There are no clouds that are silvery or golden. Noctilucent clouds. Type II, stripes
MOSCOW, June 20 - RIA Novosti. The phenomenon of the appearance of so-called noctilucent clouds in the upper layers of the Earth's atmosphere may be associated with the ancient eruption of the Krakatoa volcano, says a joint report by Roscosmos and the Moscow Planetarium.
Noctilucent clouds are the highest cloud formations in the earth's atmosphere, occurring at altitudes of 70-95 kilometers. They are also called polar mesospheric clouds (PMC) or noctilucent clouds (NLC). These are light, translucent clouds that are sometimes visible against the dark sky on a summer night in middle and high latitudes.
"The fact that this atmospheric phenomenon was not observed until 1885 has led many scientists to believe that their appearance is associated with a powerful catastrophic process on Earth - the eruption of the Krakatoa volcano in Indonesia on August 27, 1883, when about 35 million tons were released into the atmosphere volcanic dust and a huge mass of water vapor. Other hypotheses have been expressed: meteoric, technogenic, and the “solar rain” hypothesis. But many facts in this area are still incomplete and contradictory, so noctilucent clouds continue to be an exciting problem for many naturalists,” noted in the message.
How noctilucent clouds form
Noctilucent clouds form in the upper layers of the atmosphere, at altitudes of about 90 kilometers, and are illuminated by the Sun, which has descended 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, a very low temperature, and the presence of tiny dust particles on which water vapor condenses, turning into ice crystals.
"When noctilucent clouds form, the centers of moisture condensation are likely to be particles of meteorite dust. Sunlight scattered by tiny ice crystals gives the clouds their characteristic bluish-blue color. Due to their high altitude, noctilucent clouds glow only at night, scattering sunlight , which falls on them from under the horizon. During the day, even against the background of a clear blue sky, these clouds are not visible: they are very thin, “ethereal”. Only deep twilight and night darkness make them noticeable to a ground-based observer, however, with the help of equipment. raised to high altitudes, these clouds can be recorded during the daytime. It is easy to see the amazing transparency of noctilucent clouds: the stars are clearly visible through them,” the researchers note.
Noctilucent clouds in the Northern Hemisphere
Noctilucent clouds can be observed only in the summer months in the Northern Hemisphere in June-July, usually from mid-June to mid-July, and only 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 kilometers. As a rule, they are visible low above the horizon, at an altitude of 3-10 degrees in the northern part of the sky (for observers in the Northern Hemisphere). With careful observation, they are noticed every year, but they do not reach high brightness every year.
(at an altitude of 80-85 km above the earth's surface) and visible in deep dusk . Observed during the summer months in latitudes between 43° and 60° (north and south latitude).Mesosphere(from Greek μεσο- - “average” and σφαῖρα - “ball”, “sphere”) - layer atmosphere at altitudes from 40-50 to 80-90 km. Characterized by an increase in temperature with altitude; maximum (about +50° C ) temperature is located at an altitude of about 60 km, after which the temperature begins to decrease to −70° or −80° C . This decrease in temperature is associated with the energetic absorption of solar radiation (radiation) ozone Term accepted Geographical and Geophysical Union in 1951.
The gas composition of the mesosphere, like that of the underlying atmospheric layers, is constant and contains about 80% nitrogen and 20% oxygen.
The mesosphere separates from the underlying stratosphere stratopause , and from the overlying thermosphere - mesopause . Mesopause basically coincides with turbo pause.
Examples of noctilucent clouds
Noctilucent cloud at sunset. Reflection of sunlight
Noctilucent clouds at night. Reflection of sunlight.
Noctilucent clouds at night. The light source is not visible, but it is the Sun
Noctilucent clouds reflecting ground lighting.
Noctilucent clouds refracting light. And it’s unlikely that this is at an altitude of 50 km...
Noctilucent clouds create the impression of “additional” lighting (photo from my window) Photo:
This is how the sky was colored this summer (photo from my window).
First silver clouds were described by V.K. Tserasky, a private assistant professor at Moscow University, who observed them on June 12, 1885. From now on silver clouds are regularly observed by both professional and amateur astronomers. For astronomy enthusiasts, observing noctilucent clouds is of interest because... to observe them you do not need any optical instruments, moreover, a telescope silver clouds difficult to observe due to the small field of view of the instrument. Take pictures silver clouds does not present any difficulty, because shooting clouds is no different from ordinary photography, with the exception of a longer shutter speed. If you have a film or video camera, then the observation of noctilucent clouds acquires scientific value, because with the help of slow motion you can trace all the changes occurring in the silver clouds during the filming period.
Observe silver clouds in the northern hemisphere Earth possible at latitudes from 50 to 70 degrees. Silver clouds are observed on average at altitudes of 70-80 km and are visible against the background of the twilight segment. The best conditions for visibility of noctilucent clouds are the period of navigational twilight, when the Sun descends below the observer's horizon by 6-12°. At this time, against the dimly lit background of the twilight sky, luminous clouds. The best observation time is June and early July, i.e. a time when astronomical twilight in mid-latitudes does not end.
