How cloudy. General and low clouds
Humidity
Air humidity is the content of water vapor in it. Its characteristics are:
absolute humidity A - the amount of water vapor (in g) in 1 m 3 of air;
saturating (saturated) steam A - the amount of steam (in g) required to completely saturate a unit of volume (its elasticity is indicated by the letter E);
relative humidity R - the ratio of absolute humidity to saturated steam, expressed as a percentage ( R=100% × a/A);
Dew point- the temperature at which the air would reach a state of saturation for a given moisture content and constant pressure.
In the equatorial zone and subtropics, absolute humidity near the ground reaches 15–20 g/m3. In temperate latitudes in summer - 5 - 7 g/m3, in winter (as well as in the Arctic basin) it decreases to 1 g/m3 and lower. With altitude, the water vapor content in the air decreases rapidly. Humidity affects changes in air temperature, as well as the formation of clouds, fog, and precipitation.
Along with the process of evaporation of water in the atmosphere, the reverse process also occurs - the transition of water vapor as the temperature decreases into a liquid or directly into a solid state. The first process is called condensation, second - sublimation.
The decrease in temperature occurs adiabatically in rising moist air and leads to condensation or sublimation of water vapor, which is the main cause of cloud formation. The reasons for the rise of air in this case may be: 1) convection, 2) upward sliding along an inclined frontal surface, 3) wave-like movements, 4) turbulence.
In addition to the above, a decrease in temperature can also occur due to radiative cooling (from radiation) of the upper layers of inversions or the upper boundary of clouds.
Condensation occurs only if the air is saturated with water vapor and there are condensation nuclei in the atmosphere. Condensation nuclei are tiny solid, liquid and gaseous particles that are constantly present in the atmosphere. The most common nuclei are those containing compounds of chlorine, sulfur, nitrogen, carbon, sodium, calcium, and the most common nuclei are sodium and chlorine compounds, which have hygroscopic properties.
Condensation nuclei enter the atmosphere mainly from the seas and oceans (about 80%) through evaporation and splashing them from the water surface. In addition, the sources of condensation nuclei are combustion products, soil weathering, volcanic activity, etc.
As a result of condensation and sublimation, tiny droplets of water are formed in the atmosphere (with a radius of about 50 mk) and ice crystals shaped like a hexagonal prism. Their accumulation in the ground layer of air produces haze or fog in the overlying layers of the cloud. The merging of small cloud drops or the growth of ice crystals leads to the formation of various types of precipitation: rain, snow.
Clouds can consist only of drops, only of crystals, and be mixed, i.e., consist of drops and crystals. Water droplets in clouds at subzero temperatures are in a supercooled state. Droplet-liquid clouds are observed in most cases down to a temperature of -12° C, purely icy (crystalline) clouds - at temperatures below -40° C, mixed clouds - from -12 to -40° C.
Clouds are characterized by water content. Water content is the amount of water in grams contained in one cubic meter of cloud (g/m3). The water content in droplet-liquid clouds ranges from 0.01 to 4 g per cubic meter of cloud mass (in some cases more than 10 g/m 3). In ice clouds the water content is less than 0.02 g/m 3, and in mixed clouds up to 0.2-0.3 g/m 3 . Water content should not be confused with humidity.
Clouds are classified:
The height of the lower border is 3 (sometimes 4) tiers,
By origin (genetic classification) into 3 groups,
Based on appearance (morphological classification) they are divided into several forms:
The main forms are distinguished:
Cumulus clouds are white, gray, dark gray individual formations in the form of heaps of various shapes.
Cirrus- individual thin light clouds of white, transparent, fibrous or filamentous structure have the appearance of hooks, threads, feathers or stripes.
Stratus clouds- represent a homogeneous gray cover of varying transparency.
Cirrocumulus clouds, which are small white flakes or small balls (lambs), resembling lumps of snow,
Cirrostratus clouds that look like a white veil, often covering the entire sky and giving it a milky white hue.
Stratocumulus gray clouds with dark stripes - cloud shafts.
