MOSCOW, June 20 – RIA Novosti. The phenomenon of the appearance of so-called noctilucent clouds in the upper layers of the Earth's atmosphere may be associated with the ancient eruption of the Krakatoa volcano, says a joint report by Roscosmos and the Moscow Planetarium.
Noctilucent clouds are the highest cloud formations in the earth's atmosphere, occurring at altitudes of 70-95 kilometers. They are also called polar mesospheric clouds (PMC) or noctilucent clouds (NLC). These are light translucent clouds that are sometimes visible against a dark sky summer night in middle and high latitudes.
“The fact that this atmospheric phenomenon was not observed until 1885 has led many scientists to believe that their appearance is associated with a powerful catastrophic process on Earth - the eruption of the Krakatoa volcano in Indonesia on August 27, 1883, when about 35 million tons were released into the atmosphere volcanic dust and a huge mass of water vapor. Other hypotheses have been expressed: meteoric, technogenic, and the “solar rain” hypothesis. But many facts in this area are still incomplete and contradictory, so noctilucent clouds continue to be an exciting problem for many naturalists,” noted in the message.
How noctilucent clouds form
Noctilucent clouds form in the upper layers of the atmosphere, at altitudes of about 90 kilometers, and are illuminated by the Sun, which has dropped shallowly below the horizon (therefore, in the Northern Hemisphere they are observed in the northern part of the sky, and in Southern Hemisphere- in the south). For their formation, a combination of three factors is necessary: a sufficient amount of water vapor, a very low temperature, and the presence of tiny dust particles on which water vapor condenses, turning into ice crystals.
"When noctilucent clouds form, the centers of moisture condensation are likely to be particles of meteorite dust. Sunlight scattered by tiny ice crystals gives the clouds their characteristic bluish-blue color. Due to their high altitude, noctilucent clouds glow only at night, scattering sunlight , which falls on them from under the horizon. During the day, even against the background of a clear blue sky, these clouds are not visible: they are very thin, “ethereal”. Only deep twilight and night darkness make them noticeable to a ground-based observer, however, with the help of equipment. raised to high altitudes, these clouds can be recorded during the daytime. It is easy to see the amazing transparency of noctilucent clouds: the stars are clearly visible through them,” the researchers note.
Noctilucent clouds in the Northern Hemisphere
Noctilucent clouds can be observed only in the summer months in the Northern Hemisphere in June-July, usually from mid-June to mid-July, and only in geographical latitudes from 45 to 70 degrees, and in most cases they are more often visible at latitudes from 55 to 65 degrees. In the Southern Hemisphere, they are observed at the end of December and in January at latitudes from 40 to 65 degrees. At this time of year and at these latitudes, the Sun, even at midnight, does not descend very deeply below the horizon, and its sliding rays illuminate the stratosphere, where noctilucent clouds appear at an average altitude of about 83 kilometers. As a rule, they are visible low above the horizon, at an altitude of 3-10 degrees in the northern part of the sky (for observers in the Northern Hemisphere). With careful observation, they are noticed every year, but they do not reach high brightness every year.
Noctilucent clouds are the highest cloud formations in the earth's atmosphere, forming at altitudes of 70-95 km. They are also called polar mesospheric clouds (PMC) or noctilucent clouds (NLC). It is the last name that most accurately corresponds to their appearance and the conditions for their observation are accepted as standard in international practice.
Noctilucent clouds can be observed only in the summer months: in the Northern Hemisphere in June-July, usually from mid-June to mid-July, and only at latitudes from 45 to 70 degrees, and in most cases from 55 to 65 degrees. In the Southern Hemisphere - at the end of December and in January at latitudes from 40 to 65 degrees. At this time of year and at these latitudes, the Sun, even at midnight, does not descend very deeply below the horizon, and its sliding rays illuminate the stratosphere, where noctilucent clouds appear at an average altitude of about 83 km. As a rule, they are visible low above the horizon, at an altitude of 3-10 degrees in the northern part of the sky (for observers in the Northern Hemisphere). With careful observation, they are noticed every year, but they do not reach high brightness every year.
During the day, even against the background of a clear blue sky, these clouds are not visible: they are very thin, “ethereal”. Only deep twilight and night darkness make them visible to a ground observer. True, with the help of equipment raised to high altitudes, these clouds can be recorded during the daytime. It is easy to see the amazing transparency of noctilucent clouds: the stars are clearly visible through them.
