At sunset you can see the most fantastic colors and bizarre pictures. Sometimes the thought comes to mind that if you draw this truthfully, then people will not believe it - they will say that this does not happen, and that the artist exaggerated reality. We are used to thinking that all this is physics, everything is explained by the refraction of light in the layers of the atmosphere. However, there are phenomena in the sky that still do not have an exact explanation and which have been studied by meteorologists, physicists, and astronomers for a long time. One such phenomenon is noctilucent clouds.
Noctilucent clouds. Photo: mygeos.com
Noctilucent clouds are a very beautiful and relatively rare atmospheric phenomenon that can be observed at latitudes between 43° and 65° in the summer during short nights, in deep twilight. These are the highest clouds in the Earth's atmosphere, they form 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. It is quite simple to distinguish mesospheric clouds from ordinary low tropospheric clouds: the latter are visible against the background of the evening dawn as dark, and the former as light and even seemingly luminous, because the setting sun can only “illuminate” fairly “high” objects.
The optical density of mesospheric clouds is negligible, and stars often peer through them. It is not surprising that these clouds are observed mainly on the shortest nights at high latitudes: precisely under such conditions when the sun sets for a short time and not far beyond the horizon. Interestingly, noctilucent clouds move very quickly - their average speed is 100 meters per second.
The nature of noctilucent clouds is not fully understood. Noctilucent clouds were first noticed in 1885, two years after the eruption of the Krakatoa volcano. The ash ejected by this volcano produced such magnificent sunsets that viewing the pre-sunset sky became a very popular activity. One of these observers was the German scientist T.W. Backhouse, who noticed thin silver stripes shimmering with a bluish light in a completely black sky and described them in his article. Private Associate Professor of Moscow University Vitold Karlovich Tserasky, who observed noctilucent clouds on June 12, 1885, also noticed that these clouds, clearly visible against the background of the twilight sky, became completely invisible when they went beyond the twilight segment of the sky. He called them "night luminous clouds." Initially, scientists associated the appearance of noctilucent clouds with volcanic dust, but the phenomenon was observed quite often in the absence of volcanic eruptions. V.K. Tserasky, together with the astronomer from the Pulkovo Observatory A.A. Belopolsky, who was working at the Moscow Observatory at that time, studied the noctilucent clouds and determined their height, which, according to his observations, ranged from 73 to 83 km. This value was confirmed 3 years later by the German meteorologist Otto Jesse.
In 1926, researcher of the Tunguska meteorite L.A. Kulik proposed the meteorite-meteorite hypothesis of the formation of noctilucent clouds, according to which meteor particles that entered the Earth’s atmosphere are condensation nuclei of water vapor. However, this theory did not explain their characteristic fine structure, comparable to that of cirrus clouds. In 1952, I. A. Khvostikov put forward a hypothesis, called the condensation (or ice) hypothesis, according to which noctilucent clouds have a structure similar to the structure of cirrus clouds consisting of ice crystals.
Recently, the theory of meteoric origin of noctilucent clouds was confirmed by NASA. “We found particles of “meteor smoke” in the composition of noctilucent clouds. This discovery confirms the theory that particles of meteoric dust are nuclei around which crystals of noctilucent clouds form,” said NASA AIM (Aeronomy of Ice in the Mesosphere) program scientist. James Russell from Hampton University.
More than a ton of meteor dust falls on Earth every day. Flying into the atmosphere at enormous speeds, most of this dust completely burns up at altitudes of 70-100 km, leaving behind “smoke” consisting of microscopic particles. These particles form a kind of crystallization centers, around which water molecules form ice crystals. But unlike the crystals that form in ordinary clouds, the crystals of noctilucent clouds are very small. About 10-100 times finer than rain cloud crystals. This explains the unusual bluish tint of noctilucent clouds, since small ice crystals better refract light from the shorter wavelengths of the spectrum - blue and violet.
