At a certain height above the earth's surface and consist of droplets of water or ice crystals, or both. All the variety of clouds can be reduced to several types. The currently generally accepted international classification of clouds is based on two characteristics: appearance and the height of their lower boundary.

By appearance clouds are divided into three classes: separate cloud masses not connected with each other, layers with an inhomogeneous surface, and layers in the form of a homogeneous veil. All these forms can be found at different heights, differing in density and size of external elements (lambs, swellings, shafts, ripples, etc.)

According to the height of the lower base above the earth's surface, clouds are divided into 4 tiers: upper (Ci Cc Cs - height more than 6 km), middle (Ac As - height from 2 to 6 km), lower (Sc St Ns - height less than 2 km), vertical development (Cu Cb - can belong to different tiers, and for the most powerful cumulonimbus clouds (Cb) the base is located on the lower tier, and the top can reach the upper).

Cloud cover largely determines the amount of solar radiation reaching the Earth's surface and is a source of precipitation, thus influencing the formation of weather and climate.

The amount of clouds in Russia is distributed rather unevenly. The cloudiest areas are areas subject to active cyclonic activity, characterized by developed advection of humid weather. These include the north-west of the European part of Russia, the coast of Kamchatka, Sakhalin, the Kuril Islands and. The average annual amount of total cloud cover in these areas is 7 points. A significant part of Eastern Siberia is characterized by a lower average annual amount of clouds - from 5 to 6 points. This relatively cloudy area of ​​the Asian part of Russia is within the scope of the Asian one.

Distribution of the average annual amount of low cloud cover in general outline follows the distribution of total cloud cover. The largest number of low-level clouds also occurs in the north-west of the European part of Russia. Here they are predominant (only 1-2 points less than the amount of general cloudiness). The minimum amount of low-level clouds is noted, especially in (no more than 2 points), which is characteristic of the continental nature of the climate of these areas.

The annual variation in the amount of both total and lower clouds in the European part of Russia is characterized by minimum values ​​in summer and maximum late autumn and in winter, when the influence is especially pronounced. The exact opposite annual variation in the amount of total and lower cloudiness is observed on Far East, And . Here greatest number clouds occur in July, when the summer monsoon is in effect, bringing large amounts of water vapor from the ocean. The minimum cloudiness is observed in January during the period of greatest development of the winter monsoon, with which dry, cooled continental air from the mainland enters these areas.

The daily variation of the total amount of clouds throughout Russia is characterized by the following features:

1) its amplitude in most of the territory does not exceed 1-2 points (with the exception of central regions European part of Russia, where it increases to 3 points);

2) the amount of clouds during the day is greater than at night, while in January the maximum occurs in the morning hours; in the central months of spring and autumn, the diurnal cycle is smoothed, and the maximum can shift by different watches days; in April the diurnal cycle is closer to the summer type, and in October - to the winter type;

3) the diurnal variation of lower cloudiness practically repeats the diurnal variation of total cloudiness.

The distribution of cloud shapes is characterized by relative constancy in time and space. Almost throughout the entire territory of Russia, among the clouds of the upper tier, Ci of the middle tier – Ac of the lower tier – Sc and Ns predominate

In the annual course in the summer, a predominance of cumulus (Cu) and stratocumulus (Sc) clouds is noted, while the frequency of occurrence of stratus (St) and nimbostratus (Ns), which are frontal, is small, since in summer conditions for active cyclonic activity. The winter, spring and autumn periods in most of Russia are characterized by an increase in the frequency of altostratus (As), altocumulus (Ac) and stratocumulus (Sc) clouds, while in the European part of Russia there is a slight increase in the frequency of stratus and stratus clouds. -cumulus clouds (St).

Purpose of the lesson: study the classification of clouds and master the skills of determining the type of clouds using the “Cloud Atlas”

General provisions

The processes of formation of a separate cloud occur under the influence of many factors. Clouds and the precipitation that falls from them play a vital role in the formation of various types of weather. Therefore, cloud classification provides specialists with the opportunity to monitor the spatiotemporal variability of cloud formations, which is a powerful tool for studying and predicting processes occurring in the atmosphere.

The first attempt to divide clouds into different groups according to their appearance was made in 1776 by J. B. Lamarck. However, the classification he proposed, due to its imperfection, did not find wide application.

changes. The first classification of clouds included in science was developed by the English amateur meteorologist L. Howard in 1803. In 1887, scientists Hildebrandson in Sweden and Abercrombie in England, having revised the classification of L. Howard, proposed a draft of a new classification, which formed the basis for all subsequent classifications . The idea of ​​creating the first unified cloud atlas was supported by International conference directors of meteorological services in Munich in 1891. The committee she created prepared and published in 1896 the first International Cloud Atlas with 30 color lithographs. The first Russian edition of this Atlas was published in 1898. The further development of meteorology and the introduction into practice of synoptic analysis of the concepts of atmospheric fronts and air masses required a much more detailed study of clouds and their systems. This predetermined the need for a significant revision of the classification used at that time, which resulted in the publication in 1930 of a new International Cloud Atlas. This Atlas was published in Russian in 1933 in a slightly abbreviated version.

Clouds and precipitation falling from them are among the most important meteorological (atmospheric) phenomena and play a decisive role in the formation of weather and climate, in the distribution of flora and fauna on Earth. By changing the radiation regime of the atmosphere and the earth's surface, clouds have a noticeable impact on the temperature and humidity regime of the troposphere and ground layer of air, where human life and activity takes place.

A cloud is a visible collection of droplets and/or crystals suspended in the atmosphere and in the process of continuous evolution, which are products of condensation and/or sublimation of water vapor at altitudes from several tens of meters to several kilometers.

