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Polar stratospheric clouds or PSCs, also known as nacreous clouds (, from ''nacre'', or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of . They are best observed during civil twilight when the sun is between 1 and 6 degrees below the horizon.〔 They are implicated in the formation of ozone holes. The effects on ozone depletion arise because they support chemical reactions that produce active chlorine which catalyzes ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction.〔World Meteorological Organization ''(Scientific Assessment of Ozone Depletion (2002) )'', particularly section 3.2.2 (pages 3.21, i.e. 195 of the PDF file, and following).〕 ==Formation== The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their physical state and chemical composition. Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk. PSCs form at very low temperatures, below . These temperatures can occur in the lower stratosphere in polar winter. In the Antarctic, temperatures below frequently cause type II PSCs. Such low temperatures are rarer in the Arctic. In the Northern hemisphere, the generation of lee waves by mountains may locally cool the lower stratosphere and lead to the formation of PSCs. Forward-scattering of sunlight within the clouds produces a pearly-white appearance. Particles within the optically thin clouds cause colored Interference fringes by diffraction. The visibility of the colors may be enhanced with a polarising filter.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polar stratospheric cloud」の詳細全文を読む スポンサード リンク
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