For geophysicists and astronomers, noctilucent clouds are of great interest. After all, these clouds are born in the region of minimum temperature, where the atmosphere is cooled to -70 C, and sometimes to -100 C. Altitudes from 50 to 150 km are still poorly studied, since airplanes and balloons cannot fly there, and artificial Earth satellites cannot go down there for a long time. Therefore, scientists are still arguing both about the conditions at these altitudes and about the nature of the noctilucent clouds themselves, which, unlike low tropospheric clouds, are located in the zone of active interaction of the Earth’s atmosphere with outer space. Interplanetary dust, meteoric matter, charged particles of solar and cosmic origin, magnetic fields constantly participate in physical and chemical processes occurring in the upper atmosphere. The results of this interaction are observed in the form polar lights, airglow, meteor phenomena, color changes and twilight duration. It remains to be seen what role these phenomena play in the development of noctilucent clouds.
Currently, noctilucent clouds represent the only natural source of data on winds at high altitudes and wave movements in the mesopause, which significantly complements the study of its dynamics by other methods such as radar of meteor trails, rocket and laser sounding. The vast areas and significant lifetime of such cloud fields provide a unique opportunity to directly determine the parameters of atmospheric waves of various types and their time evolution.
Due to the geographical features of this phenomenon, noctilucent clouds are mainly studied in Northern Europe, Russia and Canada. Russian scientists have made and are making a very significant contribution to this work, and a significant role is played by qualified observations obtained by science enthusiasts.
Discovery of noctilucent clouds
Some references to night luminous clouds are found in the works of European scientists of the 17th and 18th centuries, but they are fragmentary and unclear. The time of discovery of noctilucent clouds is considered to be June 1885, when they were noticed by dozens of observers in different countries. The discoverers of this phenomenon are considered to be T. Backhouse, who observed them on June 8 in Kissingen (Germany), and Moscow University astronomer Witold Karlovich Tserasky, who discovered them independently and observed them for the first time on the evening of June 12 (new style). In the following days, Tserasky, together with the famous Pulkovo astrophysicist A.A. Belopolsky, who was then working at the Moscow Observatory, studied noctilucent clouds in detail and for the first time determined their height, obtaining values from 73 to 83 km, confirmed 3 years later by the German meteorologist O. Jesse.
The night luminous clouds made a great impression on Tserasky: “These clouds shone brightly in the night sky with pure, white, silvery rays, with a slight bluish tint, taking on a yellow, golden hue in the immediate vicinity of the horizon. There were cases when they made the walls light the buildings were very noticeably illuminated, and vaguely visible objects stood out sharply. Sometimes the clouds formed layers or layers, sometimes they looked like rows of waves, or resembled a sandbank covered with ripples or wavy irregularities... This is such a brilliant phenomenon that it is completely impossible to compose it. an idea of him without drawings and detailed description. Some long, dazzling silver streaks, crisscrossing or parallel to the horizon, change quite slowly and are so sharp that they can be kept in the field of view of a telescope." (For a more detailed description of the discovery of noctilucent clouds by Cerasski, see the Appendix.)
Observing noctilucent clouds
It should be remembered that noctilucent clouds can be observed from the surface of the Earth only during deep twilight, against the backdrop of an almost black sky and, of course, in the absence of lower, tropospheric clouds. It is necessary to distinguish the twilight sky from the dawn sky. Dawns are observed during the period of early civil twilight, when the center of the solar disk descends below the observer's horizon to a depth of 0 to 6 degrees. At the same time, the sun's rays illuminate the entire thickness of the layers of the lower atmosphere and the lower edge of the tropospheric clouds. Dawn is characterized by a rich variety of bright colors.
Noctilucent clouds are the highest cloud formations in the earth's atmosphere, forming at altitudes of 70-95 km. They are also called polar mesospheric clouds (PMC) or noctilucent clouds (NLC). It is the latter name, which most accurately corresponds to their appearance and the conditions of their observation, that is accepted as standard in international practice.
