Trees often become a target for lightning strikes, which sometimes leads to very serious consequences. We will talk about the danger of being struck by lightning both for the trees themselves and for the people living next to them, as well as how you can reduce the risks associated with this phenomenon.

Where does lightning strike

For a significant part of the Earth's territory, thunderstorms are quite a common occurrence. At the same time, about one and a half thousand thunderstorms rage over the Earth. For example, more than 20 thunderstorm days are observed in Moscow every year. But despite the familiarity of this natural phenomenon, its power cannot but shock. The voltage of an average lightning is about 100,000 volts, and the current is 20,000–50,000 amperes. The temperature of the lightning channel in this case reaches 25,000 - 30,000 °C. Not surprisingly, lightning strikes buildings, trees, or people and spreads its electrical charge, often with catastrophic consequences.

Although the defeat of a single ground object by lightning, be it a building, a mast or a tree, is a rather rare event, the colossal destructive force makes thunderstorms one of the most dangerous natural phenomena for humans. Thus, according to statistics, every seventh fire in rural areas starts due to a lightning strike, in terms of the number of registered deaths caused by natural disasters, lightning ranks second, second only to floods.

The probability of ground objects (including trees) being struck by lightning depends on several factors:

• on the intensity of thunderstorm activity in the region (related to the climate);
• on the height of this object (the higher, the more likely a lightning strike);
• from the electrical resistance of the object and the soil layers located under them (the lower the electrical resistance of the object and the soil layers located under it, the higher the probability of a lightning discharge into it).

From the foregoing, it is clear why trees often become a target for lightning: a tree is often the predominant element of the relief in height, living wood saturated with moisture, associated with deep layers of soil with low electrical resistance, often represents a well-grounded natural lightning rod.

Thunderstorm activity in some settlements of the Moscow region

Locality	Average annual duration of thunderstorms, hours	Specific density of lightning strikes in 1 km²	General characteristics of thunderstorm activity
Volokolamsk	40–60	4	high
Istra	40–60	4	high
New Jerusalem	40–60	4	high
Pavlovsky Posad	20–40	2	average
Moscow	20–40	2	average
Kashira	20–40	2	average

What is the danger of a tree being struck by lightning

The consequences of a lightning strike into a tree are often devastating both for itself and for nearby buildings, and also pose a significant threat to people who are nearby at that moment. At the moment of passage of a powerful electric charge through the wood, a powerful release of heat and explosive evaporation of moisture occur inside the trunk. The result of this is damage of varying severity: from superficial burns or cracks to complete splitting of the trunk or fire of the tree. In some cases, significant mechanical damage occurs inside the trunk (longitudinal cracks or splitting of wood along annual rings), which are almost imperceptible during external examination, but significantly increase the risk of a tree falling in the near future. Often serious, but imperceptible during visual inspection, damage can also be received by the roots of a tree.

In the event that lightning damage does not lead to instant destruction or death of a tree, extensive injuries received by it can cause the development of dangerous diseases, such as rot, vascular diseases, a weakened plant becomes an easy prey for stem pests. As a result, the tree may become unsafe or dry out.

Lightning strikes on trees (including living ones) often cause fires that spread to nearby buildings. Sometimes a lateral discharge from a tree is transmitted to the wall of a building, even if a lightning rod is installed on it. Finally, the electrical potential from the affected tree propagates in the surface layers of the ground, as a result of which it can be carried into the building, damage underground utilities, or cause electric shock to people or pets.

A lightning strike into a tree can cause significant material damage even if there is no emergency. After all, the assessment of the safety of such a tree, special care for it, or even the simple removal of a dried or hopelessly diseased tree can be associated with significant material costs.

Sometimes a lateral discharge from a tree is transmitted to the wall of a building, even if a lightning rod is installed on it.

Regulatory Issues

Thus, lightning protection of especially valuable trees (which are the center of landscape compositions, historical and rare) or trees growing near housing can be practically justified. However, the regulatory framework that prescribes or regulates the lightning protection of trees is completely absent in our country. This state of affairs is more a consequence of the inertia of the domestic regulatory framework than an adequate assessment of the risks associated with lightning strikes on trees in an urban environment.

The main current domestic standard for lightning protection dates back to 1987. The attitude towards lightning protection in the countryside in this document reflects the realities and positions of that time: the material value of most countryside buildings was not great, and the interests of the state were focused on the protection of public rather than private property. In addition, the compilers of domestic standards proceeded from the assumption that construction norms and rules are observed during the construction of suburban housing, but this is not always the case. In particular, the minimum distance from the tree trunk to the wall of the building must be at least 5 m. In the realities of suburban construction, houses are often located close to the trees. Moreover, the owners of such trees, as a rule, are reluctant to agree to their removal.