Silver clouds are a magnificent spectacle, because glow against the background of the sky and change appearance quite quickly and outwardly somewhat resemble auroras. To detect noctilucent clouds, you need to view the northern part of the sky every day about an hour after sunset Sun and during the night an hour before sunrise. It is during this period that you can see silver clouds, but if you did not find clouds, then you must indicate this, remembering that a negative result is also a result.
If clouds detected, then it is necessary to carry out observations and record them in the observation log.
The objectives of amateur observations of noctilucent clouds may be as follows:
1. Synoptic observations, i.e. systematic observations of the twilight segment in order to establish the presence or absence of noctilucent clouds, and if they are visible, to record some characteristic features (extent in azimuth and altitude, brightness, morphological forms). To carry out these observations, you need a site with an open northern horizon and a clock.
2. Study of the structure. Can be done through visual observation, photography or time-lapse filming. The value of observations increases as you move from the first method to the third. Necessary tools: Zenit type camera, movie camera.
3. Study of the movements of noctilucent clouds. Produced by photographing them sequentially or by slow-motion filming.
4. Determination of heights. To solve this problem you need to take photographs silver clouds at pre-agreed moments from two points separated by a distance of 20-30 km. The cameras at both points must be the same. We need an accurate clock that can be verified by radio.
Synoptic observations are intended to take into account the statistics of the appearance of noctilucent clouds. Based on synoptic observations, distributions of the appearance of noctilucent clouds are constructed by latitude, season and other characteristics (longitude, brightness points, etc.).
Opportunity to see silver clouds largely depends on the weather, more precisely, on the presence of ordinary, tropospheric clouds in the twilight segment and is determined on a letter scale:
A - the twilight sky is completely cloudless,
B - the twilight sky is partially, up to half, covered by individual clouds lower or upper tiers,
B - the twilight sky is covered by tropospheric clouds up to 4/5,
G - the twilight sky is visible only through small windows in the tropospheric clouds,
D - the twilight sky is completely covered by tropospheric clouds.
Silver clouds have a specific morphology, otherwise - structure. divided into four main types.
Type I, fleur.
Clouds almost uniform glow of individual sections of the background of the twilight sky. Fleur is very easily detected due to its fog-like structure with a soft white or bluish tint. Fleur often precedes (by about half an hour) the appearance of noctilucent clouds with a more developed structure. You can often see scallops and other noctilucent cloud features appearing in or through breaks in the flair.
Type II, stripes.
Group a (II-a). Blurry stripes arranged in groups, parallel to each other or intertwined at a slight angle.
Sometimes, the stripes seem to fan out from one distant point located on the horizon.
Group b (II - b). Stripes, sharply outlined like narrow streams, are observed mainly in noctilucent clouds with high brightness and in the presence of other well-developed forms.
Type III, scallops.
Group a (III - a). Scallops are areas with a frequent arrangement of narrow, sharply defined, parallel, usually short stripes, like light ripples on the surface of the water with a weak gust of wind.
Group b (III-6). The ridges have a more clearly defined uneven distribution of brightness in the transverse direction with clearly visible "waves"
Group c (III-c). Wave-like curves. The bends of noctilucent clouds have a clearly defined wave nature of movement.
Type IV, vortices.
Group (IV-a). Swirls and round gaps. Stripes (II), scallops (III) and sometimes flair (I) are subject to swirls.
Group b (IV-6). A twist in the form of a simple bend of one or more stripes away from the main direction.
Group c (IV-c). Powerful vortex emissions of luminous matter away from the main one clouds. This is a rare formation in silver clouds characterized by rapid variability of its shape.
Photograph silver clouds You can use any camera designed for a frame size of 24x36 mm. And such photographs are of scientific value. When shooting, the camera must be focused at infinity. It is necessary to shoot with a full hole, and the exposure time will range from a few seconds to 2-3 minutes.
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 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 in these latitudes that favorable conditions are created for their visibility, since it is at these latitudes at this time that the Sun, even at midnight, descends shallowly below the horizon, and against the background of the twilight sky beautiful silvery formations are observed, the structure reminiscent of light cirrus clouds. This happens because they glow mainly with the reflected light of the Sun, although some of the rays they send may be generated in the process of fluorescence - the re-emission of energy received from the Sun at other wavelengths. In order for this to happen, the rays of the Sun must illuminate the noctilucent clouds. Knowing their average height above 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 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 form 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 small 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. 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.
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 in the Northern Hemisphere). With careful observation, they are noticed every year, but they do not reach high brightness every year.
During the day, even against the background of a clear blue sky, these clouds are not visible: they are very thin, “ethereal”. Only deep twilight and night darkness make them visible to a ground observer. True, with the help of equipment raised to high altitudes, these clouds can be recorded during the daytime. It is easy to see the amazing transparency of noctilucent clouds: the stars are clearly visible through them.
For geophysicists and astronomers, noctilucent clouds are of great interest. After all, these clouds are born in the region of 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, 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 small 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 (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, 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, which makes it possible to predict and even control this natural phenomenon. But still many facts in this area are incomplete and contradictory.
Vladimir Surdin