As a rule, they are visible low above the horizon, at an altitude of 3-10 degrees in the northern part of the sky (for observers in the Northern Hemisphere). With careful observation, they are noticed every year, but they do not reach high brightness every year. In the book by V.A. Bronshten “Noctilucent clouds and their observation” provides data from a catalog of noctilucent clouds compiled by N.P. Fast based on 2000 observations for the years 1885-1964. This catalog gives the following distribution of observation points by latitude:
Latitude........................ 50...... 50-55..... 55-60..... 60
Number of observations (%).....3.8 .....28.1 ......57.4 .....10.8
What is the reason for this? At this time, it is in these latitudes that favorable conditions are created for their visibility, since it is at these latitudes at this time that the Sun, even at midnight, descends shallowly below the horizon, and against the background of the twilight sky beautiful silvery formations are observed, the structure reminiscent of light cirrus clouds. This happens because they glow mainly with the reflected light of the Sun, although some of the rays they send may be generated in the process of fluorescence - the re-emission of energy received from the Sun at other wavelengths. In order for this to happen, the rays of the Sun must illuminate the noctilucent clouds. Knowing their average height above earth's surface, it can be calculated that the immersion of the Sun should not exceed 19.5 degrees. At the same time, if the Sun has sunk less than 6 degrees, it is still too light (civil twilight), and clouds may not be visible in the bright sky. Thus, the most favorable conditions for observing noctilucent clouds correspond to the time of the so-called navigational and astronomical twilight, and the longer these twilights, the greater their likelihood. Such conditions are created in the summer at mid-latitudes from mid-June to mid-July (in the Southern Hemisphere - at the end of December and in January at latitudes from 40 to 65 degrees). It is at mid-latitudes from late May to mid-August that noctilucent clouds are most often observed. True, this coincidence is purely accidental. In fact, noctilucent clouds form precisely in summer period and precisely in the middle latitudes because at this time at these latitudes there is a significant cooling in the mesopause, and necessary conditions to form ice crystals.
Noctilucent clouds were first observed in 1885. Before this, there was no information about noctilucent clouds. The discoverer of noctilucent clouds is considered to be V.K. Tserasky, a private associate professor at Moscow University. He observed noctilucent clouds on June 12, 1885, when he noticed unusually bright clouds filling the twilight segment in the predawn sky. The scientist called them night luminous clouds. The scientist was especially surprised by the fact that the clouds stood out brightly against the background of the twilight segment, and completely disappeared when they went beyond its limits. He was very concerned about this because, without being visible, they could absorb starlight and distort the results of photometric measurements. But the very first measurements of luminous clouds showed that these clouds are very transparent and do not noticeably weaken the light of the stars.
The first assumptions about the nature of noctilucent clouds were associated with the eruption of the Krakatoa volcano on August 27, 1883. In the twenties of the 20th century, L.A. Kulik, a researcher of the famous Tunguska meteorite, put forward a meteorite hypothesis for the formation of noctilucent clouds. Kulik also suggested that not only giant meteorites, but also ordinary meteors are the source of the formation of noctilucent clouds. The meteor hypothesis was popular for a long time, but could not answer a number of questions:
Why do they appear in a narrow altitude range with an average value of 82-83 kilometers?
Why are they observed only in summer and only in mid-latitudes?
Why do they have a characteristic fine structure, very similar to that of cirrus clouds?
The answer to all these questions was given by the condensation (or ice) hypothesis. This hypothesis received serious justification in 1952 in the work of I.A. Khvostikov, who drew attention to the external similarity of noctilucent and cirrus clouds. Cirrus clouds are made up of ice crystals. I.A. Khvostikov suggested that noctilucent clouds have the same structure. But in order for water vapor to condense into ice, certain conditions are needed. In 1958 V.A. Bronshten gave an explanation for the seasonal and latitudinal effects of the appearance of noctilucent clouds by the fact that it is at middle latitudes in summer time During the mesopause, the temperature drops to extremely low values of 150-165 K. Thus, I.A. Khvostikov’s hypothesis about the possibility of the formation of noctilucent clouds in this area of the atmosphere was confirmed.
True, the researchers faced one more question: is there such a high altitude sufficient water vapor to form noctilucent clouds? The hypothesis of the cosmic origin of condensation nuclei is now preferred. In fact, the destruction of meteoroids penetrating the earth's atmosphere and observed in the form of meteors occurs mainly just above the mesopause, at altitudes of 120-80 km. Research shows that up to 100 tons of matter “fall” on Earth every day, and the number of particles with a mass of 10 grams suitable as condensation nuclei is quite enough to ensure the formation of noctilucent clouds. Attempts have been made to find a connection between the occurrence of noctilucent clouds and the intensity meteor showers.
