Definition of cloudiness. Determining and recording the total number of clouds. Clouds of the lower tier. These include

Cloudiness- a complex of clouds that appear in a certain place on the planet (point or territory) at a certain moment or period of time.

Types of clouds

This or that type of cloudiness corresponds to certain processes occurring in the atmosphere, and therefore portends this or that weather. Knowledge of the types of clouds from the point of view of the navigator is important for predicting weather from local characteristics. For practical purposes, clouds are divided into 10 main forms, which in turn are divided into 4 types according to height and vertical extent:

Clouds of large vertical development. These include:

Cumulus. Latin name - Cumulus(marked as Cu on weather maps)- separate thick vertically developed clouds. The upper part of the cloud is dome-shaped, with prominences, the lower part is almost horizontal. The average vertical extent of the cloud is 0.5 -2 km. The average height of the lower base from the earth's surface is 1.2 km.

- heavy masses of clouds of large vertical development in the form of towers and mountains. The upper part is a fibrous structure, often with projections to the sides in the form of an anvil. The average vertical length is 2-3 km. The average height of the lower base is 1 km. Often give heavy rainfall, accompanied by thunderstorms.

Clouds of the lower tier. These include:

- low, amorphous, stratified, almost uniform rain clouds of dark gray color. The lower base is 1-1.5 km. The average vertical extent of the cloud is 2 km. Heavy rain falls from these clouds.

- a uniform light gray foggy veil of continuous low clouds. Often formed from rising fog or turning into fog. The height of the lower base is 0.4–0.6 km. The average vertical extent is 0.7 km.

- Low cloud cover, consisting of individual ridges, waves, plates or flakes, separated by gaps or translucent areas (translucent) or without clearly visible gaps, the fibrous structure of such clouds is more clearly visible near the horizon.

Clouds of the middle tier. These include:

- a fibrous veil of gray or bluish color. The lower base is located at an altitude of 3-5 km. Vertical length - 04 - 0.8 km).

- layers or spots, consisting of strongly flattened rounded masses. The lower base is located at an altitude of 2–5 km. The average vertical extent of the cloud is 0.5 km.

Upper clouds. All of them are white, during the day they almost do not give a shadow. These include:

Cirrostratus (Cs) - a thin whitish translucent veil, gradually covering the entire sky. They do not obscure the outer contours of the Sun and Moon, leading to the appearance of a halo around them. The lower boundary of the cloud is at an altitude of about 7 km.

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


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

Cloudiness - what is this phenomenon?

Cloudiness is usually called the mass of clouds that are above the surface of a certain part of our planet at the current time or were there at a certain point in time. It is one of the main weather and climatic 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 reflects its own thermal radiation from the Earth's surface. In fact, the role of clouds is similar to that of a blanket, keeping our body temperature stable during sleep.

Cloud measurement

Aeronautical meteorologists use the so-called 8-oct scale, which divides the sky into 8 segments. The number of clouds visible in the sky and the height of their lower boundaries are indicated in layers from the lower layer to the upper one.

The quantitative expression of cloudiness is today denoted by automatic weather stations in Latin letter combinations:

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

- NSC - the absence of significant cloudiness, while the number of clouds in the sky can be any, if their lower limit 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 the smallest 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 shreds 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 varieties of clouds, which are variants or combinations of three basic forms.

What does cloudiness depend on?

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

Most often, cumulus and thunderstorm clouds form here. Subequatorial belts are characterized by seasonal cloudiness: in the rainy season, it usually increases, in the dry season it is practically absent.

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


By spring, cloudiness usually decreases, and cumulus clouds begin to appear. In summer, the sky is dominated by cumulus and cumulonimbus forms. Clouds are most abundant in autumn with a predominance of stratus and nimbostratus clouds.

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

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 number of clouds is estimated on a ten-point scale, while three states of the sky are distinguished: clear (0 ... 2 points), overcast (3 ... 7 points) and overcast (8 ... 10 points).

With all the variety of appearance, 10 main forms of clouds are distinguished. which, depending on the height, are divided into tiers. In the upper tier (above 6 km) there are three types of clouds: cirrus, cirrocumulus and cirrostratus. More dense-looking altocumulus and altostratus clouds, the bases of which are located at a height of 2 ... b km, belong to the middle tier, and stratocumulus, stratus and stratocumulus clouds belong to the lower tier. In the lower tier (below 2 km) there are also the bases of its cumulus cumulonimbus clouds. This cloud occupies several tiers vertically and constitutes a separate group of clouds of vertical development.

