Noctilucent Cloud

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noctilucent cloud

[¦näk·tə¦lü·sənt ′klau̇d]
A cloud of unknown composition which occurs at great heights and high altitudes; photometric measurements have located such clouds between 45 and 54 miles (75 and 90 kilometers); they resemble thin cirrus, but usually with a bluish or silverish color, although sometimes orange to red, standing out against a dark night sky.

Noctilucent Cloud


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.


Bronshten, V. A., and N. I. Grishin. Serebristye oblaka. Moscow, 1970.


References in periodicals archive ?
Winter to summer changes in the Antarctic lower atmosphere sparked a complex series of responses throughout the atmosphere 6 one of which is an earlier noctilucent cloud season," NASA said.
Prof Gary Thomas, of the University of Colorado's laboratory for atmospheric and space physics and a world expert on noctilucent clouds, said: "This is a big event.
The researchers contend that the massive amount of water vapor spewed into the atmosphere by the comet's icy nucleus was caught up in swirling eddies with tremendous energy by a process called two-dimensional turbulence, which explains why the noctilucent clouds formed a day later many thousands of miles away.
This theory appears to explain the mysterious fact that noctilucent clouds, as far as we know, have been around for only about 100 years.
For the last five years observers who have formed a network in Canada and the United States have searched the skies for the elusive and mysterious noctilucent clouds.
The Director has given talks about aurora and noctilucent clouds to several astronomical societies during 2010 '11.
But higher up, above all the usual clouds, at a height of about 50 miles is a layer known as the mesosphere - and it's here that we can observe a phenomenon called noctilucent clouds.
The program intends to send trained astronauts to the Mesosphere altitude (at 83 Km) to study noctilucent clouds.
Later we're shown an image one of the staff members, keen astrophotographer Matt Robinson, took of noctilucent clouds from his home in Sunderland, an image of such significance a film crew are due to follow him round for a week.
Noctilucent clouds in Earth's high-latitude upper mesosphere form at temperatures below 150 K (Rapp and Thomas 2006) and it has been suggested that they are composed of cubic ice (Murray and Plane 2003a,b, 2005; Murray and Jensen 2010).
Using measurements of noctilucent clouds as a guide, Schilling suggested that the mesosphere's height might vary from roughly 100 km at the equator down to 82 km at latitude 60[degrees] north or south.
the director of northumberland's kielder observatory has said that the stunning noctilucent clouds are likely to be visible in clear skies for the rest of july.