Changes in the phase structure of the cloud - the ratio of droplets and crystals by mass, number of particles and other parameters per unit volume of air - occur under the influence of temperature, humidity and vertical movements both inside and outside the cloud. In turn, the release and absorption of heat as a result of phase transitions of water and the presence of particles themselves in the air flow have a reverse effect on the parameters of the cloud environment.

Based on their phase structure, clouds are divided into three groups.

1. Water, consisting only of droplets with a radius of 1-2 microns or more. Drops can exist not only at positive, but also at negative temperatures. The purely droplet structure of the cloud is maintained, as a rule, to temperatures of the order of –10...–15 °C (sometimes lower).

2. Mixed, consisting of a mixture of supercooled drops and ice crystals at temperatures of –20...–30 °C.

3. Ice, consisting only of ice crystals at fairly low temperatures (about –30...–40 °C).

Cloud cover during the day reduces the influx of solar radiation to the earth's surface, and at night it noticeably weakens its radiation and, consequently, cooling, very significantly reduces the daily amplitude of air and soil temperatures, which entails a corresponding change in other meteorological quantities and atmospheric phenomena.

Regular and reliable observations of cloud forms and their transformation contribute to the timely detection of dangerous and unfavorable hydrometeorological phenomena accompanying a particular type of cloud.

The meteorological observation program includes monitoring the dynamics of cloud development and determining the following cloud characteristics:

a) the total number of clouds,

b) the number of low-level clouds,

c) the shape of the clouds,

d) the height of the lower boundary of the lower or middle level clouds (in the absence of lower level clouds).

The results of observations of cloudiness from meteorological observation units in real time using code KN-01 (national version of the international code FM 12-IX SYNOP) are regularly transmitted to local forecasting authorities (organizations and divisions of UGMS) and the Hydrometeorological Research Center of the Russian Federation (Hydrometcenter Russia) for synoptic analysis and preparation of weather forecasts at various lead times. In addition, these data are calculated over different time intervals and are used for climate assessments and generalizations.

The amount of clouds is defined as the total proportion of the sky covered by clouds from the entire visible surface of the sky and is assessed in points: 1 point is 0.1 share (part) of the entire sky, 6 points is 0.6 sky, 10 points is the entire sky is covered by clouds .

Long-term observations of clouds have shown that they can be located at different heights, both in the troposphere and in the stratosphere and even in the mesosphere. Tropospheric clouds are usually observed as isolated, isolated cloud masses or as a continuous cloud cover. Depending on their structure, clouds are divided by appearance into shapes, types and varieties. Noctilucent and nacreous clouds, in contrast to tropospheric clouds, are observed quite rarely and are characterized by relatively little diversity. The classification of tropospheric clouds by appearance currently used is called the international morphological classification.

Along with the morphological classification of clouds, genetic classification is also used, i.e. classification according to the conditions (reasons) for the formation of clouds. In addition, clouds are classified according to their microphysical structure, i.e., by their state of aggregation, type and size of cloud particles, as well as by their distribution within the cloud. According to the genetic classification, clouds are divided into three groups: stratus, wavy and cumuliform (convective).

The main distinguishing features when determining the shape of clouds are their appearance and structure. Clouds can be located at different heights in the form of separate isolated masses or a continuous cover, their structure can be different (homogeneous, fibrous, etc.), and the lower surface can be smooth or dissected (and even torn). In addition, clouds can be dense and opaque or thin - the blue sky, moon or sun shines through them.

The height of clouds of the same shape is not constant and may vary somewhat depending on the nature of the process and local conditions. On average, cloud heights are greater in the south than in the north, and greater in summer than in winter. Clouds are lower over mountainous regions than over plains.

An important characteristic of clouds is the precipitation that falls from them. Clouds of some forms almost always produce precipitation, while others either do not produce precipitation at all, or precipitation from them does not reach the surface of the earth. The fact of precipitation, as well as its type and nature of precipitation, serve as additional signs for determining the shapes, types and varieties of clouds. Fall out of clouds of certain shapes the following types precipitation:

– showers – from cumulonimbus clouds (Cb);

– covered – from nimbostratus (Ns) in all seasons, from altostratus (As) – in winter and sometimes weak – from stratocumulus (Sc);

– drizzling – from stratus clouds (St).

In the process of development and decay of a cloud, its appearance and structure change and it can transform from one form to another.

When determining the number and shape of clouds, only clouds visible from the earth's surface are taken into account. If the entire sky or part of it is covered with clouds of the lower (middle) tier, and clouds of the middle (upper) tier are not visible, this does not mean that they are absent. They may be above underlying cloud layers, but this is not taken into account in cloud observations.

Clouds floating across the sky attract our gaze from early childhood. Many of us liked to peer at their outlines for a long time, figuring out what the next cloud looked like - a fairy-tale dragon, the head of an old man, or a cat running after a mouse.


How I wanted to climb onto one of them to roll around in the soft cotton mass or jump on it like on a springy bed! But at school, during natural history lessons, all children learn that in reality they are just large accumulations of water vapor floating at a great height above the ground. What else is known about clouds and cloud cover?

Cloudiness - what is this phenomenon?

Cloudiness is usually called the mass of clouds that are above the surface of a certain area of ​​our planet at the current time or were there at a certain point in time. It is one of the main weather and climate factors that prevents both too much heating and cooling of the surface of our planet.

Cloudiness scatters solar radiation, preventing overheating of the soil, but at the same time it reflects the own thermal radiation of the Earth's surface. In fact, the role of cloudiness is similar to the role of a blanket in keeping our body temperature stable during sleep.