A double assessment of cloudiness is usually made: first, the total cloudiness is determined and all clouds that are visible in the sky are taken into account, then the lower cloudiness, where only the clouds of the lower tier (stratus, stratocumulus, stratocumulus) and clouds of vertical development are taken into account.

Circulation plays a decisive role in cloud formation. 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. The frequent passage of cyclones at this time, and along with them the fronts, usually causes a significant increase in the lower cloudiness, a decrease in the height of the lower boundary of the clouds, and frequent precipitation. In November and December, the amount of cloudiness is the largest in the year and averages 8.6 points for the general and 7.8 ... 7.9 points for the lower cloudiness (Table 60). Starting from January, cloudiness (total and lower) gradually decreases, reaching the lowest values ​​in May-June. But for a lady at this time, the sky is on average more than half covered with clouds of various shapes (6.1 ... 6.2 points for total cloudiness). The share of low-level clouds in the total cloud cover is large throughout the year and has a clearly defined annual variation (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 cloudiness increases.

The diurnal variation of the total and lower cloudiness in winter is rather weakly expressed. More distinctly oh in the warm period of the year. At this time, two maxima are noted: the main one is in the afternoon hours, due to the development of convective clouds, and less pronounced - in the early morning hours, when clouds of layered forms form under the influence of radiative cooling (see Table 45 of the appendix).

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

The stability of cloudy weather can be judged by the number of cloudy days during which cloudiness of 8 ... 10 points prevails. In Leningrad, 171 such days are observed per year for general and 109 for lower cloudiness (see Table 47 of the Appendix). Depending on the nature of the atmospheric circulation, the number of cloudy days varies over a very wide range.

So, in 1942, in terms of lower cloudiness, they 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 for total cloudiness and 19 for lower). During the warm period, their number sharply decreases to 2 ... 4 per month, although in some years even in the lower cloudiness in the summer months there are up to 10 cloudy days (June 1953, August 1964).

Clear weather in autumn and winter in Leningrad is a rare phenomenon. It is usually set during the invasion of air masses from the Arctic and there are only 1 ... 2 clear days per month. Only in spring and summer, the frequency of clear sky increases to 30% in terms of total cloudiness.

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

The warm period is also characterized by a semi-clear state of the sky (20 ... 25%) both in terms of total cloud cover and in the lower one 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 from the standard deviations, which are given in Table. 46, 47 applications.

Clouds of various forms do not have the same effect on the arrival of solar radiation, the duration of sunshine and, accordingly, on the temperature of air and soil.

For Leningrad in the autumn-winter period, a continuous coverage of the sky with clouds of the lower tier of stratocumulus and stratocumulus forms is typical (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, respectively (see Table 49 of the Appendix). Under them, at altitudes of about 300 m, patches of broken clouds can be located. 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%.

In the warm period, cumulus clouds 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 you to see clouds of the middle and upper tiers. As a result, the recurrence of altocumulus and cirrus clouds in summer is more than twice as high as their recurrence in the winter months and reaches 40 ... 43%.

The frequency of individual cloud forms varies not only during the year, but also during the day. The changes during the warm period are especially significant for cumulus and cumulonimbus clouds. They reach their greatest development, as a rule, in the daytime 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 forms of clouds from time to time during the cold period varies slightly.

6.2. Visibility

The visibility range of real objects is the distance at which the apparent 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 the background, the illumination of the transparency of the atmosphere. The meteorological visibility range is one of the characteristics of the transparency of the atmosphere, it is associated with other optical characteristics.

The meteorological visibility range (MDV) Sm is the greatest distance from which in daylight it is possible to distinguish with the naked eye against the sky near the horizon (or against the background of air haze) an absolutely black object of sufficiently large angular dimensions (more than 15 arc minutes), at night time - the greatest distance at which a similar object could be detected with an increase in illumination to daylight levels. It is this value, expressed in kilometers or meters, that is determined in weather stations either visually or with the help of special instruments.

