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noctilucent cloud[¦näk·tə¦lü·sənt ′klau̇d]
a type of luminous, transparent cloud that sometimes appears in the upper part of the mesosphere at altitudes of 70–90 km. The structure of noctilucent clouds is somewhat similar to that of light cirrus clouds. Noctilucent clouds consist of aggregations of particles 10-4–10-5 cm in size that scatter sunlight. Such particles may consist of ice crystals formed upon condensation of water vapor that has been borne upward to high altitudes, or they may be volcanic or cosmic (meteoric) dust. It is possible that ice crystals form only around dust particles.
Noctilucent clouds were first investigated by V. K. Tseraskii in 1885. They are observed in the northern hemisphere between the latitudes of 45° and 70° and in the southern hemisphere between the latitudes of 40° and 65°. They occur only during the warm part of the year—from May through August in the northern hemisphere, with a maximum number of occurrences in July. The annual number of recorded occurrences observed from a given point may be as high as 20 to 30. Noctilucent clouds exist for periods of several minutes to several hours. In appearance they assume four basic configurations: gauze, bands, combs, and whirls. For observations from the ground, the best visibility is during nautical twilight, when the sun is 6°-12° below the horizon. Noctilucent clouds can also be observed during daylight if the observation equipment is raised aloft to high altitudes. Observations of noctilucent clouds are used to obtain data on the winds prevailing at the altitude of formation.
REFERENCEBronshten, V. A., and N. I. Grishin. Serebristye oblaka. Moscow, 1970.
Iu. L. TRUTTSE