At present, the nature of the appearance at an altitude of 80 km in sufficient quantities of water vapor necessary for the formation of noctilucent clouds is not completely clear. In 2012, after 5 years of operation of the AIM satellite, a new hypothesis was published about the nature of the appearance of water in the mesosphere, which could explain why clouds appeared 130 years ago, and had not been observed before. According to this theory, the source of water formation is methane gas, with which the Earth’s atmosphere began to be intensively enriched, starting from the end of the century before last. The increase in methane content in the atmosphere is largely facilitated by industrial development of oil and gas fields, disposal of household and industrial waste, etc. In terms of its greenhouse effect, methane is tens of times greater than carbon dioxide. But CO 2 is heavier than air and therefore accumulates directly at the surface of the Earth and is also “utilized” by plants. Methane is lighter than air and rises up to 10-12 km. At the same time, part of the methane molecules, under the influence of solar radiation and atmospheric oxygen and ozone, decompose into water molecules, which, under the influence of convective currents, rise even higher, up to 70-80 km. There they condense on meteor dust and give rise to noctilucent clouds. Thus, scientists believe that noctilucent clouds may be a kind of indicator of excessive accumulation of methane and subsequent global warming due to the greenhouse effect.
Research into noctilucent clouds continues. “Nocturnal luminous clouds,” or “polar mesospheric clouds,” as they are also called, serve as the main source of information about the movement of air masses in the upper atmosphere, which makes their study an even more pressing and important task. This is precisely the goal of the PoSSUM (Polar Suborbital Science in the Upper Mesosphere) project led by Jason Reimuller. The researcher explains: “The idea is to create a laboratory to study noctilucent clouds. We are talking about a portable laboratory that would be located on board an aircraft and would make the measurements we need during suborbital flight. One of the most important instruments in this laboratory is a laser radar. Scattering of laser pulses on molecules of ozone, nitrogen, oxygen, argon and carbon dioxide, which are very rare at this altitude, will make it possible to monitor thermodynamic processes occurring in the mesosphere." The PoSSUM project involves spraying trimethylaluminum into the mesosphere - and it is planned to record luminous plumes not from the surface of the earth, as happened previously within the ATREX project, but from aircraft at an altitude of about 6.5 thousand meters.
The content of the article
noctilucent clouds, 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.
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°, and in most cases from 55° to 65°. In the Southern Hemisphere at the end of December and in January at latitudes from 40° to 65°. 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° to 15° degrees in the northern part of the sky (for observers of 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 temperature minimum, where the atmosphere is cooled to –70° C, and sometimes to –100° C. Altitudes from 50 to 150 km have been poorly studied, since airplanes and balloons cannot rise there, and artificial Earth satellites cannot capable of staying 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 are constantly involved in physical and chemical processes occurring in the upper atmosphere. The results of this interaction are observed in the form of auroras, airglow, meteor phenomena, changes in color and the duration of twilight. 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 (T.W. 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 the noctilucent clouds in detail and determined their height for the first time, obtaining values from 73 to 83 km, confirmed 3 years later by the German meteorologist Otto Jesse (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 light appear, the walls of buildings were very noticeably illuminated and vaguely visible objects protruded 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 absolutely impossible to get an idea about it without drawings and a detailed description. Some long, dazzling silver streaks, crossing or parallel to the horizon, change quite slowly and are so sharp that they can be kept in the field of view of the telescope.”
Observation of 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°. 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.
In the second half of civil twilight (solar depth 3–6°), the western part of the sky still has quite bright dawn illumination, but in neighboring areas the sky already acquires deep dark blue and blue-green shades. The region of greatest brightness of the sky during this period is called the twilight segment.