Other features of appearance (presence of waviness, specific cloud shapes) and connections with precipitation are also noted. In total there are 10 main forms of clouds and 70 varieties of them.
The shape of clouds is determined when they are observed in accordance with the accepted classification using a specially published Cloud Atlas.
Clouds that arise within air masses are called intra-mass, formed on atmospheric fronts – frontal, arising over the mountains when air currents flow over obstacles (mountains) – orographic.
| Groups | Education process | Tier | ||
| Lower (0 – 2000m). | Clouds of vertical development. | Medium (2000 – 6000 m). | ||
| Upper (above 6000m). | cumuliform | Convection in the presence of a delay layer. | Cumulus (flat clouds). | Altocumulus: - flocculus; |
| - tower-shaped. | Cirrocumulus flocculus | |||
| Vertical development: invasion of cold air under warm air. | Cumulonimbus. | Powerful cumulus (upper limit – up to the tropopause). | Layered | The upward sliding of warm air along gently sloping frontal sections or over a cold underlying surface. |
| Nimbostratus. | Rupture-nimbus (stratus or stratocumulus) | Highly layered: - thin. | - dense | Cirrus. |
| Cirrostratus | Wavy | Above-inversion: upward sliding of warm air along the inversion layer with a weak slope. |
Stratocumulus dense Altocumulus dense Cirrocumulus undulate
Sub-inversion: turbulence, radiation, mixing in the boundary layer.
Stratocumulus translucent.
The amount of clouds is assessed on a ten-point scale, and three states of the sky are distinguished: clear (0... 2 points), and cloudy (3... 7 points) and cloudy (8... 10 points).
With all the variety 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. The 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 is on average 8.6 points for general cloudiness and 7.8... 7.9 points for lower cloudiness (Table 60). Starting from January, cloudiness (total and low) gradually decreases, reaching its lowest values in May-June. But at this time the sky is on average more than half covered with clouds of various shapes (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). In the warm period, their number sharply decreases to 2... 4 per month, although in some years, even with lower clouds 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. Usually it is 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 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 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) can be distinguished by the naked eye against the background of the sky near the horizon (or against the background of air haze), at night time - the greatest distance at which a similar object could be detected when 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, snowstorms, 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, 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. Deterioration of visibility within the specified limits due to the presence of other phenomena 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.
A 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. In addition, relative humidity close to 100% 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 the atmospheric circulation of 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, associated with the cooling of the air layer from the 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 transport of warm, moist air by western and western currents to 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 their total number during 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 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. 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 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 can be seen that in the climatic conditions of 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 weather phenomena that are no less dangerous for the national economy, 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. Most often, thunderstorms occur in the afternoon: between 12 and 18 hours they account for 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 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 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 the warm season from powerful convection clouds with great instability of the atmosphere. It consists of 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.
Hail often causes great damage to landscape gardening and agriculture in the 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 most often associated with the passage of fronts, mostly cold, and less often with the heating of the 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). The largest number of days with hail (four days) was observed in May 1975 and June 1957.
In the daily cycle, hail 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.
Determination and recording of the total number of clouds, as well as determination and recording of the number of low and middle clouds and their heights.
Determining and recording the total number of clouds
The number of clouds is expressed in points on a 10-point scale from 0 to 10. It is estimated by eye how many tenths of the sky are covered with clouds.
If there are no clouds or cloudiness covers less than 1/10 of the sky, cloudiness is assessed with a score of 0. If clouds cover 1/10, 2/10, 3/10 of the sky, etc., marks are given respectively 1, 2, 3, etc. d. The number 10 is placed only when the entire sky is completely covered with clouds. If even very small gaps are observed in the sky, 10 is recorded.
If the number of clouds is more than 5 points (i.e. half the sky is covered by clouds), it is more convenient to estimate the area not occupied by clouds and subtract the resulting value, expressed in points, from 10. The remainder will show the number of clouds in points.