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

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

Dangerous reduced DMV for road transport: with a visibility of less than one kilometer, there are two and a half times more accidents on average than on days with good visibility. In addition, when visibility deteriorates, the speed of vehicles is significantly reduced.

The decrease in visibility also affects the working conditions of industrial enterprises and construction sites, especially those with a network of access roads.

Poor visibility limits the ability of tourists to view the city and its surroundings.

DMV in Leningrad has a well-defined annual course. 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 phenomena that worsen visibility in the warm season, as well as to more intense turbulence than in the cold period, which contributes to the transfer of various impurities to higher air layers.

The worst visibility in the city is observed in winter (December-February), when only about half of the observations fall on good visibility, and the visibility frequency of less than 4 km increases to 11%. In this season, the frequency of atmospheric phenomena that worsen visibility is high - smoke and precipitation, cases of inversion temperature distribution are not uncommon. contributing to the accumulation of various impurities in the surface 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 gradation (4 ... 10 km) especially increases compared to summer, which is associated with an increase in the number of cases with haze in the city.

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

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

Comparison of observational 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 number of emissions of pollution products on their territory, dust raised by urban transport.

6.3. Fog and haze

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

Fog in the city is one of the dangerous atmospheric phenomena. The deterioration of visibility during fogs greatly complicates the normal operation of all modes of transport. In addition, close to 100% relative humidity in fogs contributes to increased corrosion of metals and metal structures and aging of paint coatings. The water droplets that form fog dissolve harmful impurities emitted by industrial enterprises. Settling then on the walls of buildings and structures, they greatly pollute them and shorten their service life. Due to the 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 the 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, advective fogs are formed. In addition, radiation fogs of local origin may occur in Leningrad, associated with the cooling of the air layer from the earth's surface at night in clear weather. Other types of fogs, as a rule, are special cases of these two main ones.

In Leningrad, an average of 29 days with fogs are observed per year (Table 63). In some years, depending on the characteristics of atmospheric circulation, the number of days with fog can 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, an average of 3 ... 4 days with fog is observed per month. As a rule, advective fogs predominate, due to the intensive and frequent removal of warm moist air by western and togo-western flows to the cold land surface. The number of days during the cold period with advective fogs, according to G. I. Osipova, is about 60% of their total number during this period.

Fogs form in Leningrad much less frequently during the warm half of the year. The number of days with them per month varies from 0.5 in June, July to 3 in September, and in 60 ... 70% of years in ion, 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 the most characteristic. They account for about 65% of days with fogs during the warm period, and they usually form in stable air masses in 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 most often observed, ohms do not occur every year. In December, for example, they are not observed about once every 10 years, and in February - once every 7 years.

The average total duration of fogs in Leningrad for a 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 observed in June (Table 65).

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

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

The diurnal course of the duration of fogs in all months of the year is quite clearly expressed: 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-year, fogs most often (35 hours) are observed from 6 to 12 hours (Table 67), and in the warm half-year, after midnight and reach their greatest development in the predawn hours. Their greatest duration (14 hours) falls on the night hours.

The lack of wind has a significant effect on the formation and especially on the persistence of fog in Leningrad. Strengthening the wind leads to the dispersal 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 inflow of air masses with a westerly flow. Fog is less likely to occur with northerly and northeasterly winds.

The repetition of fogs and their duration are highly variable in space. In addition to weather conditions, the formation of OH is influenced by the nature of the underlying surface, relief, and the proximity of a reservoir. even within Leningrad, in its various districts, 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 st. Neva, 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 more than in the city.

Haze in Leningrad is observed much more often than fog. It is observed every second day on average for the year (Table 68) and can be not only a continuation of the fog during its dispersal, 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 the formation of haze are the same. as for the fog,. therefore, most often it occurs in the cold half-year (62% of the total number of days with haze). Monthly at this time there can be 17 ... 21 days with a king, 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... suburban areas remote from the bay (Voeykovo, Pushkin, etc.) (Table b8).

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

6.4. Icy frost deposits.

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

Ice deposits differ in their structure and appearance, but practically distinguish such types of icing as ice, frost, wet snow deposition and complex deposition. Each of them, at any intensity, significantly complicates the work of many branches of the urban economy (energy systems and communication lines, landscape gardening, aviation, rail and road transport), and if it is significant, it is one of the hazardous 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 deposition 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 intramass formation that occurs most often in anticyclones.