Cloud measurement

Aviation meteorologists use the so-called 8-octant scale, which consists of dividing the sky into 8 segments. The number of clouds visible in the sky and the height of their lower boundaries are indicated layer by layer from the bottom layer to the top.

Automatic weather stations today denote the quantitative expression of cloudiness using Latin letter combinations:

— FEW – slight scattered cloudiness in 1-2 octants, or 1-3 points on the international scale;

— NSC – absence of significant cloudiness, while the number of clouds in the sky can be any, if their lower boundary is located above 1500 meters, and there are no powerful cumulus and cumulonimbus clouds;


- CLR - all clouds are above 3000 meters.

Cloud shapes

Meteorologists distinguish three main forms of clouds:

- cirrus, which are formed at an altitude of more than 6 thousand meters from tiny ice crystals into which droplets of water vapor turn, and have the shape of long feathers;

- cumulus, which are located at an altitude of 2-3 thousand meters and look like pieces of cotton wool;

- layered, located one above the other in several layers and, as a rule, covering the entire sky.

Professional meteorologists distinguish several dozen types of clouds, which are variants or combinations of three main forms.

What does cloudiness depend on?

Cloudiness directly depends on the moisture content in the atmosphere, since clouds are formed from evaporated water molecules condensed into tiny droplets. Significant amount clouds form in equatorial zone, since the evaporation process is very active there due to high temperature air.

The most common types of clouds that form here are cumulus and thunderstorm clouds. Subequatorial belts are characterized by seasonal cloudiness: in the rainy season it, as a rule, increases, in the dry season it is practically absent.

Cloudiness temperate zones depends on the transport of sea air, atmospheric fronts and cyclones. It is also seasonal in both the number and shape of clouds. In winter, stratus clouds most often form, covering the sky with a continuous veil.


By spring, cloud cover usually decreases and cumulus clouds begin to appear. In summer, the skies are dominated by cumulus and cumulonimbus forms. In autumn, clouds are at their most abundant, with stratus and nimbostratus clouds predominating.

For the entire planet as a whole, the quantitative indicator of cloudiness is approximately equal to 5.4 points, with cloudiness over land being lower - about 4.8 points, and above the sea - higher - 5.8 points. The heaviest cloudiness occurs over the northern part Pacific Ocean and the Atlantic, where its value reaches 8 points. Over deserts it does not exceed 1-2 points.

As you know, many industries, agriculture, and transport services are very dependent on the efficiency, timeliness and reliability of forecasts from the federal meteorological service. Advance warning of dangerous and especially dangerous weather phenomena, timely submission of storm warnings - all these are necessary conditions for the successful and safe operation of many sectors of the economy and transport. For example, long-term meteorological forecasts play a decisive role in organizing agricultural production.

One of the most important parameters that determine the ability to predict dangerous weather conditions, is an indicator such as the height of the lower boundary of the clouds.

In meteorology, cloud height is the height of the cloud base above the earth's surface.

To understand the importance of conducting research to determine the height of clouds, it is worth mentioning the fact that clouds can be of different types. For different types of clouds, the height of their lower boundary can vary within certain limits, and the average value of the cloud height has been identified.

So, clouds can be:

Stratus clouds (average altitude 623 m)

Rain clouds (average height 1527 m)

Cumulus (apex) (1855)

Cumulus (base) (1386)

Grozovye (summit) (average height 2848 m)

Thunderstorms (base) (average height 1405 m)

False cirrus (average altitude 3897 m)

Stratocumulus (average altitude 2331 m)

Altocumulus (below 4000 m) (average altitude 2771 m)

Altocumulus (above 4000 m) (average altitude 5586 m)

Cirrocumulus (average altitude 6465 m)

Low cirrostratus (average altitude 5198 m)

Tall cirrocumulus (average altitude 9254 m)

Cirrus (average altitude 8878 m)

As a rule, the height of clouds of the lower and middle tiers is measured, not exceeding 2500 m. At the same time, the height of the lowest clouds from their entire mass is determined. In fog, the cloud height is considered to be zero, and in this case, “vertical visibility” is measured at airports.

To determine the height of the lower boundary of clouds, the light-location method is used. In Russia, a meter is produced for these purposes, in which a flash lamp is used as a source of pulses and light.

The height of the lower boundary of the clouds using the light-location method using DVO-2 is determined by measuring the time it takes for a light pulse to travel from the light emitter to the cloud and back, as well as converting the resulting time value into a cloud height value proportional to it. Thus, a light pulse is sent by the emitter and, after reflection, is received by the receiver. In this case, the emitter and receiver must be located in close proximity to each other.

Structurally, the DVO-2 meter is a complex of several individual devices:

Transmitter and receiver,

Communication lines,

Measuring block,

Remote control.

The cloud height meter DVO-2 can work autonomously with a measuring unit, complete with a remote control and as part of automated meteorological stations.

The transmitter consists of a flash lamp, capacitors that supply it, and a parabolic reflector. The reflector, together with the lamp and capacitors, is installed in a gimbal suspension enclosed in a housing with an opening lid.

The receiver consists of a parabolic mirror, a photodetector, and a photoamplifier, also installed in a gimbal and housed in a housing with an opening lid.

The transmitter and receiver should be located near the main observation point. On runways, the transmitter and receiver are installed on the nearest locator beacons at both ends of the runway.

The measuring unit, intended for collecting and processing information, consists of a measuring board, a high-voltage unit and a power supply.

The remote control includes a keyboard and display board and a control board.

The signal from the receiver is transmitted via a two-wire potentially isolated communication line with unipolar signals and rated current (20±5) mA to the measuring unit, and from there to the remote control. Depending on the configuration, instead of a remote control for processing and displaying on the operator’s display, the signal can be transmitted to the central system of the weather station.

