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 appear 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.
Contents 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 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 minimum temperature, 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 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 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 from the surface of the Earth, noctilucent clouds can be observed 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°. In this case, 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, densest, dust-laden 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 found its way 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 (W. 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, the meteor-condensation hypothesis of noctilucent clouds has been developed in the works of V.A. Bronshten, I.A. Khvostikov and others; 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, which will make it possible to predict and even control this natural phenomenon. But still many facts in this area are incomplete and contradictory.
Vladimir Surdin
Just a few hundred years ago, the Earth was full of the unknown, and in order to paint over the blank spots, hypothetical aborigines with dog heads and human faces on their stomachs were drawn on geographical maps. Since then, mysteries on our planet have diminished. All the more interesting are those that modern science still cannot solve...
Sergey Sysoev
Polarization of Light Light is an electromagnetic wave. Polarization for electromagnetic waves is the phenomenon of directional oscillation of the electric and magnetic field strength vectors. Linear polarization is a special case of polarization when the oscillations of the electric field strength vector lie in the same plane
Today, lidar installations (LIDAR, English Light Identification, Detection and Ranging), in which a laser serves as the source of the light beam, are widely used to study the atmosphere. A small part of its radiation, scattered in the atmosphere, returns back and is captured by the receiver. This makes it possible to calculate the distance from the installation to the area of the atmosphere that scattered the signal from the time of arrival of the reflected signal. Pictured is the lidar of the Pierre Auger Observatory (Argentina)
The diagram clearly shows the principle of operation of the lidar installation. Unfortunately, the method has an insurmountable limitation: it requires a clear sky - in dense clouds the laser beam is lost almost completely
Noctilucent clouds form at an altitude of approximately 80 km, in the region bordering the meso- and thermosphere, the so-called mesopause. The mesosphere is cold—the temperature in it drops to -150°C. The thermosphere is characterized by very high temperatures - the air (if this monstrously rarefied substance can be called that) under the influence of solar radiation sometimes heats up to 1500 K. The concentration of gas molecules in the thermosphere is so low that the usual mechanisms for transferring thermal energy practically do not work, and the only way cool down - radiate energy. Noctilucent clouds “live” in such difficult conditions
The reason why noctilucent clouds are observed at night and not during the day is clear from the diagram above. While the observer is still in the “night territory”, noctilucent clouds fall into the sunlit zone;. Noctilucent clouds “love” not just the night, but the summer night. The reason for this is simple. Oddly enough, the upper mesosphere cools most strongly in the summer: the dynamics of air flows in the atmosphere are to blame for this. There are also no problems with crystallization centers - after all, microparticles of meteoric origin are actually present in the mesosphere
In June 1885, with an interval of several days, several European astronomers noticed an unusual phenomenon: strange clouds of a previously unseen structure, glowing in the evening or early morning twilight, when the Sun was below the horizon. In Germany, this phenomenon was observed by astronomers Otto Jesse and Thomas William Backhaus, in Austria-Hungary by Vaclav Laska, in Russia by Witold Karlovich Cerasky. Since all the first observations were made independently of each other, it would be unfair to consider one person the discoverer. Jesse and Tserasky paid the most serious attention to the new phenomenon. The latter managed to establish with acceptable accuracy the height of the new clouds above the Earth's surface - about 75 versts. He was the first to establish the negligible optical density of clouds - the brilliance of the stars “closed” by them almost did not lose power! Jesse also carried out corresponding measurements, but with slightly less accuracy. But it was he who came up with the name that has become widespread since then - “noctilucent clouds”. In English-language literature, this phenomenon is usually called noctilucent clouds or (especially in NASA materials) polar mesospheric clouds - PMC.
Conditions of existence
By the end of the 19th century, there were many astronomers in Europe who regularly observed the sky. Until the summer of 1885, none of them described anything resembling noctilucent clouds. Perhaps observations of clouds were not recorded in scientific history due to triviality? But by 1885, the same Witold Cerasky had already been engaged in photometry of the twilight sky for about ten years. This painstaking task required close attention to any clouds that could distort the data. Tserasky wrote: “It would be quite difficult for me not to notice a phenomenon that sometimes covers no more than the entire vault of heaven.” Otto Jesse shared the same opinion. Therefore, we will proceed from the fact that noctilucent clouds were actually not observed before the summer of 1885 and, probably, did not exist. Of course, attempts to explain the novelty of nature were made very quickly. The most logical explanation at that moment seemed to be the catastrophic eruption of the Krakatoa volcano on the territory of modern Indonesia, which led to a powerful explosion that literally lifted the entire island into the air. There were other theories - we will look at them below. But before we say anything about the noctilucent clouds themselves, it is worth paying attention to the conditions in which they exist.