In order to estimate what part of the sky is free from clouds, you need to mentally sum up all those clear sky gaps (windows) that exist between individual clouds or banks of clouds. But those gaps that exist inside several clouds (cirrus, cirrocumulus and almost all types of altocumulus) are inherent in their internal structure and are very small in size and cannot be summed up. If such clouds with gaps cover the entire sky, the number 10 is set
Determine and record the number of low and middle clouds and their heights.
In addition to the total number of clouds N, it is necessary to determine the total number of stratocumulus, stratus, cumulus, cumulonimbus and fractus clouds Nh (forms recorded in the “CL” line) or, if there are none, then the total number in altocumulus, altostratus and nimbostratus clouds (forms recorded in the “CM” line). The number of these clouds Nh is determined by the same rules as the total number of clouds.
The height of the clouds must be assessed by eye, aiming for an accuracy of 50-200 m. If this is difficult, then at least with an accuracy of 0.5 km. If these clouds are located at the same level, then the height of their base is recorded in line “h”; if they are located at different levels, the height h of the lowest clouds is indicated. If there are no clouds of the form recorded in the “CL” line, and clouds of the form recorded in “Cm” are observed, the height of the base of these clouds is recorded in line h. If individual fragments or shreds of clouds recorded in the “CL” line (in quantities less than 1 point) are located under a more extensive layer of other clouds of the same shapes or forms recorded in the “Sm” line, the height of the base of this is recorded in the “h” line a layer of clouds, not wisps or scraps.
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.
Based on their appearance, clouds are divided into three classes: separate, unconnected cloud masses, layers with a heterogeneous 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 cloud amount - from 5 to 6 points. This relatively cloudy area of the Asian part of Russia is within the scope of the Asian one.
The distribution of the average annual amount of low cloud cover generally 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 values in late autumn and winter, when the influence is especially pronounced. The exact opposite annual variation in the amount of total and lower cloudiness is observed in the Far East, and. Here, the greatest number of clouds occurs 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 the central regions of the 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 to different hours of the day; 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).
According to the international classification, there are 10 main types of clouds of different tiers.
> UPPER LEVEL CLOUDS(h>6km) 
Spindrift clouds(Cirrus, Ci) are individual clouds of a fibrous structure and a whitish hue. Sometimes they have a very regular structure in the form of parallel threads or stripes, sometimes on the contrary, their fibers are tangled and scattered across the sky in separate spots. Cirrus clouds are transparent because they consist of tiny ice crystals. Often the appearance of such clouds heralds a change in the weather. From satellites, cirrus clouds are sometimes difficult to see.
Cirrocumulus clouds(Cirrocumulus, Cc) - a layer of clouds, thin and translucent, like cirrus, but consisting of individual flakes or small balls, and sometimes as if from parallel waves. These clouds usually form, figuratively speaking, a “cumulus” sky. They often appear along with cirrus clouds. Sometimes visible before storms.
Cirrostratus clouds(Cirrostratus, Cs) - a thin, translucent whitish or milky cover, through which the disk of the Sun or Moon is clearly visible. This cover can be uniform, like a layer of fog, or fibrous. On cirrostratus clouds, a characteristic optical phenomenon is observed - a halo (light circles around the Moon or Sun, false Sun, etc.). Like cirrus, cirrostratus clouds often indicate the approach of severe weather.
> MIDDLE LEVEL CLOUDS(h=2-6 km)
They differ from similar lower-level cloud forms in their high altitude, lower density, and greater likelihood of having an ice phase. 
Altocumulus clouds(Altocumulus, Ac) - a layer of white or gray clouds consisting of ridges or individual “blocks”, between which the sky is usually visible. The ridges and “blocks” that form the “feathery” sky are relatively thin and are arranged in regular rows or in a checkerboard pattern, less often - in disorder. "Cirrus" skies are usually a sign of pretty bad weather.
Altostratus clouds(Altostratus, As) - a thin, less often dense veil of a grayish or bluish tint, in places heterogeneous or even fibrous in the form of white or gray shreds all over the sky. The Sun or Moon shines through it in the form of light spots, sometimes quite faint. These clouds are a sure sign of light rain.