The most favorable conditions for detecting noctilucent clouds are created during the period of navigational twilight, when the Sun dives below the horizon by 6–12° (at the end of June in mid-latitudes this happens 1.5–2 hours before true midnight). At this time, the earth's shadow covers the lower, most dense, dusty layers of the atmosphere, and only rarefied layers are illuminated, starting with the mesosphere. Sunlight scattered in the mesosphere forms a faint glow in the twilight sky; Against this background, the glow of noctilucent clouds is easily detected, which attract the attention of even casual witnesses. Various observers define their color as pearl-silver with a bluish tint or blue-white.
At dusk, the color of noctilucent clouds appears unusual. Sometimes the clouds seem to phosphorescent. Barely noticeable shadows move along them. Certain areas of the cloud field become significantly brighter than others. After a few minutes, neighboring areas may appear brighter.
Despite the fact that the wind speed in the stratosphere is 100–300 m/s, the high altitude of noctilucent clouds makes them almost motionless in the field of view of a telescope or camera. Therefore, the first photographs of these clouds were obtained by Jesse back in 1887. Several groups of researchers around the world are systematically studying noctilucent clouds in both the Northern and Southern Hemispheres. The study of noctilucent clouds, like other difficult-to-predict natural phenomena, involves the widespread involvement of science enthusiasts. Every naturalist, regardless of his main profession, can contribute to the collection of facts about this remarkable atmospheric phenomenon. A high-quality photograph of noctilucent clouds can be obtained using a simple amateur camera. For example, you can use a Zenit camera with a standard Helios-44 lens; with an aperture of 2.8–3.5 and a film sensitivity of 100–200 units. GOST recommends shutter speeds from 2–3 to 10–15 seconds. It is very important that the camera does not shake during exposure; For this, it is advisable to use a reliable tripod, but in extreme cases, it is enough to press the camera with your hand to a window frame, tree or stone; When releasing the shutter, be sure to use a cable.
In order for the resulting images to be of not only aesthetic interest, but also have a scientific meaning and provide material for subsequent analysis, it is necessary to accurately record the circumstances of the shooting (time, parameters of equipment and photographic materials), and also use the simplest devices: light filters, polarizing filters, a mirror for determining the speed of movement of contrasting cloud details.
In appearance, noctilucent clouds have some similarities with high cirrus clouds. To describe the structural forms of noctilucent clouds during their visual observation, an international morphological classification has been developed:
Type I. Fleur, the simplest, even form, fills the space between more complex, contrasting details and has 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.
The zone of maximum frequency of observation of noctilucent clouds in the Northern Hemisphere lies at latitude 55–58°. Many large cities of Russia fall into this band: Moscow, Yekaterinburg, Izhevsk, Kazan, Krasnoyarsk, Nizhny Novgorod, Novosibirsk, Chelyabinsk, etc., and only a few cities in Northern Europe and Canada.
Properties and nature of noctilucent clouds.
The altitude range at which noctilucent clouds form is generally quite stable (73–95 km), but in some years it narrows to 81–85 km, and sometimes expands to 60–118 km. Often a cloud field consists of several rather narrow layers. The main reason for the glow of clouds is their scattering of sunlight, but it is possible that the effect of luminescence under the influence of ultraviolet rays from the Sun also plays some role.
The transparency of noctilucent clouds is extremely high: a typical cloud field blocks only about 0.001% of the light passing through it. It was the nature of the scattering of sunlight by noctilucent clouds that made it possible to establish that they are clusters of particles 0.1–0.7 microns in size. A variety of hypotheses have been expressed about the nature of these particles: it was assumed that they could be ice crystals, small particles of volcanic dust, salt crystals in an ice “coat,” cosmic dust, particles of meteoric or cometary origin.