The DVO-2 cloud height meter can operate either continuously or as needed. The remote control has a serial RS-232 interface, designed to work with a computer. Information from DVO-2 meters can be transmitted via a communication line over a distance of up to 8 km.

Processing of measurement results on the DVO-2 measuring unit includes:

Averaging results over 8 measured values;

Exclusion from measurements of those results in which a short-term loss of the reflected signal is observed. Those. eliminating the “gap in the clouds” factor;

Issuing a signal about “no clouds” if among the 15 observations made there are not 8 significant ones;

Elimination of so-called localists - false reflection signals.

Clouds are a visible collection of suspended drops of water or ice crystals at a certain height above the earth's surface. Cloud observations include determining the amount of clouds. their shape and the height of the lower boundary above the station level.

The amount of clouds is estimated by ten point scale, in this case, three states of the sky are distinguished: clear (0... 2 points), and cloudy (3... 7 points) and cloudy (8... 10 points).

With all the diversity of appearance, there are 10 main forms of clouds. which, depending on the height, are divided into tiers. In the upper tier (above 6 km) there are three forms of clouds: cirrus, cirrocumulus and cirrostratus. Denser-looking altocumulus and altostratus clouds, the bases of which are at an altitude of 2... b km, belong to the middle tier, and stratocumulus, stratus and nimbostratus - to the lower tier. The bases of cumulonimbus clouds are also located in the lower tier (below 2 km). This cloud occupies several vertical layers and constitutes a separate group of clouds of vertical development.

Typically, a double assessment of cloudiness is made: first, the total cloudiness is determined and all clouds visible in the vault of the sky are taken into account, then the lower cloudiness, where only lower-level clouds (stratus, stratocumulus, nimbostratus) and vertical clouds are taken into account.

Circulation plays a decisive role in the formation of cloudiness. As a result of cyclonic activity and the transfer of air masses from the Atlantic, cloudiness in Leningrad is significant throughout the year and especially in the autumn-winter period. Frequent passage of cyclones at this time, and with them fronts, usually causes a significant increase in lower cloud cover, a decrease in the height of the cloud base and frequent precipitation. In November and December, the amount of cloudiness is the highest in the year and amounts to a long-term average of 8.6 points for total cloudiness and 7.8... 7.9 points for lower cloudiness (Table 60). Starting from January, cloudiness (total and low) gradually decreases, reaching lowest values in May-June. But at this time the sky is on average more than half covered with clouds different forms(6.1... 6.2 points in total cloudiness). The share of low-level clouds in the total cloudiness is high throughout the year and has a clearly defined annual cycle (Table 61). In the warm half of the year it decreases, and in winter, when the frequency of stratus clouds is especially high, the proportion of lower clouds increases.

The diurnal variation of general and lower cloudiness in winter is rather weakly expressed. The oh is more pronounced in the warm season. At this time, two maxima are observed: the main one in the afternoon, due to the development of convective clouds, and a less pronounced one in the early morning hours, when clouds of layered forms form under the influence of radiative cooling (see Table 45 of the Appendix).

In Leningrad, cloudy weather prevails throughout the year. Its frequency of occurrence in terms of total cloudiness is 75... 85% in the cold period, and -50... 60% in the warm period (see Table 46 of the Appendix). According to the lower cloudiness, a cloudy state of the sky is also observed quite often (70... 75%) and only by summer it decreases to 30%.

The stability of cloudy weather can be determined by the number of cloudy days during which cloudiness of 8...10 points prevails. In Leningrad, during the year there are 171 such days in total cloudiness and 109 in lower cloudiness (see Table 47 of the Appendix). Depending on the nature of atmospheric circulation, the number of cloudy days varies within very wide limits.

Thus, in 1942, according to the lower cloudiness, there were almost two times less, and in 1962, one and a half times more than the average value.

The most cloudy days are in November and December (22 in total cloudiness and 19 in lower cloudiness). During the warm period, their number sharply decreases to 2... 4 per month, although individual years even with low cloud cover, there are up to 10 cloudy days in the summer months (June 1953, August 1964).

Clear weather in autumn and winter in Leningrad is a rare phenomenon. It is usually established when air masses invade from the Arctic and there are only 1...2 clear days per month. Only in spring and summer does the frequency of clear skies increase to 30% of total cloud cover.

Much more often (50% of cases) this state of the sky is observed due to lower clouds, and in summer there can be an average of nine clear days per month. In April 1939 there were even 23 of them.

The warm period is also characterized by a semi-clear sky (20...25%) both in overall cloudiness and in lower cloudiness due to the presence of convective clouds during the day.

The degree of variability in the number of clear and cloudy days, as well as the frequency of clear and cloudy sky conditions, can be judged by the standard deviations, which are given in Table. 46, 47 applications.

Clouds of different shapes have different effects on the arrival of solar radiation, the duration of sunshine and, accordingly, on the temperature of the air and soil.

Leningrad in the autumn-winter period is characterized by continuous coverage of the sky with clouds of the lower tier of stratocumulus and nimbostratus forms (see Table 48 of the Appendix). The height of their lower base is usually at the level of 600... 700 m and about 400 m above the ground surface, respectively (see Table 49 of the Appendix). Below them, at altitudes of about 300 m, there may be shreds of torn clouds. In winter, the lowest (200...300 m high) stratus clouds are also frequent, the frequency of which at this time is the highest in the year, 8...13%.

During the warm period, clouds of cumulus forms often form with a base height of 500... 700 m. Along with stratocumulus clouds, cumulus and cumulonimbus clouds become characteristic, and the presence of large gaps in the clouds of these forms allows one to see clouds of the middle and upper tiers. As a result, the frequency of altocumulus and cirrus clouds in summer is more than twice as high as their frequency in the winter months and reaches 40... 43%.