In other countries, there are standards for lightning protection: for example, American - ANSI A 300 part 4 or British - british standard 6651 also regulates the lightning protection of trees.

The minimum distance from the tree trunk to the wall of the building must be at least 5 m.

When is protection needed?

In what cases does it make sense to think about the lightning protection of a tree? We list the factors on the basis of which such a decision can be recommended.

The tree grows in open areas or noticeably higher than neighboring trees, buildings, structures and landforms. Objects that dominate in height are more likely to be struck by lightning.

An area with high thunderstorm activity. With a high frequency of thunderstorms, the probability of damaging trees (as well as other objects) increases. The main characteristics of thunderstorm activity are the average annual number of thunderstorm hours, as well as the average specific density of lightning strikes to the ground (average annual number of lightning strikes per 1 km²) of the earth's surface. The latter indicator is used to calculate the expected number of lightning strikes of an object (including a tree) per year. For example, in the case of an area with an average duration of 40–60 thunderstorm hours per year (in particular, some areas of the Moscow region), one can expect a tree 25 m high to be damaged once every 20 years.

Location of the site near water bodies, underground springs, high soil moisture on the site . This arrangement further increases the risk of a tree being struck by lightning.

A tall tree grows at a distance of three meters or less from the building. This arrangement of the tree does not affect the probability of being hit by lightning. However, the defeat of trees located near buildings poses significant threats both to the buildings themselves and to the people in them. At the same time, the risk of damage to the building by a side discharge increases, the risk of damage to the roof when a tree falls is very high, and if it ignites, a fire can spread to the building.

The branches of the tree hang over the roof of the building, touch its walls, canopies, gutters or decorative elements of the facade. In this case, the risk of damage to the building, fires, and transfer of the discharge to the house also increases.

The tree belongs to a species that is often or regularly struck by lightning strikes. . Some tree species are more likely to be struck by lightning than others. Oak trees are the most commonly affected by lightning.

The roots of a tree growing near the building may come into contact with an underground foundation or communications suitable for the house. In this case, when a tree is struck by lightning, the probability of the discharge “drifting” into the premises or damage to communications (for example, sensors of the irrigation system and electrical networks) increases.

Specialists in lightning protection of buildings recommend the installation of a free-standing lightning rod, while at a distance of 3 to 10 m there are trees that are suitable in height and other parameters for installing a lightning rod and down conductor. Installing a separate mast can be quite expensive. For many owners of country houses, such masts are also aesthetically unacceptable. And finally, placing a mast in a forest area in such a way that tree roots are not damaged during its construction or stretch marks do not interfere with the movement of people can be very difficult.

Exposure to unprotected trees of some species
(from standard ANSI A 300, part 4)

Operating principle

The principle of operation of the lightning protection system is that the lightning discharge is "intercepted" by the lightning rod, safely carried out by the down conductor and transmitted to the deep layers of the soil by means of grounding.

The components of a tree lightning protection system are: a lightning rod (one or more), an overhead down conductor, an underground down conductor and a grounding system consisting of several grounding rods or plates.

When developing our own lightning protection schemes, we were faced with the need to combine domestic standards for lightning protection of buildings and structures and Western standards governing the lightning protection of trees. The need for such a combination is due to the fact that in the current domestic standards there are no recommendations for installing lightning protection systems on trees, and older prescriptions include instructions that pose a threat to the health of a tree. At the same time, the American standard ANSI A 300, which contains detailed information about mounting the system on a tree and the principles of its installation and maintenance, imposes lower requirements on the electrical safety of the system compared to domestic standards.

Lightning protection components are made of copper or stainless steel. At the same time, in order to avoid corrosion, only one of the selected materials is used in all connections and contacts between conductive elements. However, when using copper, the use of bronze fasteners is allowed. Copper components are more expensive, but have greater conductivity, allowing components to be smaller, less visible, and reduce system installation costs.

According to statistics, every seventh fire in rural areas starts due to a lightning strike, in terms of the number of registered deaths caused by natural disasters, lightning ranks second, second only to floods.

System Components

The lightning rod is a metal tube closed at the end. The down conductor enters the lightning rod and is attached to it with bolts.

For trees with a spreading crown, additional pantographs are sometimes necessary, since in this case the lightning discharge can strike branches or peaks that are far from the lightning rod. If a mechanical branch support system based on metal cables is installed on a tree, then it must also be grounded when performing lightning protection. To do this, with the help of a bolted contact, an additional down conductor is attached to it. It should be borne in mind that direct contact of copper with a galvanized cable is unacceptable, as it leads to corrosion.

