Noctilucent clouds: Difference between revisions

Revision as of 14:45, 20 February 2012

noctilucent clouds

(Rarely called luminous clouds.) Thin silvery-blue cirrus-like clouds frequently seen during summer twilight conditions at high latitudes (above 50°) in both hemispheres.

They are the highest visible clouds in the atmosphere, occurring in the upper mesosphere at heights of about 85 km, and are closely related to the polar mesospheric clouds seen in satellite observations at similar altitudes over the summer polar cap. Noctilucent clouds are now known to consist of tiny ice particles with dimensions of the order of tens of nanometers, growing in the extreme cold of the summer polar mesopause region. The condensation nuclei on which the particles grow are thought to be either smoke and dust particles of meteoric origin or large hydrated positive ions. Strong upwelling of air from below, associated with a pole-to-pole meridional circulation in the upper mesosphere, is responsible for both the extreme cold and the upward flux of water vapor. Although water-vapor mixing ratios are very low (less than 10 parts per million by volume) in the region, the temperatures are also low enough to produce a high degree of supersaturation at times. Anomalously strong radar echoes from the region, known as polar summer mesospheric echoes, are also associated with the clouds.
Compare nacreous clouds, polar stratospheric clouds.

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-<div class="definition"><div class="short_definition">(Rarely called luminous clouds.) Thin silvery-blue cirrus-like clouds frequently  seen during summer twilight conditions at high latitudes (above 50&deg;) in both hemispheres.</div><br/> <div class="paragraph">They are the highest visible clouds in the [[atmosphere]], occurring in the upper [[mesosphere]]  at heights of about 85 km, and are closely related to the [[polar mesospheric clouds]] seen in satellite  observations at similar altitudes over the summer polar cap. Noctilucent clouds are now known  to consist of tiny [[ice]] particles with [[dimensions]] of the order of tens of nanometers, growing in  the extreme cold of the summer polar [[mesopause]] region. The [[condensation nuclei]] on which  the [[particles]] grow are thought to be either [[smoke]] and [[dust]] particles of meteoric origin or large  hydrated positive ions. Strong upwelling of air from below, associated with a pole-to-pole [[meridional  circulation]] in the upper mesosphere, is responsible for both the extreme cold and the upward  [[flux]] of [[water vapor]]. Although water-vapor [[mixing ratios]] are very low (less than 10 parts per  million by volume) in the region, the temperatures are also low enough to produce a high degree  of [[supersaturation]] at times. Anomalously strong radar [[echoes]] from the region, known as polar  summer mesospheric echoes, are also associated with the clouds. <br/>''Compare'' [[nacreous clouds]], [[polar  stratospheric clouds]].</div><br/> </div>
+<div class="definition"><div class="short_definition">(Rarely called luminous clouds.) Thin silvery-blue cirrus-like clouds frequently  seen during summer twilight conditions at high latitudes (above 50&#x000b0;) in both hemispheres.</div><br/> <div class="paragraph">They are the highest visible clouds in the [[atmosphere]], occurring in the upper [[mesosphere]]  at heights of about 85 km, and are closely related to the [[polar mesospheric clouds]] seen in satellite  observations at similar altitudes over the summer polar cap. Noctilucent clouds are now known  to consist of tiny [[ice]] particles with [[dimensions]] of the order of tens of nanometers, growing in  the extreme cold of the summer polar [[mesopause]] region. The [[condensation nuclei]] on which  the [[particles]] grow are thought to be either [[smoke]] and [[dust]] particles of meteoric origin or large  hydrated positive ions. Strong upwelling of air from below, associated with a pole-to-pole [[meridional  circulation]] in the upper mesosphere, is responsible for both the extreme cold and the upward  [[flux]] of [[water vapor]]. Although water-vapor [[mixing ratios]] are very low (less than 10 parts per  million by volume) in the region, the temperatures are also low enough to produce a high degree  of [[supersaturation]] at times. Anomalously strong radar [[echoes]] from the region, known as polar  summer mesospheric echoes, are also associated with the clouds. <br/>''Compare'' [[nacreous clouds]], [[polar  stratospheric clouds]].</div><br/> </div>
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