User:Mathwhiz 29/clouds

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There are many rare clouds we can encounter in Alaska, due to either location in the polar region (or subpolar, to be more exact) and the existance of mountains in the Rocky Mountain Range. Following is an overview of noctilucent and nacreous clouds, lenticular clouds, and relevant atmospheric effects.

Polar Clouds[edit]

Noctilucent Clouds[edit]

Helkivad ööpilved Kuresoo kohal.jpg

Polar Mesopheric Clouds (PMCs), or noctilucent clouds, are diffuse water-ice clouds found near the summer polar mesopause, and often only visible from space due to geometrical considerations restricting ground observation to twilight hours. The clouds occur about 76-84 km above the ground, and as such are the highest clouds in the atmosphere. They are usually colorless or pale blue (absorption of ozone) although rocket exhausts that seed these clouds may iridesce, due to the uniformity of crystal sizes. The clouds can be featureless, more defined bands than cirrus clouds, but often are streaked, whirled, or undulating.

Noctilucent clouds can be directly water ice crystals, of around 30-100 nm, or form on dust from micrometeors, volcanoes, the surface, or rockets. The water vapor may be lifted from gaps in the troposphere (lower atmosphere) or form directly from the reaction of methane with stratospheric hydroxyl radicals. However, the mesosphere is extremely dry, containing about 10-8 the water of the Sahara desert environment. Coupled with the very thin air of the mesosphere, water crystallization requires temperatures of -120 °C.

They occur for a season of around 80 days, peaking 20 days after summer solstice. They are usually frequent around 70°-85° latitude in both hemispheres, but are visible from the ground at around 50°-70°, where ground observers view the edge of the clouds illuminated by the set sun. Ideally, the sun will be 6°-16° below horizon. This is due to the constraint that the clouds reflect sunlight so dimly (due to the diffuse nature of the ice crystals and the ineffectual scattering from especially small crystals, ~30 nm) that from the position of the observer, the sun cannot be in the sky but must be still illuminating the higher layers of the atmosphere. In any more polar regions, the sky never gets dark enough in the summer to see noctilucent clouds.

Formation of PMCs are indepedent of longitude and auroral activity, and, indeed, varies inversely with solar activity due to the effect on the temperature of the mesosphere. The clouds are usually geographically dependent for formation of ice crystals in the correct conditions. Curiously, it appears that the southern hemisphere (due to geographical considerations limiting high altitude water ice, probably) has fainter and less frequent clouds. Moreover, UV radiation breaks apart water molecules, a process of dessication that lags the solar variability by a year (for unknown reasons). The extremely high radar reflectivity may be due to vapor deposition of sodium and iron atoms from micrometeor debris in a layer above that of the clouds.

Noctilucent clouds were first observed in 1885, two years after the great Krakatoa explosion, possibly as an effect of more dust in the upper atmosphere from the volcano. The occurrance of noctilucent clouds appears to be increasing in frequency, brightness and extent. They may be indicators for anthropogenic climate change. Climate models predict increased greenhouse gases cause a cooling of the mesosphere, or that larger methan emissions produce higher concentrations of water vapor in the upper atmosphere.

Nacreous Clouds[edit]

Polar stratospheric clouds (PSCs), also known as nacreous clouds, are winter, polar stratospheric clouds (15-25 km altitude). They are best observed in civil twilight, when the sun will be 1°-6° below the horizon, for the same reasons as noctilucent clouds. Nacreous clouds directly catalyze ozone depletion, by supporting reactions that produce active chlorine that catalyzes ozone destruction, by removing nitric acid(g), and thus perturbing nitrogen and chlorine cycles to increase ozone destruction. Forward-scattering of sunlight produces a pearly white cloud, but within optically thin clouds, diffraction causes interference fringes and cloud iridescence.

Most nacresous clouds contain not only water ice but nitric acid and/or sulfuric acid and only iridesce if optically thin, etc. They form in the lower stratosphere, in dry conditions, which require -78 °C to crystallize. This only happens in the stratosphere in the polar winter. If temperatures drop below -86 °C, which is very rare for the Arctic but sometimes possible in the Antarctic, purely water ice clouds can form, which are necessarily iridescent. In the Artic, however, mountainous lee winds may cause local cooling and more nacreous clouds.

Cloud Iridescence and Coronas[edit]

Solar Halo, 3 May 2014, Pasadena, CA, USA

Rare cloud iridescence is due to small, uniform ice crystals or water droplets and the diffraction of sunlight. Only seen in optically thin clouds, or at edges of clouds, especially altocumulus, cirrocumulus, cirrus, and lenticular clouds, as well as the polar clouds mentioned above. Most light rays encounter only one prism. When a thin cloud has droplets of the same size over a large angular area, the iridescence takes the structured form of a corona. A corona is pretty common, as dispersed, small droplets or crystals are everywhere in the lower atmosphere, and the effect is basically an Airy disk through a thin cloud.

Halos[edit]

Unlike iridescence or coronas, a halo (or icebow, nimbus, or gloriole) is formed by refraction by large ice crystals. These ice crystals are in the upper troposphere, in cirrostatus clouds at an altitude of 5-10 km. The rings have a size of 22°, which corresponds with the minimum deviation angle of refraction through a hexagonal (60°) water ice crystal. In winter or snowy and icy conditions, they occur much more often. Halos are much larger and less bright than coronas, but the wavelength dependent refraction can result in a rainbow spread of colors. The inner edge is reddish; the outer edge bluish.

Lenticular Clouds[edit]

Very low altitude lenticular clouds in Homer, Alaska. Note the lenticular clouds cap two different mountains in the middle right, but the second cycle resulted in simly roll clouds of cumulus or bits of cumulus, cumulus fractus, in the left side of the image.
The wind flows towards a mountain and produces a first oscillation (A) followed by more waves. The following waves will have lower amplitude because of the natural damping. Lenticular clouds stuck on top of the flow (A) and (B) will appear immobile despite the strong wind.

At the crest of a lee wave, adiabatic expansion cooling of moist air can form lens-shaped clouds, lenticular clouds. These clouds can be stacked if there are layers of alternating dry and moist air in the current. Away from peaks, the water will evaporate back into vapor and the clouds dissapear. As damping takes over when the current goes farther away from the mountain ranges, the clouds become more dispersed and of smaller amplitude, creating roll clouds of broken cumulus clouds and then dissapating.

Lee Waves[edit]

Lee waves are standing waves in the atmosphere, changes in pressure, height, and temperature of a current of air caused by vertical air displacement. The most common form is mountain waves, which are internal atmospheric gravity waves. A gravity wave is generated in a fluid at the interface of two densities, with a resultant density-based buoyancy, where restoration to equilibrium results in an oscillation around that equilibrium state (wave orbit). These waves can be observed through lenticular clouds or periodic windows of roll clouds and empty sky, caused by stable moist air flowing over mountains carried high and cold enough to the dew point.