Bright noctilucent clouds, first observed in 1885–1892 and apparently not noticed before, suggested that their appearance was associated with some powerful catastrophic process. Such a phenomenon was the eruption of the Krakatoa volcano in Indonesia on August 27, 1883. In fact, it was a colossal explosion with an energy equal to the explosion of twenty hydrogen bombs (20 Mt TNT). About 35 million tons of volcanic dust, rising to a height of 30 km, and a huge mass of water vapor were thrown into the atmosphere. After the Krakatoa explosion, optical anomalies were noticed: bright dawns, a decrease in atmospheric transparency, polarization anomalies, Bishop's ring (a brown-red crown around the Sun with an outer angular radius of about 22° and a width of 10°; the sky inside the ring is light with a bluish tint). These anomalies lasted for about two years, gradually weakening, and noctilucent clouds appeared only towards the end of this period.
The hypothesis about the volcanic nature of noctilucent clouds was first expressed by the German researcher W. Kohlrausch in 1887; he considered them to be condensed water vapor released during the eruption. Jesse in 1888–1890 developed this idea, believing that it was not water, but some unknown gas (possibly hydrogen) that was ejected by the volcano and frozen into small crystals. It has been suggested that volcanic dust also plays a role in the formation of noctilucent clouds by serving as nuclei for water vapor crystallization.
The gradual accumulation of observational data provided facts that clearly spoke against the volcanic hypothesis. Analysis of light anomalies after major volcanic eruptions (Mont Pele, 1902; Katmai, 1912; Cordillera, 1932) showed that only in rare cases were they accompanied by the appearance of noctilucent clouds; most likely these were random coincidences. Currently, the volcanic hypothesis, which at the beginning of the 20th century. considered generally accepted and even penetrated into meteorology textbooks, has only historical significance.
The emergence of the meteor hypothesis of the origin of noctilucent clouds is also associated with a grandiose natural phenomenon - the Tunguska disaster on June 30, 1908. From the point of view of observers, among whom were very experienced astronomers and meteorologists (V. Denning, F. Bush, E. Esclangon, M. Wolf, F. Arkhengold, D.O. Svyatsky, etc.), this phenomenon manifested itself mainly as various optical anomalies observed in many European countries, in the European part of Russia and Western Siberia, right up to Krasnoyarsk. Along with bright dawns and “white nights” that occurred in places where they usually do not occur even at the end of June, many observers noted the appearance of noctilucent clouds. However, in 1908, none of the eyewitnesses of optical anomalies and luminous clouds knew anything about the Tunguska meteorite. Information about him appeared in print only about 15 years later.
In 1926, the idea of a connection between these two phenomena was independently expressed by the first researcher of the Tunguska disaster site, L.A. Kulik, and meteorologist L. Apostolov. Leonid Alekseevich Kulik developed his hypothesis in detail, proposing a very specific mechanism for the formation of noctilucent clouds. He believed that not only large meteorites, but also ordinary meteors, which completely collapse at altitudes of 80–100 km, deliver their sublimation products into the mesosphere, which then condense into particles of the finest dust that form clouds.
In 1930, the famous American astronomer H. Shapley, and in 1934, independently of him, the English meteorologist F. J. Whipple (not to be confused with the American astronomer F. L. Whipple) hypothesized that the Tunguska meteorite was the nucleus of a small comet with a dust tail. The penetration of tail matter into the earth's atmosphere led, in their opinion, to the appearance of optical anomalies and the appearance of noctilucent clouds. However, the idea that the cause of the optical anomalies of 1908 was the passage of the Earth through a cloud of cosmic dust was expressed back in 1908 by one of the eyewitnesses of the “bright nights” of that period, F. de Roy, who, of course, knew nothing about the Tunguska meteorite.
In subsequent years, the meteor hypothesis was supported and developed by many astronomers, trying to explain with its help the observed features of noctilucent clouds - their morphology, latitudinal and temporal distribution, optical properties, etc. But the meteor hypothesis in its pure form failed to cope with this task, and since 1960 its development has practically ceased. But the role of meteoric particles as condensation nuclei and growth of ice crystals that make up noctilucent clouds is still undisputed.