Observations of icing have been carried out in Leningrad visually since 1936. In addition to them, since 1953, observations have been made of ice-rime deposits on the wire of an icing machine. 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 they appear on the icing machine to their complete disappearance.

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

During the season, the city experiences an average of 31 days with all types of icing (see Table 50 of the appendix). However, in the 1959-60 season, the number of days with deposits was almost twice the long-term average and was the largest (57) for the entire period of instrumental observations (1963-1977). There were also such seasons when icing and hoarfrost phenomena were observed relatively rarely, at] 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 annually.

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

Usually, 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). Longer than other deposits (on average 37 hours), a complex deposit is retained on the wires (Table 72). Ice duration is usually 9 hours, but in December 1960 r. 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 deposition (161 hours) was noted in January 1960, and crystalline frost - in January 1968 h).

The degree of danger of icing is characterized not only by the frequency of repetition of icy frost deposits and the duration of their impact, but also by the magnitude 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 you know, ice load is the main one and its underestimation leads to frequent accidents on the lines. In Leningrad, according to the data of observations on an icing machine, the size and mass of icy frost deposits are usually small. In all cases, in the central part of the city, the diameter of 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 an ice machine, once every 10 years, the load from ice-frost deposits does not exceed 60 g / m (Table 73), which corresponds to area I of ice according to the work.

In fact, the formation of ice and frost on real objects and on the wires of existing power transmission and communication lines does not fully correspond to the conditions of icing on an ice 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 the height increases in the lower layer of the atmosphere, the formation of ice and frost, as a rule, proceeds much more intensively than at the level of the ice machine, and the size and mass of deposits increase with height. Since in Leningrad there are no direct measurements of the magnitude of ice-frost deposits at heights, the ice load in these cases is estimated by various calculation methods.

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

For high-rise structures 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 a height of 100 ... 600 m and was not taken into account. From the table. 74 data it follows that in Leningrad at a height of 100 m, the load from ice-frost deposits, which is possible once every 10 years, reaches 1.5 kg / m, and at a height of 300 and 500 m it exceeds this value by two and three times, respectively . Such a distribution of ice loads over heights is due to the fact that with height the wind speed and the duration of the existence of lower clouds increase, and in connection with this, the number of supercooled drops applied to the object increases.

In the practice of building design, however, a special climatic parameter is used to calculate ice loads - ice wall thickness. Ice wall thickness 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 icing conditions in the current regulatory documents is 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 recurrence cycle of deposits once every 5 and 10 years. According to this map, Leningrad belongs to the low-icing area I, in which, with the indicated probability, there may be icy-hoarfrost deposits corresponding to an ice wall thickness of 5 mm. for the transition to other wire diameters, heights and other repeatability, the appropriate coefficients are introduced.

6.5. Thunderstorm and hail

Thunderstorm - 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 a fire, cause various kinds of damage to power transmission and communication lines, but they are especially dangerous for aviation. Thunderstorms are often accompanied by weather phenomena no less dangerous for the national economy, such as squally winds and intense heavy rainfall, and in some cases hail.

Thunderstorm activity is determined by the processes of atmospheric circulation and, to a large extent, by local physical and geographical conditions: the terrain, the proximity of a reservoir. 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 a strong instability of air stratification at a 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) is the formation of convective thunderstorms possible, most often in summer. Despite the frontal origin of thunderstorms, summer heating is of significant additional importance. Most often, thunderstorms occur in the afternoon hours: in the period from 12 to 18 hours, they account for 50% of all days. Thunderstorms are least likely between 24:00 and 06:00.

Table 1 gives an idea of ​​the number of days with a thunderstorm in Leningrad. 75. 3a year in the central part of the city there are 18 days with a thunderstorm, while at st. 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 increase in terrain and remoteness from a reservoir lead to an increase in the number of days with a thunderstorm in the vicinity of the city up to 20 (Voeykovo, Pushkin).

The number of days with thunderstorms is also very variable in time. In 62% of cases, the number of days with a thunderstorm for a particular year deviates from the long-term average by ±5 days, in 33%o - 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. So, in 1937 there were 32 days with a thunderstorm, and in 1955 there were only nine of them.

The most intense thunderstorm activity develops 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.