The frequency of individual cloud forms varies not only throughout the year, but also throughout the day. Changes are especially significant during the warm period for cumulus and cumulonimbus clouds. They reach their greatest development, as a rule, in daytime hours and their frequency at this time is maximum per day. In the evening, cumulus clouds dissipate, and oohs are rarely observed during the night and morning hours. The frequency of occurrence of the prevailing cloud forms varies slightly from time to time during the cold period.

6.2. Visibility

The visibility range of real objects is the distance at which the visible contrast between the object and the background becomes equal to the threshold contrast of the human eye; it depends on the characteristics of the object and background, illumination and transparency of the atmosphere. Meteorological visibility range is one of the characteristics of atmospheric transparency; it is related to other optical characteristics.

Meteorological visibility range (MVR) Sm is the greatest distance from which, during daylight hours, an absolutely black object of sufficiently large angular dimensions (more than 15 arc minutes), at night - the greatest distance at which a similar object could be detected when the illumination increased to daylight levels. It is this value, expressed in kilometers or meters, that is determined at weather stations either visually or using special instruments.

In the absence of meteorological phenomena that impair visibility, the MDV is at least 10 km. Haze, fog, snowstorm, precipitation and others meteorological phenomena reduce meteorological visibility range. So, in fog it is less than one kilometer, in heavy snowfalls - hundreds of meters, in snowstorms it can be less than 100 m.

A decrease in MDV negatively affects the operation of all types of transport, complicates sea and river navigation, and complicates operations in the port. For takeoff and landing of aircraft, the MDV should not be below the established limit values ​​(minimums).

A reduced MLV is dangerous for road transport: when visibility is less than one kilometer, vehicle accidents occur on average two and a half times more than on days with good visibility. In addition, when visibility deteriorates, the speed of cars decreases significantly.

Reduced visibility also affects the operating conditions of industrial enterprises and construction sites, especially those with a network of access roads.

Poor visibility limits tourists' ability to view the city and surrounding area.

The MDV in Leningrad has a well-defined annual cycle. The atmosphere is most transparent from May to August: during this period, the frequency of good visibility (10 km or more) is about 90%, and the proportion of observations with visibility less than 4 km does not exceed one percent (Fig. 37). This is due to a decrease in the frequency of occurrence of phenomena that impair visibility in the warm season, as well as more intense turbulence than in the cold season, which contributes to the transfer of various impurities to higher layers of air.

The worst visibility in the city is observed in winter (December-February), when only about half of the observations occur in good visibility, and the frequency of visibility less than 4 km increases to 11%. During this season, there is a high frequency of atmospheric phenomena that impair visibility - haze and precipitation, and there are frequent cases of inverted temperature distribution. promoting the accumulation of various impurities in the ground layer.

Transitional seasons occupy an intermediate position, which is well illustrated by the graph (Fig. 37). In spring and autumn, the frequency of lower visibility gradations (4...10 km) especially increases compared to summer, which is associated with an increase in the number of cases of haze in the city.

Deterioration in visibility to values ​​less than 4 km, depending on atmospheric phenomena, is shown in table. 62. In January, such a deterioration in visibility most often occurs due to haze, in summer - in precipitation, and in spring and autumn in precipitation, haze and fog. Reduced visibility in within specified limits due to the presence of other phenomena, it is much less common.

In winter, a clear diurnal variation of the MDV is observed. Good visibility (Sm, 10 km or more) has the greatest frequency in the evening and at night, and the lowest frequency in the daytime. A similar course of visibility is less than four kilometers. The visibility range of 4...10 km has a reverse diurnal cycle with a maximum in the daytime. This can be explained by an increase in the concentration of air-clouding particles emitted into the atmosphere by industrial and energy enterprises and urban transport during the daytime hours. During transition seasons, the diurnal cycle is less pronounced. The increased frequency of visibility deteriorations (less than 10 km) shifts to the morning hours. In summer, the daily cycle of the MDV mail is not traceable.

Comparison of observation data in large cities and in rural areas shows that in cities the transparency of the atmosphere is reduced. This is caused by a large amount of emissions of pollution products on their territory, dust raised by city transport.

6.3. Fog and haze

Fog is a collection of water droplets or ice crystals suspended in the air that reduce visibility to less than 1 km.

Fog in the city is one of the dangerous atmospheric phenomena. Deterioration of visibility during fog significantly complicates the normal operation of all types of transport. Moreover, close to 100% relative humidity air in fogs increases the corrosion of metals and metal structures and the aging of paint and varnish coatings. Harmful impurities emitted by industrial enterprises dissolve in drops of water that form fog. Then deposited on the walls of buildings and structures, they heavily pollute them and shorten their service life. Due to high humidity and saturation with harmful impurities, urban fogs pose a certain danger to human health.

Fogs in Leningrad are determined by the peculiarities of atmospheric circulation in the North-West of the European Union, primarily by the development of cyclonic activity throughout the year, but especially during the cold period. When relatively warm and humid sea air moves from the Atlantic to the colder underlying land surface and cools, advection fogs are formed. In addition, radiation fogs of local origin may occur in Leningrad due to the cooling of the air layer from earth's surface at night in clear weather. Other types of fogs are usually special cases of these two main ones.

