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The Research
Definitions
Lenticular Cloud
A commonly used term for clouds of the species lenticularis. A cloud species the elements of which have the form of more or less isolated, generally smooth lenses or almonds; the outlines are sharp and sometimes show irisation. These clouds appear most often in formations of orographic origin, the result of lee waves, in which cases they remain nearly stationary with respect to the terrain (standing cloud), but they also occur in regions without marked orography. This species is found mainly in the genera cirrocumulus, altocumulus, and (rarely) stratocumulus. Altocumulus lenticularis differs from cirrocumulus lenticularis in that, when smooth and without elements, it has shadowed parts while the latter is very white throughout. When undulated or subdivided, the altocumulus species differs from stratocumulus lenticularis in that its elements subtend an angle of less than 5° when viewed at an angle of more than 30° above the horizon. (from: AMS Glossary of Meteorology, 2nd Edition)
Nacreous Clouds(Also known as Polar Stratospheric Clouds.)
Rarely seen clouds that form in the stratosphere at altitudes of 25-30 kilometers; resemble cirrus or altocumulus lenticularis and may be composed of ice crystals or supercooled water droplets. Alson known as mother-of-pearl clouds because of their soft, pearly luster. (from: Glossary of Weather and Climate I.W.Geer, Editor)
PerlanPerlan is the Icelandic word for Pearl.
Polar Stratospheric Clouds(Abbreviated PSC; also called nacreous clouds, mother-of-pearl clouds; rarely, luminous clouds.)
Clouds are cirrus or altocumulus lenticularis, and show very strong irisation similar to that of mother-of-pearl, especially when the sun is several degrees below the horizon. They occur at heights about 20-30 km above the earth. These clouds are rarely seen, and it would appear that they can be observed only in certain regions. They have been observed mainly over Antarctica due to the colder temperatures present there in the circumpolar vortex. They also form over Scotland and Scandinavia in winter during periods with an intense, broad, deep, and homogeneous westerly to northwesterly flow of air over northern Europe; they are also observed in Alaska. Less frequent sightings have been reported at lower latitudes. The simultaneous occurrence of various colors of the spectrum in more or less irregular patterns strongly suggests diffraction by spherical particles. The exact physical constitution of cloud particles has been determined by aircraft (e.g., the NASA ER2 aircraft) penetration showing the presence of nitric acid hydrates (in particular nitric acid tri-hydrate, type I) with the addition of water ice at temperatures a few degrees colder (type II). Nuclei for clouds are thought to be sulfuric acid aerosol, possibly of volcanic origin. The clouds form in regions where dynamic lifting or radiational cooling lowers the air to temperatures below saturation for these different constituents (about -95ÕC). PSC are thought to play a major role in the formation of the "ozone hole" because they absorb odd nitrogen from the atmosphere, which allows the catalytic destruction of ozone to occur. Nacreous clouds appear stationary and, by day, often resemble pale cirrus. At sunset, all the colors of the spectrum appear; as the sky darkens after sunset, they increase in brilliance. As the sun drops lower and lower below the horizon and the clouds are lighted by last rays, the various colors are replaced by a general coloration that is first orange and then becomes pink, contrasting vividly with the darkening sky. The clouds next become gray and the colors of the spectrum reappear but very weakly, then fade out rapidly. Later, up until about two hours after sunset, the nacreous clouds can still be distinguished standing out against the starry sky as tenuous and gray clouds. They can even be observed all night if there is moonlight. Before dawn, the same series of aspects appear, but in reverse order. (from: AMS Glossary of Meteorology, 2nd addition)
Polar Vortex(Also called polar cyclone, polar low, circumpolar whirl.)
The planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere. The westerly airflow is largely a manifestation of the thermal wind above the polar frontal zone of middle and subpolar latitudes. The vortex is strongest in winter when the pole-to-equator temperature gradient is strongest. In the Northern Hemisphere, the vortex has two centers in the mean, one near Baffin Island and the other over northeast Siberia. (from: AMS Glossary of Meteorology, 2nd addition)