The condensation (ice) hypothesis itself has been developing independently since 1917, but for a long time did not have sufficient experimental foundations. In 1925, the German geophysicist A. Wegener, based on this hypothesis, calculated that for steam to condense into ice crystals at an altitude of 80 km, the air temperature should be about –100 ° C; as it turned out during rocket experiments 30 years later, Wegener turned out to be very close to the truth. Since 1950, in the works of V.A. Bronshten, I.A. Khvostikov and others, the meteor-condensation hypothesis of noctilucent clouds was developed; in it, meteoric particles play the role of condensation nuclei, without which the formation of droplets and crystals from steam in the atmosphere is extremely difficult. This hypothesis is partly based on the results of rocket experiments, during which microscopic solid particles with an ice “coat” frozen on them were collected at altitudes of 80–100 km; when rockets were launched into the zone of observed noctilucent clouds, the number of such particles turned out to be a hundred times greater than in the absence of clouds.
In addition to the mentioned “classical” hypotheses, other, less traditional ones have been put forward; The connection of noctilucent clouds with solar activity, with auroras, and with other geophysical phenomena was considered. For example, the source of water vapor in the mesosphere was considered to be the reaction of atmospheric oxygen with solar wind protons (the “solar rain” hypothesis). One of the latest hypotheses links noctilucent clouds to the formation of ozone holes in the stratosphere. The area of formation of these clouds is being studied more and more actively in connection with space and stratospheric transport: on the one hand, launches of powerful rockets with hydrogen-oxygen engines serve as an important source of water vapor in the mesosphere and stimulate the formation of clouds, and on the other hand, the appearance of clouds in this area creates problems when returning spacecraft to Earth. It is necessary to create a reliable theory of noctilucent clouds, making it possible to predict and even control this natural phenomenon. But still many facts in this area are incomplete and contradictory.
Vladimir Surdin
Noctilucent clouds, forming almost at the border of the earth's atmosphere and space, which greatly complicates their study, still keep many secrets about their nature and origin.
The first documented evidence of the observation of noctilucent clouds can be found in the astronomical works of scientists from the Old World. These records date back to the mid-17th century and are characterized by extreme scarcity, unsystematicity and contradictory facts. Only in the summer of 1885 did this strange phenomenon attract the attention of several astronomers from different countries of the Northern Hemisphere. The honor of discovering unusual clouds based on the results of independent observations was shared between the Russian scientist V.K. Tserasky and the German scientist T.W. Backhouse. It was the domestic astronomer who approached the study of a new phenomenon in science most responsibly. He was able to determine the approximate distance to the boundaries of the manifestation of a unique atmospheric process (about 80 km) and the negligible optical density of these formations. Over the next three years, noctilucent clouds were studied by another German scientist, Otto Jesse. He confirmed the data obtained by Tserasky and gave the newly discovered phenomenon its current name.
General information
Noctilucent (night luminous, polar mesomorphic) clouds are record holders of the earth's atmosphere, the altitude of their formation varies between 70-95 km. The formation of phenomena of this kind is possible only in regions of the stratosphere with minimal temperature regimes ranging from -70 to -120°C. The time of appearance of noctilucent clouds is evening and predawn twilight. The zonal features in which the processes of their formation take place have for many years made it practically impossible to obtain objective information about this amazing atmospheric phenomenon. Additional negative factors included the proximity of space, penetrating particles of meteoric matter and interstellar dust, the effect of magnetic fields, various physical and chemical reactions, and the dependence of observations on the position of the Earth and time of day. In addition, the altitude of noctilucent clouds in the mesosphere turned out to be difficult to reach for many modern aircraft (too high for airplanes, low for satellites). Today, the study and research of a unique phenomenon is dominated by representatives of geophysical and astronomical trends in science.
Properties and types
Online image of noctilucent clouds from the AIM satellite
The basis of noctilucent clouds is made up of crystals of frozen moisture that condenses and then forms an ice shell around microscopic particles (0.1-0.7 microns) of terrestrial or cosmic origin. This explains the maximum transparency of such formations, which block only a thousandth of the light flux.