In Leningrad, there are an average of 29 days with fog per year (Table 63). In some years, depending on the characteristics of atmospheric circulation, the number of days with fog may differ significantly from the long-term average. For the period from 1938 to 1976, the largest number of days with fog per year was 53 (1939), and the smallest was 10 (1973). The variability in the number of days with fog in individual months is represented by the standard deviation, the values ​​of which range from 0.68 days in July to 2.8 days in March. The most favorable conditions for the development of fogs in Leningrad are created during the cold period (from October to March), coinciding with the period of increased cyclonic activity,

which accounts for 72% of the annual number of days with fog. At this time, there are an average of 3...4 days with fog per month. As a rule, advective fogs predominate, due to the intense and frequent removal of warm humid air western and togo-western flows onto the cold surface of the land. The number of days during the cold period with advective fogs, according to G.I. Osipova, is about 60% of them total number in this period.

Fogs in Leningrad form much less frequently in the warm half of the year. The number of days with them per month varies from 0.5 in June and July to 3 in September, and in 60...70% of the years in June and July, fogs are not observed at all (Table 64). But at the same time, there are years when in August there are up to 5... 6 days with fog.

For the warm period, in contrast to the cold period, radiation fogs are most characteristic. They account for about 65% of days with fogs during the warm period, and they usually form in stable air masses during calm weather or light winds. As a rule, summer radiation fogs in Leningrad occur at night or before sunrise; during the day, such fog quickly dissipates.

The largest number of days with fog in a month, equal to 11, was observed in September 1938. However, even in any month of the cold period, when fogs are observed most often, fog does not occur every year. In December, for example, they are not observed approximately once every 10 years, and in February - once every 7 years.

The average total duration of fogs in Leningrad per year is 107 hours. In the cold period, fogs are not only more frequent than in the warm period, but also longer. Their total duration, equal to 80 hours, is three times longer than in the warm half of the year. In the annual course, fogs have the longest duration in December (18 hours), and the shortest (0.7 hours) is noted in Nyun (Table 65).

The duration of fogs per day with fog, which characterizes their stability, is also slightly longer in the cold period than in the warm period (Table 65), and on average for the year it is 3.7 hours.

The continuous duration of fogs (average and greatest) in various months is given in Table. 66.

The diurnal variation in the duration of fogs in all months of the year is expressed quite clearly: the duration of fogs in the second half of the night and the first half of the day is longer than the duration of fogs in the rest of the day. In the cold half of the year, fogs most often (35 hours) are observed from 6 to 12 hours (Table 67), and in the warm half of the year, after midnight and reach their greatest development in the predawn hours. Their longest duration (14 hours) occurs at night.

The absence of wind has a significant influence on the formation and especially on the persistence of fog in Leningrad. Increasing wind leads to the dispersion of fog or its transition to low clouds.

In most cases, the formation of advective fogs in Leningrad, both in the cold and in the warm half of the year, is caused by the arrival of air masses with the westerly flow. Fog is less likely to occur with northerly and northeasterly winds.

The frequency of fogs and their duration are highly variable in space. In addition to weather conditions, the formation of oxo is influenced by the nature of the underlying surface, relief, and proximity to a reservoir. Even within Leningrad, in different areas, the number of days with fog is not the same. If in the central part of the city the number of days with p-khan per year is 29, then at the station. Nevskaya, located near the Neva Bay, their number increases to 39. In the rugged, elevated terrain of the suburbs of the Karelian Isthmus, which is especially favorable for the formation of fog, the number of days with fog is 2... 2.5 times greater than in the city.

Haze in Leningrad is observed much more often than fog. It is observed on average every second day per year (Table 68) and can not only be a continuation of fog when it dissipates, but also arise as an independent atmospheric phenomenon. Horizontal visibility during haze, depending on its intensity, ranges from 1 to 10 km. The conditions for haze formation are the same. as for fog,. therefore, most often it occurs in the cold half of the year (62% of the total number of days with haze). Every month at this time there can be 17...21 days with fog, which exceeds the number of days with fog by five times. The fewest days with haze are in May-July, when the number of days with them does not exceed 7... 9. In Leningrad there are more days with haze than in the coastal strip (Lisiy Nos, Lomonosov), and almost as many as in the elevated regions suburban areas remote from the bay (Voeikovo, Pushkin, etc.) (Table B8).

The duration of the haze in Leningrad is quite long. Its total duration per year is 1897 hours (Table 69) and varies significantly depending on the time of year. In the cold period, the duration of the haze is 2.4 times longer than in the warm period, and is 1334 hours. The most hours with haze are in November (261 hours), and the least in May-July (52... 65 hours).

6.4. Ice-frost deposits.

Frequent fogs and liquid precipitation during the cold season contribute to the appearance of ice deposits on parts of structures, television and radio towers, on branches and tree trunks, etc.

Ice deposits vary in their structure and appearance, but practically distinguish types of icing such as black ice, rime, wet snow deposits and complex deposits. Each of them, at any intensity, significantly complicates the work of many sectors of the urban economy (energy systems and communication lines, gardening, aviation, railway and road transport), and if they are significant in size, they are considered dangerous atmospheric phenomena.

A study of the synoptic conditions for the formation of icing in the North-West of the European territory of the USSR, including in Leningrad, showed that ice and complex deposits are mainly of frontal origin and are most often associated with warm fronts. Ice formation is also possible in a homogeneous air mass, but this rarely happens and the icing process here usually proceeds slowly. Unlike ice, frost is, as a rule, an intra-mass formation that most often occurs in anticyclones.

Observations of icing have been carried out visually in Leningrad since 1936. In addition, since 1953, observations of ice-frost deposits on the wire of the icing machine have been carried out. In addition to determining the type of icing, these observations include measuring the size and mass of deposits, as well as determining the stages of growth, steady state and destruction of deposits from the moment of their appearance on the icing platform until complete disappearance.

Icing of wires in Leningrad occurs from October to April. Dates of icing formation and destruction for various types are indicated in table. 70.