Structure of noctilucent clouds.
In 1955 N.I. Grishin proposed a morphological classification of the forms of noctilucent clouds. Later it became an international classification. Combination various forms noctilucent clouds formed the following main types:
Type I. Fleur, the simplest, even form, filling the space between more complex, contrasting details and having a foggy structure and a weak, soft white glow with a bluish tint.
Type II. Stripes resembling narrow streams, as if carried away by air currents. They are often located in groups of several, parallel to each other or intertwined at a slight angle. The stripes are divided into two groups - blurred (II-a) and sharply defined (II-b).
Type III. Waves are divided into three groups. Scallops (III-a) - areas with a frequent arrangement of narrow, sharply defined parallel stripes, like light ripples on the surface of the water with a slight gust of wind. Ridges (III-b) have more noticeable signs of a wave nature; the distance between adjacent ridges is 10–20 times greater than that of scallops. Wave-like bends (III-c) are formed as a result of the curvature of the cloud surface, occupied by other forms (stripes, ridges).
Type IV. Vortexes are also divided into three groups. Small radius vortices (IV-a): from 0.1° to 0.5°, i.e. no larger than the lunar disk. They bend or completely curl stripes, combs, and sometimes flairs, forming a ring with a dark space in the middle, reminiscent of a lunar crater. Swirls in the form of a simple bend of one or more stripes away from the main direction (IV-b). Powerful vortex emissions of “luminous” matter away from the main cloud (IV-c); This rare formation is characterized by rapid variability of its shape.
But even within a type, noctilucent clouds are different. Therefore, in each type of clouds, groups are identified that indicate a specific structure of the clouds (blurry stripes, sharply defined stripes, ridges, ridges, wavy bends, etc.) Usually, when observing noctilucent clouds, you can see several of their forms at once different types and groups.
Noctilucent clouds have been studied both from the ground and from space, as well as by rocket probes; they are too high for stratospheric balloons. The AIM satellite, launched in April 2007, studies noctilucent clouds from orbit.
Studies of noctilucent clouds are necessary for a deeper understanding of the circulation of the Earth's atmosphere, as well as many processes occurring outside the Earth, on the Sun.
It is noteworthy that noctilucent clouds are one of the main sources of information about the movement of air masses in the upper layers of the atmosphere. Noctilucent clouds move extremely quickly in the upper atmosphere - their average speed is about 100 meters per second. 
Sources: http://www.astrogalaxy.ru/775.html
http://ru.wikipedia.org/wiki/Noctilucent_clouds
http://www.astronet.ru/db/msg/1214909
http://www.cloudappreciationsociety.org 
Cloud view
Noctilucent clouds (also known as mesospheric clouds) are a rare phenomenon, usually observed during the summer months at latitudes between 50° and 60° (north and south latitude). Highlighted as an independent phenomenon by V.K. Tserasky. The study of noctilucent clouds was carried out by V.V. Sharonov.
How atmospheric optical phenomenon, noctilucent clouds are luminous silver color clouds of various bizarre shapes observed at dusk. Not observed during daylight hours.
Mesospheric clouds are the highest clouds in the Earth's atmosphere; formed in the mesosphere at an altitude of about 85 km, and are visible only when illuminated by the sun from above the horizon, while the lower layers of the atmosphere are in the earth's shadow; they are not visible during the day. Moreover, their optical density is so insignificant that stars often peer through them. Noctilucent clouds have not been fully studied. It has been suggested that they consist of volcanic or meteoric dust, but they are known from data from the UARS satellite to consist mainly of water ice. This is a relatively young phenomenon - they were first reported in 1885, shortly after the Krakatoa eruption, and there was speculation. They have been studied from the ground and from space, as well as by rocket probes; they are very high for stratospheric balloons. The AIM satellite, launched in April 2007, studies noctilucent clouds from orbit. It is noteworthy that noctilucent clouds are one of the main sources of information about the movement of air masses in the upper layers of the atmosphere. Noctilucent clouds move extremely quickly in the upper atmosphere - their average speed is about 100 meters per second. Quite a lot of people are photographing noctilucent clouds. There are sections on astronomy forums where observers share their photographs.