> LOWER CLOUDS(h According to many scientists, nimbostratus clouds are illogically assigned to the lower tier, since only their bases are located in this tier, and the tops reach a height of several kilometers (middle tier cloud levels). These heights are more typical for clouds of vertical development, and therefore, some scientists classify them as middle-tier clouds.
Stratocumulus clouds(Stratocumulus, Sc) - a cloud layer consisting of ridges, shafts or individual elements thereof, large and dense, gray in color. There are almost always darker areas.
The word “cumulus” (from the Latin “heap”, “heap”) means a crowded, piled-up cloud. These clouds rarely bring rain, only sometimes they turn into nimbostratus clouds, from which rain or snow falls.
Stratus clouds(Stratus, St) - a rather homogeneous layer of low gray clouds, devoid of regular structure, very similar to fog that has risen a hundred meters over the ground. Stratus clouds cover large areas and look like torn rags. In winter, these clouds often remain throughout the day; precipitation usually does not fall on the ground; sometimes there is drizzle. In summer they quickly dissipate, after which good weather sets in.
Nimbostratus clouds(Nimbostratus, Ns, Frnb) are dark gray clouds, sometimes threatening in appearance. Often, low dark fragments of broken rain clouds appear below their layer - typical harbingers of rain or snowfall.
> VERTICAL CLOUDS
Cumulus clouds (Cumulus, Cu)- dense, sharply defined, with a flat, relatively dark base and a dome-shaped white, as if swirling, top, reminiscent of cauliflower. They begin in the form of small white fragments, but soon they form a horizontal base, and the cloud begins to rise imperceptibly. With little humidity and weak vertical ascent of air masses, cumulus clouds foretell clear weather. Otherwise, they accumulate throughout the day and can cause a thunderstorm.
Cumulonimbus (Cb)- powerful cloud masses with strong vertical development (up to a height of 14 kilometers), giving heavy rainfall with thunderstorm phenomena. They develop from cumulus clouds, differing from them in the upper part, consisting of ice crystals. These clouds are associated with squally winds, heavy precipitation, thunderstorms, and hail. The lifespan of these clouds is short - up to four hours. The base of the clouds is dark in color, and the white top goes far above. In the warm season, the peak can reach the tropopause, and in the cold season, when convection is suppressed, the clouds are flatter. Usually clouds do not form a continuous cover. As a cold front passes, cumulonimbus clouds can form a swell. The sun does not shine through the cumulonimbus clouds. Cumulonimbus clouds are formed when the air mass is unstable, when active upward movement of air occurs. These clouds also often form on a cold front when cold air hits a warm surface.
Each genus of clouds, in turn, is divided into species according to the characteristics of their shape and internal structure, for example, fibratus (fibrous), uncinus (claw-shaped), spissatus (dense), castellanus (tower-shaped), floccus (flaky), stratiformis (stratified). ), nebulosus (foggy), lenticularis (lenticular), fractus (torn), humulus (flat), mediocris (medium), congestus (powerful), calvus (bald), capillatus (hairy). Types of clouds, further, have varieties, for example, vertebratus (ridge-shaped), undulatus (wavy), translucidus (translucent), opacus (non-translucent), etc. Further, additional features of clouds are distinguished, such as incus (anvil), mamma (snake-shaped) , vigra (fall stripes), tuba (trunk), etc. And finally, evolutionary features are noted that indicate the origin of clouds, for example, Cirrocumulogenitus, Altostratogenitus, etc.
When observing cloudiness, it is important to determine by eye the degree of sky coverage on a ten-point scale. Clear sky - 0 points. It's clear, there are no clouds in the sky. If the sky is covered with clouds no more than 3 points, partly cloudy. Partly cloudy 4 points. This means that clouds cover half the sky, but at times their amount decreases to "clear". When the sky is half covered, cloudiness is 5 points. If they say “sky with gaps,” they mean that the cloudiness is at least 5, but not more than 9 points. Cloudy - the sky is completely covered with clouds of a single blue sky. Cloud cover 10 points.