The Earth's atmosphere is a complex object characterized by various conditions. By height, it is usually divided into the troposphere (up to 10 km), stratosphere (10−50 km), mesosphere (50−85 km), thermosphere and exosphere. Noctilucent clouds form in the region bordering the meso- and thermosphere - the so-called mesopause.
The physical conditions above and below the mesopause are different. The mesosphere is cold—the temperature in it drops to -150°C. The thermosphere, on the contrary, is characterized by very high temperatures - the air under the influence of solar radiation sometimes heats up to 1500K. The concentration of gas molecules in the thermosphere is so low that the usual mechanisms for transferring thermal energy do not work, and the only way to cool down is to radiate energy.
Now imagine what kind of clouds can appear in such “harsh” conditions? Ordinary cirrocumulus clouds “live” in the troposphere, at an altitude of 5-6 km, and are something like water fog. A cloud that can form at an altitude of 70 km can be compared to a person who has adapted to existence without protective equipment, for example, on Jupiter...
Where did they come from?
Above we mentioned the volcanic hypothesis of the formation of noctilucent clouds, proposed by the German physicist Friedrich Kohlrausch at the end of the 19th century. Alas, subsequent studies showed that the properties of clouds and the properties of volcanic aerosols suspended in the atmosphere are very different.
In the 1920s, meteorite researcher Leonid Kulik proposed a hypothesis of the meteorite origin of noctilucent clouds - according to it, they consist of tiny particles of meteorite matter dispersed in the upper layers of the atmosphere. Indeed, studies of the mesosphere by meteorological rockets back in the 1960s showed that noctilucent clouds contain a certain amount of substance clearly of meteorite origin. But by that time another theory was already in the scientific mainstream - the condensation theory, which was started by the Soviet physicist Ivan Andreevich Khvostikov.
An important feature of noctilucent clouds is that they are observed from year to year at the same altitudes (about 80 km), the same latitudes (50−70 degrees) and only in the summer, and all these rules are followed in the North , and in the Southern Hemispheres. Neither volcanic nor meteoric hypotheses could explain these facts. The condensation theory suggests that noctilucent clouds are composed of tiny ice crystals frozen onto aerosol particles. The zone where these nano-ice flakes appear is at an altitude of about 90 km, from there they gradually drift downward under the influence of gravity, increasing in size. At an altitude of about 85 km, their clusters become visible at dusk when illuminated by the sun from below - clouds appear. For the formation of such ice floes, at least three conditions are needed: low temperature, sufficient humidity and the presence of crystallization centers.
The biggest problem is air humidity. The upper kilometers of the mesosphere are drier than the Sahara - there is negligible water there and it comes there mainly from two sources. This is, firstly, water vapor from below, and secondly, the destruction of methane molecules under the influence of solar ultraviolet radiation, after which water is formed with the participation of atmospheric oxygen. The difficulty is that water molecules also disintegrate under the influence of solar radiation - their average life time in the mesopause is several days. It is not yet completely clear as to under what conditions and in what time frame a sufficient amount of water can accumulate in the mesopause, therefore, while the condensation version is plausible, the question is far from closed.
Study Tools
Studying noctilucent clouds is not easy. The air above the stratosphere is so rarefied that neither an airplane nor a balloon can stay in it; the only aircraft capable of reaching such heights is a rocket. This creates considerable inconvenience for researchers: a rocket flying at high speed is in the studied area for a few seconds and has very limited contact with the environment. Its launch is not possible from anywhere and is quite expensive.
In the first half of the 20th century, it was proposed to use optical sensing to study the atmosphere. At first, a powerful spotlight was used for this. The observed scattering of the light beam provided information about the composition and state of air masses. In the USA, searchlight sounding was used mainly to determine air density and temperature; in the USSR, the study of atmospheric aerosols was also considered an important task, for which the searchlight beam was polarized and then the distribution of polarization with height was studied. Of course, the searchlight as a light source was not very convenient - the sounding ceiling never exceeded 70 km.