Structure of noctilucent clouds
In 1955 N.I. Grishin proposed a morphological classification of the forms of noctilucent clouds. Later it became an international classification. The combination of different forms of noctilucent clouds formed the following main types:- Type I. Fleur, the simplest, even form, filling the space between more complex, contrasting details and having a foggy structure and a weak, soft white glow with a bluish tint.
- Type II. Stripes resembling narrow streams, as if carried away by air currents. They are often located in groups of several, parallel to each other or intertwined at a slight angle. The stripes are divided into two groups - blurred (II-a) and sharply defined (II-b).
- Type III. Waves are divided into three groups. Scallops (III-a) - areas with a frequent arrangement of narrow, sharply defined parallel stripes, like light ripples on the surface of the water with a slight gust of wind. Ridges (III-b) have more noticeable signs of a wave nature; the distance between adjacent ridges is 10–20 times greater than that of scallops. Wave-like bends (III-c) are formed as a result of the curvature of the cloud surface, occupied by other forms (stripes, ridges).
- Type IV. Vortexes are also divided into three groups. Small radius vortices (IV-a): from 0.1° to 0.5°, i.e. no larger than the lunar disk. They bend or completely curl stripes, combs, and sometimes flairs, forming a ring with a dark space in the middle, reminiscent of a lunar crater. Swirls in the form of a simple bend of one or more stripes away from the main direction (IV-b). Powerful vortex emissions of “luminous” matter away from the main one
Noctilucent clouds are the highest cloud formations in the earth's atmosphere, occurring at altitudes of 70-95 km. They are also called polar mesospheric clouds (PMC) or noctilucent clouds (NLC). These are light translucent clouds that are sometimes visible against the dark sky on a summer night in mid and high latitudes.
“These clouds shone brightly in the night sky with clean, white, silvery rays, with a slight bluish tint, taking on a yellow, golden hue in the immediate vicinity of the horizon” - this is how Vitold Karlovich TSERASKY describes the night luminous clouds, who first observed them on June 12, 1885 in Moscow .
Noctilucent clouds form in the upper layers of the atmosphere, at altitudes of 80-90 km and are illuminated by the Sun, which has fallen shallowly below the horizon (therefore, in the Northern Hemisphere they are observed in the northern part of the sky, and in the Southern Hemisphere - in the southern). For their formation, a combination of three factors is necessary: a sufficient amount of water vapor; very low temperature; the presence of tiny dust particles on which water vapor condenses, turning into ice crystals.
During the formation of noctilucent clouds, the centers of moisture condensation are likely to be particles of meteorite dust. Sunlight scattered by tiny ice crystals gives clouds their characteristic bluish-blue color. Due to their high altitude, noctilucent clouds glow only at night, scattering sunlight that hits them from below the horizon. During the day, even against the background of a clear blue sky, these clouds are not visible: they are very thin, “ethereal”. Only deep twilight and night darkness make them visible to a ground observer. True, with the help of equipment raised to high altitudes, these clouds can be recorded during the daytime. It is easy to see the amazing transparency of noctilucent clouds: the stars are clearly visible through them.
Noctilucent clouds can be observed only in the summer months in the Northern Hemisphere in June-July, usually from mid-June to mid-July, and only at latitudes from 45 to 70 degrees, and in most cases they are more often visible at latitudes from 55 to 65 degrees. In the Southern Hemisphere, they are observed at the end of December and in January at latitudes from 40 to 65 degrees. At this time of year and at these latitudes, the Sun, even at midnight, does not descend very deeply below the horizon, and its sliding rays illuminate the stratosphere, where noctilucent clouds appear at an average altitude of about 83 km. As a rule, they are visible low above the horizon, at an altitude of 3-10 degrees in the northern part of the sky (for observers in the Northern Hemisphere). With careful observation, they are noticed every year, but they do not reach high brightness every year.
Until now in scientific community There is no consensus regarding the origin of noctilucent clouds. The fact that this atmospheric phenomenon was not observed until 1885 led many scientists to believe that their appearance was associated with a powerful catastrophic process on Earth - the eruption of the Krakatoa volcano in Indonesia on August 27, 1883, when about 35 million tons of volcanic dust and a huge mass of water vapor. Other hypotheses have also been expressed: meteoric, man-made, the “solar rain” hypothesis, etc. But so far, many facts in this area are incomplete and contradictory, so noctilucent clouds continue to be an exciting problem for many naturalists.
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