Thunderstorms are observed annually 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. Thunderstorms occur annually in the central part of the city and in August, but in areas located on the coast of the bay, the probability of thunderstorms at this time is 98% (Table 76).

From April to September, the number of days with a thunderstorm in Leningrad varies from 0.4 in April to 5.8 in July, while 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 are usually the longest. The largest total duration of thunderstorms per month, equal to 8.4 hours, occurs in July. The shortest are spring and autumn thunderstorms.

An individual thunderstorm in Leningrad lasts continuously on average for 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 noted in June 1960 and 1973. In summer, during the day, the longest thunderstorms (from 2 to 5 hours) are observed during the day (Table 79).

The climatic parameters of thunderstorms according to the data of statistical visual observations at the point (at weather stations with a viewing radius of about 20 km) give somewhat underestimated characteristics of thunderstorm activity compared to areas that are large in area. It is accepted that in summer the number of days with a thunderstorm at the 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 of radar stations. Radar observations make it possible to identify the centers of thunderstorm activity one to two hours before the approach of a thunderstorm to the station, as well as to follow their movement and evolution. Moreover, the reliability of radar information is quite high.

For example, on June 7, 1979, at 5:50 pm, the MRL-2 radar of the Weather Information Center recorded a thunderstorm center associated with the tropospheric front at a distance of 135 km northwest of Leningrad. Further observations showed that this thunderstorm center was moving at a speed of about 80 km/h in the direction of Leningrad. In the city, the beginning of the thunderstorm was baked visually in an hour and a half. The availability of radar data made it possible to warn interested organizations (aviation, power grid, etc.) about this dangerous phenomenon in advance.

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

Often, hail causes great damage to landscape gardening and suburban agriculture, damaging crops, fruit and park trees, and garden crops.

In Leningrad, hail is a rare, short-term phenomenon and is of a local local character. The size of the hailstones is mostly small. According to the observations of meteorological stations, there were no cases of especially dangerous hail falling with a diameter of 20 mm or more in the city itself.

The formation of hail clouds in Leningrad, as well as thunderstorms, is more often associated with the passage of fronts, mostly cold ones, and less often with the heating of the air mass from the underlying surface.

During the year, an average of 1.6 days with hail is observed, and in some years an increase up to 6 days is possible (1957). Most often hail falls in Leningrad in June and September (Table 80). The largest number of days with hail (four days) was recorded in May 1975 and June 1957.

In the diurnal course, hail falls mainly in the afternoon hours with a maximum frequency from 12:00 to 14:00.

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

Due to the shielding effect, it prevents both the cooling of the Earth's surface due to its own thermal radiation and its heating by solar radiation, thereby reducing seasonal and daily fluctuations in air temperature.

Cloud Characteristics

Number of clouds

The amount of clouds - the degree of sky coverage clouds(at a certain moment or on average over a certain period of time), expressed on a 10-point scale or as a percentage of coverage. The modern 10-point cloud scale was adopted at the first Maritime International Meteorological Conference ( Brussels, G.).

When observing at meteorological stations, the total amount of clouds and the amount of lower clouds are determined; these numbers are recorded in the weather diaries through a fractional line, for example 10/4 .

In aviation meteorology, an 8-oct scale is used, which is easier for visual observation: the sky is divided into 8 parts (that is, in half, then in half and again), cloudiness is indicated in octants (eighths of the sky). In aviation meteorological weather reports ( METAR , SPECI , TAF) the amount of clouds and the height of the lower boundary are indicated by layers (from the lowest to the highest), while the gradations of the amount are used:

• FEW - minor (scattered) - 1-2 octants (1-3 points);
• SCT - scattered (separate) - 3-4 octants (4-5 points);
• BKN - significant (broken) - 5-7 oktants (6-9 points);
• OVC - solid - 8 octants (10 points);
• SKC - clear - 0 points (0 octants);
• NSC - no significant clouds (any amount of clouds with a base height of 1500 m and above, in the absence of cumulonimbus and powerful cumulus clouds);
• CLR - no clouds below 3000 m (abbreviation used in reports generated by automatic weather stations).

cloud shapes

The observed forms of clouds are indicated (in Latin designations) in accordance with the international classification of clouds.