Since the 1960s, so-called lidar systems, in which a laser serves as the source of the light beam, have been increasingly used to study the atmosphere. A small part of its radiation, scattered in the atmosphere, returns back and is captured by the receiver. Laser radiation is coherent, its wavelength and polarization can be determined with great accuracy. The laser beam can be emitted for a period of time determined with high accuracy. This sets the length of the light beam. This allows the time of arrival of the reflected signal to be used to calculate the distance from the installation to the area of the atmosphere that scattered the signal with an accuracy of several meters. Well, the characteristics of reflected (scattered) radiation carry information about the environment from which it was reflected.
The second important tool is the study of light polarization. The fact that the sunlight we see is polarized was discovered by Francois Arago back in 1809; he also established that the maximum polarization is at an angular distance of 90 degrees from the Sun. The degree of polarization of light is influenced by the properties of the medium on which it is scattered. This is what the method is based on. What is especially remarkable is that at twilight, when the Sun below the horizon illuminates the earth’s atmosphere from below, polarimetry provides information about the properties of a particular layer of air that is brightest at that moment. Thus, by measuring polarization during twilight, the distribution of properties over height can be obtained.
With the beginning of the space age, the question arose that noctilucent clouds could be observed from space. The first apparatus created specifically for the study of the mesosphere and noctilucent clouds was the American satellite AIM (The Aeronomy of Ice in the Mesosphere), launched in 2007 and still operating in orbit.
...and the Tunguska meteorite
The most famous case of mass observation of noctilucent clouds occurred in the summer of 1908, immediately after the fall of the Tunguska meteorite and, logically, in connection with it. “White nights” began almost throughout Europe because of luminous clouds, even where no one had ever heard of them. Eyewitnesses recalled that there was enough light in the middle of the night to read a newspaper. Unfortunately, almost no reliable instrumental measurements have been carried out, and modern estimates differ greatly - the illumination of those nights is estimated to be 10-8000 times higher than the natural background.
Contemporaries, as a rule, did not associate unusual clouds with the Tunguska meteorite, since they did not know about its existence. The very fact of the fall of some celestial body somewhere in the Yenisei province was known - they even tried to look for it, but scientists were able to assess the true scale of what happened only two decades later. In addition, it was in those places that no atmospheric anomalies, at least obvious ones, were observed. The night illumination was explained by volcanism, which sounded plausible at that time.
From the point of view of today's ideas, the noctilucent clouds of the summer of 1908 are still more likely associated with Tunguska - but how? Although there are about a hundred versions of what happened in 1908, scientists have the greatest confidence in two: meteorite and comet. Meteorite stumbles upon a fundamental problem - where did the pebble go? Comet seems better in all respects, but the appearance of noctilucent clouds within it seems difficult to explain. The substance dispersed in the atmosphere should have flown away from Vanavara to the east, and noctilucent clouds would have been visible in Vladivostok and Tokyo - but nothing like that happened. In addition, the size of the cometary “aura” reaches hundreds of thousands and sometimes millions of kilometers. Approaching the Earth approximately from the direction of the Sun, the tailed guest should have deposited dust in the atmosphere a couple of days before the fall, and the rotation of the Earth would have distributed all the matter evenly around the circumference in a completely natural way.
So it turns out that the mysterious Tunguska phenomenon significantly increases the number of questions about noctilucent clouds. 125 years after privatdozent Witold Karlovich Tserasky saw unusual clouds in the sky in the morning, we still cannot say with certainty that we understand where and how they came from.
Noctilucent clouds - what are they?
General information about noctilucent clouds.
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. Noctilucent clouds form at altitudes from 73 to 97 km, with a maximum of their distribution at 83-85 km, when the temperature drops to 150-165 K. Although this phenomenon is atmospheric, historically its studies are considered astronomical, since a number of phenomena in our atmosphere are so or otherwise associated with processes occurring on the Sun, with meteor showers. In addition, the study of the atmospheres of other planets is inextricably linked with the study of our own atmosphere. In addition, noctilucent clouds, unlike other clouds, are observed at night, and their observation and registration of their appearances can be carried out simultaneously with the observation of other astronomical phenomena or objects.
Noctilucent clouds can be observed from March to October in the northern hemisphere and from November to April in the southern hemisphere. But most often in the northern hemisphere they are observed from late May to mid-August (with a peak in June-July), in the southern hemisphere in the winter months.