During the season, the city experiences an average of 31 days with icing of all types (see Table 50 of the Appendix). However, in the 1959-60 season, the number of days with deposits was almost twice as high as the long-term average and was the largest (57) for the entire period of instrumental observations (1963-1977). There were also seasons when ice-frost phenomena were observed relatively rarely, approximately 17 days per season (1964-65, 1969-70, 1970-71).

Most often, icing of wires occurs in December-February with a maximum in January (10.4 days). During these months, icing occurs almost every year.

Of all the types of icing in Leningrad, crystalline frost is most often observed. On average, there are 18 days with crystalline frost per season, but in the 1955-56 season the number of days with frost reached 41. Glaze is observed much less frequently than crystalline frost. It accounts for only eight days per season and only in the 1971-72 season there were 15 days with ice. Other types of icing are relatively rare.

Typically, icing of wires in Leningrad lasts less than a day, and only in 5 °/o cases does the duration of icing exceed two days (Table 71). Complex deposits remain on the wires longer than other deposits (on average 37 hours) (Table 72). The duration of ice is usually 9 hours, but in December 1960. ice was observed continuously for 56 hours. The process of ice growth in Leningrad lasts on average about 4 hours. The longest continuous duration of complex sedimentation (161 hours) was noted in January 1960, and crystalline frost - in January 1968 (326 h) .

The degree of danger of icing is characterized not only by the frequency of repetition of ice-frost deposits and the duration of their impact, but also by the size of the deposit, which refers to the size of the deposit in diameter (large to small) and mass. With an increase in the size and mass of ice deposits, the load on various types of structures increases, and when designing overhead power transmission and communication lines, as is known, the ice load is the main one and its underestimation leads to frequent accidents on the lines. In Leningrad, according to observations at a glaze machine, the size and mass of glaze-frost deposits are usually small. In all cases in the central part of the city, the diameter of the ice did not exceed 9 mm, taking into account the diameter of the wire, crystalline frost - 49 mm, . complex deposits - 19 mm. The maximum weight per meter of wire with a diameter of 5 mm is only 91 g (see Table 51 of the Appendix). It is practically important to know the probabilistic values ​​of ice loads (possible once in a given number of years). In Leningrad, on a glaze machine, once every 10 years, the load from glaze-frost deposits does not exceed 60 g/m (Table 73), which corresponds to region I of glaze according to the work.

In fact, the formation of ice and frost on real objects and on the wires of existing power and communication lines does not fully correspond to the conditions of icing on an ice-covered machine. These differences are determined primarily by the height of the location of the volume n wires, as well as a number of technical features (configuration and size of the volume,
the structure of its surface, for overhead lines - the diameter of the wire, the voltage of the electric current and r. P.). As altitude increases in the lower layer of the atmosphere, the formation of ice and frost, as a rule, occurs much more intensely than at the level of the ice dam, and the size and mass of deposits increase with altitude. Since in Leningrad there are no direct measurements of the amount of ice-frost deposits at heights, the ice load in these cases is estimated by various calculation methods.

Thus, using observational data on ice conditions, the maximum probabilistic values ​​of ice loads on the wires of existing overhead power lines were obtained (Table 73). The calculation was made for the wire that is most often used in the construction of lines (diameter 10 mm at a height of 10 m). From the table 73 it is clear that in climatic conditions Leningrad, once every 10 years, the maximum icy load on such a wire is 210 g/m, and exceeds the value of the maximum load of the same probability on an icy machine by more than three times.

For high-rise buildings and structures (above 100 m), the maximum and probabilistic values ​​of ice loads were calculated based on observational data on low-level clouds and temperature and wind conditions at standard aerological levels (80) (Table 74). In contrast to cloudiness, supercooled liquid precipitation plays a very insignificant role in the formation of ice and frost in the lower layer of the atmosphere at an altitude of 100...600 m and was not taken into account. From those given in table. 74 data shows that in Leningrad at an altitude of 100 m the load from ice-frost deposits, possible once every 10 years, reaches 1.5 kg/m, and at an altitude of 300 and 500 m it exceeds this value by two and three times, respectively. . This distribution of ice loads over heights is caused by the fact that wind speed and the duration of existence of lower-tier clouds increase with height and, therefore, the number of supercooled drops deposited on an object increases.

In the practice of construction design, however, a special climatic parameter is used to calculate ice loads - ice wall thickness. The thickness of the ice wall is expressed in millimeters and refers to the deposition of cylindrical ice at its highest density (0.9 g/cm3). The zoning of the territory of the USSR according to ice conditions in the current regulatory documents was also carried out for the thickness of the ice wall, but reduced to a height of 10 m and
to a wire diameter of 10 mm, with a repeat cycle of deposits once every 5 and 10 years. According to this map, Leningrad belongs to low-ice region I, in which, with the indicated probability, there may be ice-frost deposits corresponding to an ice wall thickness of 5 mm. to move to other wire diameters, heights and other repeatability, appropriate coefficients are introduced.

6.5. Thunderstorm and hail

A thunderstorm is an atmospheric phenomenon in which multiple electrical discharges (lightning) occur between individual clouds or between a cloud and the ground, accompanied by thunder. Lightning can cause fires and cause various types of damage to power and communication lines, but they are especially dangerous for aviation. Thunderstorms are often accompanied by such equally dangerous National economy weather phenomena such as squally winds, intense rainfall, and in some cases hail.

Thunderstorm activity is determined by atmospheric circulation processes and, to a large extent, by local physical and geographical conditions: terrain, proximity to a body of water. It is characterized by the number of days with near and distant thunderstorms and the duration of thunderstorms.