The stars are clearly visible through the noctilucent clouds. The core of crystals can be invisible fragments of meteoric or cometary matter, volcanic or interplanetary dust, frozen particles of water vapor. Since the discovery of this phenomenon, scientists have put forward various assumptions about its causes and origin. The hypotheses evolved as follows: volcanic (since 1887), meteoric (since 1926), condensation (since 1950). Other theories appeared periodically, trying to explain the atmospheric phenomenon with the help of various geophysical phenomena, but they did not gain support in scientific circles.
Noctilucent clouds have a varied structure, based on which they are classified according to these characteristics into several types:
- Fleur– the most primitive form, characterized by a blurred structure and a dull whitish glow.
- Stripes– line up in small parallel or intertwining lines, reminiscent of jets. They can be sharply defined or blurred.
- Waves- visually very similar to the surface of water distorted by small ripples. They are divided into 3 subspecies.
- Vortexes– represent twisted ring-shaped swirls with a dark central part. Based on the radius and complexity of the structure, 3 subgroups are distinguished, the last of which includes the rarest phenomenon - clouds resembling a luminous substance scattering from an explosion.
Today, noctilucent clouds are unique and one-of-a-kind formations that carry scientifically important information about the processes occurring in the mesopause. Research into this phenomenon is carried out using rocket, laser and radar sounding methods, providing new information about wave atmospheric movements, high-altitude winds and processes affecting their temporal changes.
Image gallery
Conditions and time of observation
During daylight hours, it is unlikely that noctilucent clouds will be found and seen in the sky. Their time is a dark, clear sky in the deep evening or predawn twilight, when the earth's star drops 6-12° below the horizon. During this period, the sun's rays cease to illuminate the lower atmospheric masses, continuing their impact on the rarefied upper regions: the stratosphere and mesosphere. The background created under such conditions is optimal for observing the beauty of noctilucent clouds. Despite the significant wind force at high altitudes, the objects formed are quite static, which makes them easier to study and photograph, creating an excellent opportunity to examine all the details of a rare phenomenon. Residents of both the Southern and Northern Hemispheres can enjoy the fantastic shapes and colors of noctilucent clouds. For the former, this is possible in January-February at 40°-65° latitude, for the latter - June-July, 45°-70°. The most possible place for objects to appear is the northern part of the sky at a height above the horizon from 3 to 15 degrees.
Travels of noctilucent clouds in the sky over Belarus in the summer of 2013!
The first high-quality photographs of noctilucent clouds were obtained by the German scientist Otto Jesse back in 1887.
Unique atmospheric formations of this type are very difficult to distinguish from their feathery counterparts, so confusion periodically arises among lovers of celestial light shows on this issue.
For residents of Russia, the optimal area for observing this interesting phenomenon will be latitudes from 55° to 58°.
In our hemisphere, the study and research of noctilucent clouds is available only to astronomers and meteorologists from the Russian Federation, Canada and Northern Europe. Moreover, the maximum contribution of discoveries in this area belongs not to professional scientists, but to amateurs.
The altitude range in which the formation processes of the phenomenon occur is inexplicably capable of compressing to 80-85 km, expanding afterward to 60-120 km.
The main reason for the colorful glow of noctilucent clouds is the scattering effect of the ultraviolet spectrum of sunlight.
By 2007, NASA specialists developed and launched the AIM project. The mission was made up of a satellite whose equipment records the main processes occurring in the mesosphere of our planet. High-precision instruments have expanded knowledge of the chemical composition of noctilucent clouds by analyzing and measuring ice crystals, gas molecules and cosmic dust particles.
Lecture by O.S. Ugolnikov about noctilucent clouds
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.
As an atmospheric optical phenomenon, noctilucent clouds are clouds glowing with a silvery color 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 middle 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.
To date, there is no consensus in the scientific community 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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