths. At these wavelengths, however, water in the clouds acts as an efficient absorber. The water reacts by radiating, also in the infrared, both upward and downward, and the downward long-wave radiation results in some warming at the surface. This is analogous to the greenhouse effect of greenhouse gases and water vapor.<ref name="cloud-heating">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

High-étage tropospheric genus-types, cirrus, cirrocumulus, and cirrostratus, particularly show this duality with both short-wave albedo cooling and long-wave greenhouse warming effects. On the whole though, ice-crystal clouds in the upper troposphere tend to favor net warming.<ref name="Clouds and the greenhouse effect"/><ref name="Nucleation">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> However, the cooling effect is dominant with low-étage stratocumuliform and stratiform clouds made of very small water droplets that have an average radius of about 0.002 mm (0.00008 in).,<ref name="cloud drops"/> especially when they form in extensive sheets that block out more of the sun. These include middle-étage layers of altocumulus and altostratus as well as low stratocumulus, and stratus. Small-droplet aerosols are not good at absorbing long-wave radiation reflected back from Earth, so there is a net cooling with almost no long-wave effect. This effect is particularly pronounced with low-étage clouds that form over water.<ref name="Clouds and the greenhouse effect"/> Low and vertical heaps of cumulus, towering cumulus, and cumulonimbus are made of larger water droplets ranging in radius from 0.005 to about 0.015 mm. Nimbostratus cloud droplets can also be quite large, up to 0.015mm radius.<ref name="large droplets in precipitating clouds">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These larger droplets associated with vertically developed clouds are better able to trap the long-wave radiation thus mitigating the cooling effect to some degree. However, these large often precipitating clouds are variable or unpredictable in their overall effect because of variations in their concentration, distribution, and vertical extent. Measurements taken by NASA indicate that on the whole, the effects of low and middle étage clouds that tend to promote cooling are outweighing the warming effects of high layers and the variable outcomes associated with multi-étage or vertically developed clouds.<ref name="Clouds and the greenhouse effect">Ackerman, p. 124</ref>

As difficult as it is to evaluate the effects of current cloud cover characteristics on climate change, it is even more problematic to predict the outcome of this change with respect to future cloud patterns and events. As a consequence, much research has focused on the response of low and vertical clouds to a changing climate. Leading global models can produce quite different results, however, with some showing increasing low-étage clouds and others showing decreases.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

In the stratosphere, Type I non-nacreous clouds are known to have harmful effects over the polar regions of Earth. They become catalysts which convert relatively benign man-made chlorine into active free radicals like chlorine monoxide which are destructive of the stratospheric ozone layer.<ref name="PSC"/>

Polar mesospheric clouds are not common or widespread enough to have a significant effect on climate.<ref name="simulation studies"/> However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.

Global brightening

New research indicates a global brightening trend.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The details are not fully understood, but much of the global dimming (and subsequent reversal) is thought to be a consequence of changes in aerosol loading in the atmosphere, especially sulfur-based aerosol associated with biomass burning and urban pollution.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Changes in aerosol burden can have indirect effects on clouds by changing the droplet size distribution<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> or the lifetime and precipitation characteristics of clouds.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Cloud sections
Intro  Etymology   History of cloud science and nomenclature   Tropospheric clouds   Polar stratospheric clouds    Polar mesospheric clouds    Clouds throughout the homosphere    Extraterrestrial    See also    References    Bibliography    External links   

Clouds throughout the homosphere
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