The occurrence of a thunderstorm is associated with the development of powerful cumulonimbus clouds, with strong instability of air stratification with high moisture content. There are thunderstorms that form at the interface between two air masses (frontal) and in a homogeneous air mass (intramass or convective). Leningrad is characterized by the predominance of frontal thunderstorms, in most cases occurring on cold fronts, and only in 35% of cases (Pulkovo) the formation of convective thunderstorms is possible, most often in summer. Despite the frontal origin of thunderstorms, summer heating has a significant additional significance. Thunderstorms most often occur in the afternoon: between 12 and 6 p.m. they occur on 50% of all days. Thunderstorms are least likely between 24 and 6 hours.

Table 1 gives an idea of ​​the number of days with thunderstorms in Leningrad. 75. In the 3rd year in the central part of the city there were 18 days with thunderstorms, while at the station. Nevskaya, located within the city, but closer to the Gulf of Finland, the number of Days is reduced to 13, just like in Kronstadt and Lomonosov. This feature is explained by the influence of the summer sea breeze, which brings relatively cool air during the day and prevents the formation of powerful cumulus clouds in the immediate vicinity of the bay. Even a relatively small elevation of the terrain and distance from the reservoir lead to an increase in the number of days with thunderstorms in the vicinity of the city to 20 (Voeikovo, Pushkin).

The number of days with thunderstorms is a very variable value over time. In 62% of cases, the number of days with thunderstorms in a particular year deviates from the long-term average by ±5 days, in 33% - by ±6... 10 days, and in 5% - by ±11... 15 days. In some years, the number of thunderstorm days is almost twice the long-term average, but there are also years when thunderstorms are extremely rare in Leningrad. Thus, in 1937 there were 32 days with thunderstorms, and in 1955 there were only nine.

Thunderstorm activity develops most intensely from May to September. Thunderstorms are especially frequent in July, the number of days with them reaches six. Rarely, once every 20 years, thunderstorms are possible in December, but they have never been observed in January and February.

Every year thunderstorms are observed only in July, and in 1937 the number of days with them in this month was 14 and was the largest for the entire observation period. In the central part of the city, thunderstorms occur annually in August, but in areas located on the Gulf coast, the probability of thunderstorms occurring at this time is 98% (Table 76).

From April to September, the number of days with thunderstorms in Leningrad varies from 0.4 in April to 5.8 in July, and the standard deviations are 0.8 and 2.8 days, respectively (Table 75).

The total duration of thunderstorms in Leningrad averages 22 hours per year. Summer thunderstorms usually last the longest. The longest total monthly duration of thunderstorms, equal to 8.4 hours, occurs in July. The shortest thunderstorms are spring and autumn.

An individual thunderstorm in Leningrad lasts continuously for an average of about 1 hour (Table 77). In summer, the frequency of thunderstorms lasting more than 2 hours increases to 10...13% (Table 78), and the longest individual thunderstorms - more than 5 hours - were recorded in June 1960 and 1973. During the day in summer, the longest thunderstorms (from 2 to 5 hours) are observed during the day (Table 79).

Climatic parameters of thunderstorms according to statistical visual observations at a point (at weather stations with a viewing radius of approximately 20 km) give somewhat underestimated characteristics of thunderstorm activity compared to large areas. It is accepted that in summer the number of days with thunderstorms at an observation point is approximately two to three times less than in an area with a radius of 100 km, and approximately three to four times less than in an area with a radius of 200 km.

The most complete information about thunderstorms in areas with a radius of 200 km is provided by instrumental observations from radar stations. Radar observations make it possible to identify foci of thunderstorm activity one to two hours before a thunderstorm approaches a station, as well as to monitor their movement and evolution. Moreover, the reliability of radar information is quite high.

For example, on June 7, 1979, at 17:50, the MRL-2 radar of the Weather Information Center detected a thunderstorm center associated with the tropospheric front at a distance of 135 km northwest of Leningrad. Further observations showed that this thunderstorm was moving at a speed of about 80 km/h in the direction of Leningrad. In the city, the beginning of the thunderstorm was visible visually after an hour and a half. The availability of radar data made it possible to warn interested organizations (aviation, power grid, etc.) in advance about this dangerous phenomenon.

hail falls in warm time year from powerful convection clouds with great instability of the atmosphere. It represents precipitation in the form of particles dense ice various sizes. Hail is observed only during thunderstorms, usually during. showers. On average, out of 10...15 thunderstorms, one is accompanied by hail.

Hail often causes great damage to landscape gardening and agriculture suburban area, damaging crops, fruit and park trees, and garden crops.

In Leningrad, hail is a rare, short-term phenomenon and has a local character. The hailstones are generally small in size. There were no cases of particularly dangerous hail with a diameter of 20 mm or more, according to observations from weather stations in the city itself.

The formation of hail clouds in Leningrad, like thunderstorms, is more often associated with the passage of fronts, mostly cold, and less often with warming up air mass from the underlying surface.

An average of 1.6 days with hail is observed per year, and in some years an increase to 6 days is possible (1957). Most often in Leningrad, hail falls in June and September (Table 80). Largest number days with hail (four days) were recorded in May 1975 and June 1957.

IN diurnal course Hailfall occurs mainly in the afternoon hours with a maximum frequency of occurrence from 12 to 14 hours.

The period of hail in most cases ranges from several minutes to a quarter of an hour (Table 81). Hailstones that fall usually melt quickly. Only in some rare cases, the duration of hail can reach 20 minutes or more, while in the suburbs and surrounding areas it is longer than in the city itself: for example, in Leningrad on June 27, 1965, hail fell for 24 minutes, in Voeikovo on September 15, 1963 city ​​- 36 minutes with breaks, and in Belogorka on September 18, 1966 - 1 hour with breaks.