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 ab
out 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 lower 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 lower (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 95C). 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. (Abbreviated PSC; also called nacreous clouds, mother-of-pearl clouds; rarely, luminous clouds.)
High altitude clouds found in the stratosphere where the temperature is less than -85 Celsius. Commonly found over Antarctica. Have a role in the creation of the ozone hole over Antarctica.
In tropical meteorology, a wave trough in the circumpolar westerlies having sufficient amplitude to reach the Tropics in the upper air. At the surface it is manifest as a trough in the tropical easter
lies, but at moderate elevations it is characterized by westerly winds. It moves generally from west to east, accompanied by considerable cloudiness. Cumulus congestus and cumulonimbus clouds are usually found in and around the trough lines. Early- and late-season (June and October) hurricanes of the western Caribbean frequently form in polar troughs.
The term polar vortex is used to describe several different features in the atmosphere. It most commonly refers to a planetary-scale mid- to high-latitude circumpolar circulation. There are distinct t
ropospheric and stratospheric circumpolar vortices. The tropospheric polar vortex is usually defined by geopotential contours that lie within the core of the tropospheric westerlies.1 The tropospheric vortex edge is generally between 40˚ and 50˚ latitude (Fig. 1), and the vortex exists throughout the year but is strongest during winter when the air within the polar vortex is the coldest. The term 'polar vortex' is sometimes used in reference to smaller-scale (meso- to synoptic scale) vortices that usually occur within the tropospheric polar vortex in polar regions near the tropopause - for example, tropopause polar vortices. 2. The stratospheric polar vortex exists from spring to fall and usually extends from just above tropopause to the upper stratosphere (see Fig. 1). The stratospheric vortex generally increases in size from the lower stratosphere to the upper stratosphere where its edge is located around 50˚ latitude. The stratospheric vortex breaks down, and the circumpolar flow reverses, during summer. 3. Circumpolar vortices have also been observed on other planetary bodies (e.g., Mars, Venus, Saturn, and Titan). 4.
High pressure system located in the upper atmosphere at the polar regions. In this system, air in the upper troposphere moves into the vortex center and then descends to the Earth's surface to create
the polar highs.
The steady motion of the rotational axis relative to coordinates fixed in the earth. The pole moves at a rate of about 10 cm per year in the general direction of Philadelphia, Pennsylvania. Polar wand
er is believed to be due to the redistribution of matter within the earth. It is sometimes called "apparent polar wander" because it is difficult to separate from continental drift.
Stream of plasma escaping (dispersing) from the terrestrial atmosphere above the regions of the geomagnetic poles and moving along the magnetospheric lines of force.