The observation range is limited to latitudes from 50 to 65 degrees. But there are rare cases of their observation at lower latitudes - up to 45 degrees. 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 at 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 the earth's surface, it can be calculated that the sun's immersion 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. 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 are formed precisely in the summer and precisely in the middle latitudes because at this time at these latitudes there is a significant cooling in the mesopause, and the necessary conditions are created for the formation of ice crystals.
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 the summer season in the mesopause that the temperature drops to extremely low values of 150-165 K. Thus, I.A. Khvostikov’s hypothesis about the possibility of formation in this area atmosphere of noctilucent clouds was confirmed.
However, the researchers were faced with another question: does water vapor exist at such a high altitude in quantities sufficient to form noctilucent clouds? The work of scientists in this area has yielded unexpected results. A clear maximum of water vapor content was established in July-August and a minimum in January-February (in the northern hemisphere). That is, the fact of an increase in humidity in those seasons, over those latitudes and at the level where noctilucent clouds form, has been established. This fact has a simple explanation: above 25-30 kilometers at mid-latitudes in the summer, ascending air currents are observed that carry water vapor to the mesopause region. There the water vapor freezes, forming noctilucent clouds. Its deficiency is compensated by a new influx of steam from below. At other latitudes and in other seasons, upward air currents either do not arise or are suppressed by the absence of freezing. There is another explanation. It consists in the fact that water vapor at high altitudes is formed by the interaction of hydrogen atoms flying towards the Earth from the Sun with oxygen atoms of the upper layers of the Earth's atmosphere. This idea was expressed by the Norwegian scientist L. Vegard in 1933 and received quantitative substantiation in 1961 in the work of the French scientist C. de Tourville. True, this “solar rain” hypothesis has weaknesses and cannot fully explain the increased humidity in the mesopause. In recent years, some researchers have put forward another source of supplying the mesopause with water vapor. This hypothesis is supported, for example, by Iowa State University professor L. Frank, Russian scientist V.N. Lebedinets and some other scientists. They believe that the mesopause region supplies enough water vapor to form noctilucent clouds on the mini-comet. What particles serve as condensation nuclei in the formation of noctilucent clouds? Various assumptions have been made: particles of volcanic dust, crystals of sea salt, meteor particles. The hypothesis that it is meteoric particles that serve as condensation nuclei was put forward by L.A. Kulik in 1926 in his meteoric-meteorite hypothesis of the origin of noctilucent clouds. In 1950, this hypothesis was again independently put forward by V.A. Bronshten.
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 appearance of noctilucent clouds and the intensity of 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. 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 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 distinguished that indicate a specific structure of the clouds (blurry stripes, sharply defined stripes, ridges, crests, wavy bends, etc.). You can learn more about this classification of the forms of noctilucent clouds in the book by V.A. Bronshten "Noctilucent clouds and their observations." Usually, when observing noctilucent clouds, you can see several of their forms of different types and groups at once.
Types and methods of observing noctilucent clouds.
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 possible that the weather on Earth depends not only on conditions in the troposphere, but also on the state of higher layers of the atmosphere. Observations of noctilucent clouds are different; their organization, methodology and implementation depend on the objectives. The following types of observations of noctilucent clouds can be distinguished:
- 1. Synoptic observations are systematic observations of the twilight segment with the aim of establishing the presence or absence of noctilucent clouds, and if they are visible, recording some characteristic features.
- 2. Study of the structure. Can be done through visual observation, photography or time-lapse filming.
- 3. Study of the movements of noctilucent clouds. It is produced by photographing them sequentially or by slow-motion filming. A theodolite may be needed here.
- 4. Determination of heights. To solve this problem, you need to photograph noctilucent clouds at pre-agreed moments from two points separated by a distance of 20-0 km. The cameras must be the same in both cases. We need an accurate clock. To process observations you will need a special palette.
- 5. Photometry and polarimetry. Produced from photographs. But to perform these tasks, special devices are needed.
These are the main types of observations. Some of the above tasks can be performed using the same observations. The same photographs can be used to study the structure, movements, height determination and photometry of noctilucent clouds. The weather observer can take photographs of noctilucent clouds between recordings. The synoptic method is most suitable for amateur observations of noctilucent clouds. It involves patrolling the twilight segment, statistics of noctilucent clouds, description of their structure and brightness. In my work, I mainly used the synoptic method of observing noctilucent clouds. To study the structure of noctilucent clouds, a photographic method was used. The azimuth and height of noctilucent clouds above the horizon were also measured.
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 appear 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