Cloud base height (CLB)

The VNGO of the lower tier is determined in meters. At a number of weather stations (especially aviation) this parameter is measured by the device (error 10-15%), on the rest - visually, approximately (in this case, the error can reach 50-100%; visual VNGO is the most unreliably determined weather element). Cloudiness can be divided into 3 tiers (lower, middle and upper) depending on the VNGO. The lower tier includes (up to about a height of 2 km): stratus (precipitation may fall in the form of drizzle), nimbostratus (overdose precipitation), stratocumulus (in aviation meteorology, stratified and ruptured rain are also noted) clouds. Middle layer (approximately from 2 km to 4-6 km): altostratus and altocumulus. Upper layer: cirrus, cirrocumulus, cirrostratus clouds.

Cloud top height

Can be determined from data aircraft and radar atmospheric sounding. It is usually not measured at weather stations, but in aviation weather forecasts on routes and flight areas, the expected (predicted) height of the upper boundary of the clouds is indicated.

see also

Sources

Seasons tropical seasons Weather conditions Precipitation Forecast and weather diary

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An excerpt characterizing Cloudiness

Finally, the headman Dron entered the room and, bowing low to the princess, stopped at the lintel.
Princess Mary walked across the room and stopped in front of him.
“Dronushka,” said Princess Mary, seeing in him an undoubted friend, that very Dronushka who, from his annual trip to the fair in Vyazma, brought her every time and served his special gingerbread with a smile. “Dronushka, now, after our misfortune,” she began and fell silent, unable to speak further.
“We all walk under God,” he said with a sigh. They were silent.
- Dronushka, Alpatych has gone somewhere, I have no one to turn to. Are they telling me the truth that I can't even leave?
“Why don’t you go, your excellency, you can go,” said Dron.
- I was told that it was dangerous from the enemy. My dear, I can’t do anything, I don’t understand anything, there’s no one with me. I certainly want to go at night or tomorrow early in the morning. Drone was silent. He glanced frowningly at Princess Marya.
“There are no horses,” he said, “I also told Yakov Alpatych.
- Why not? - said the princess.
“All from God’s punishment,” said Dron. - What horses were dismantled under the troops, and which died, now what a year. Not to feed the horses, but not to die of hunger ourselves! And so they sit for three days without eating. There is nothing, ruined completely.
Princess Mary listened attentively to what he was saying to her.
Are the men ruined? Do they have any bread? she asked.
“They die of starvation,” said Dron, “let alone carts…
“But why didn’t you say, Dronushka?” Can't help? I will do everything I can ... - It was strange for Princess Mary to think that now, at such a moment when such grief filled her soul, there could be people rich and poor and that the rich could not help the poor. She vaguely knew and heard that there was master's bread and that it was given to peasants. She knew, too, that neither her brother nor her father would have denied the need to peasants; she was only afraid to make a mistake somehow in her words about this distribution of bread to the peasants, which she wanted to dispose of. She was glad that she had an excuse for caring, one for which she was not ashamed to forget her grief. She began to ask Dronushka for details about the needs of the peasants and about what is masterful in Bogucharov.
“We have the master’s bread, bro?” she asked.
“The Lord’s bread is whole,” Dron said proudly, “our prince did not order to sell it.
“Give him to the peasants, give him everything they need: I give you permission in the name of your brother,” said Princess Mary.
Drone did not answer and took a deep breath.
- You give them this bread, if it will be enough for them. Distribute everything. I command you in the name of a brother, and tell them: whatever is ours, so is theirs. We will spare nothing for them. So you say.
Drone gazed at the princess intently while she spoke.
“Fire me, mother, for God’s sake, send me the keys to accept,” he said. - He served twenty-three years, did not do anything bad; quit, for God's sake.
Princess Mary did not understand what he wanted from her and why he asked to be fired. She answered him that she never doubted his devotion and that she was ready to do everything for him and for the peasants.

An hour later, Dunyasha came to the princess with the news that Dron had come and all the peasants, on the orders of the princess, had gathered at the barn, wanting to talk to the mistress.
“Yes, I never called them,” said Princess Marya, “I only told Dronushka to distribute bread to them.
- Only for God's sake, Princess Mother, order them to drive away and do not go to them. It’s all a deception,” Dunyasha said, “but Yakov Alpatych will come, and we’ll go ... and you don’t mind ...