Down conductors from lightning rods and additional contacts are connected using special clamp contacts or bolted connections. In accordance with the ANSI A 300 standard for lightning protection of trees, down conductors are used in the form of all-metal steel cables of various weaving. In accordance with domestic standards, the minimum effective cross section of a down conductor made of copper is 16 mm², the minimum effective cross section of a down conductor made of steel is 50 mm. When conducting down conductors on wood, it is necessary to avoid their sharp bends. It is not allowed to bend down conductors at an angle less than 900, the radius of curvature of the bend should not be less than 20 cm.

Down conductors are attached to the trunk with metal clips, buried in the wood of the trunk for several centimeters. The material of the clamps must not lead to contact corrosion when connected to the down conductor. It is impossible to fix the down conductors by tying them to the tree with wire, since the radial growth of the trunk will lead to ring injuries and the drying of the tree. Rigid fixation of down conductors on the surface of the trunk (with staples) will lead to their growing into the trunk, reducing the durability and safety of the system and the development of extensive stem rot. The best option for mounting the system is to install dynamic clamps. In this case, when the diameter of the trunk increases, the holders with cables are automatically pressed to the end of the rod by the pressure of the wood tissues. It should be noted that the deepening of the pins of the clamps a few centimeters into the wood and their subsequent partial encapsulation by the wood practically does not cause any harm to it.

Down conductors go down the shaft to its base and go deep into the trench.

The minimum trench depth for the underground part of the down conductor, prescribed by the ANSI A 300 standard, is 20 cm. The trench is dug manually while maintaining the maximum number of roots. In cases where root damage is particularly undesirable, special equipment should be used to make a trench. For example, an air knife is a compressor tool designed to perform earthworks in the near-trunk zone of trees. This device, using a strong focused air stream, is able to remove soil particles without damaging even the thinnest tree roots.

The type and parameters of the grounding device and the distance to which the down conductor must extend to it are determined by the properties of the soil. This is due to the need to reduce the ground impulse resistance to the required level - the electrical resistance to the spreading of an electric current pulse from the ground electrode. According to domestic standards, in places regularly visited by people, such resistance should not exceed 10 ohms. This value of ground resistance should exclude spark breakdowns of current from the underground down conductor and ground electrode to the soil surface and, therefore, prevent electric shock to people, buildings and communications. The main indicator of the soil, which determines the choice of the grounding scheme, is the soil resistivity - the resistance between two faces of 1 m³ of earth when current passes through it.

The higher the resistivity of the soil, the more extensive the grounding system must be in order to ensure the safe flow of electrical charge. On soils with low resistivity - up to 300 Ohm (loams, clays, wetlands), - as a rule, a grounding system is used from two vertical grounding rods connected by a down conductor. A distance of at least 5 m is maintained between the rods. The length of the rods is 2.5–3 m, the upper end of the rod is deepened by 0.5 m.

On soils with high values ​​of resistivity (sandy loam, sand, gravel), multi-beam grounding systems are used. When limiting the possible depth of grounding, grounding plates are used. For the convenience of inspections and testing of the reliability of grounding, small wells are installed above the grounding elements.

Soil resistivity is not a constant value, its value strongly depends on soil moisture. Therefore, in the dry season, the reliability of grounding may decrease. Several methods are used to prevent this. First, ground rods are placed in the irrigation zone whenever possible. Secondly, the upper part of the rod is buried 0.5 m below the soil surface (the top 0.5 m of the soil is most prone to drying out). Thirdly, if necessary, bentonite is added to the soil - a natural moisture-retaining component. Bentonite is a small colloidal mineral clay particles, the pore space of which retains moisture well and stabilizes soil moisture.

Moisture-saturated living wood, bonded to deep, low-resistance ground layers, is often a well-grounded natural lightning rod.

Common Mistakes

In domestic practice, lightning protection of trees is rarely used, and in cases where it is nevertheless performed, a number of serious mistakes are made during its construction. So, as lightning rods, as a rule, metal rods are used, fixed on a tree with wire or metal hoops. This mounting option leads to serious ring injuries of the trunk, which eventually lead to the complete drying of the tree. A certain danger is also represented by the ingrowth of the down conductor into the trunk of a tree, leading to the appearance of extensive open longitudinal wounds on the trunk.

Since the installation of lightning protection on trees is carried out by electricians, they usually use hafs (cats) to climb a tree - boots with metal spikes that cause serious injuries to a tree.

Unfortunately, the features of the tree crown are also ignored: as a rule, the need to install several lightning rods on multi-top trees with wide crowns is not taken into account, structural defects in the branching of the tree are also not taken into account, which often leads to breaking and falling of the top with the installed lightning rod.

Lightning protection of trees cannot be called a common practice. Indications for its implementation are quite rare in areas with moderate thunderstorm activity. Nevertheless, in cases where lightning protection of trees is necessary, its correct implementation is extremely important. When designing and installing such systems, it is important to take into account not only the reliability of the lightning rod itself, but also the safety of the system for the protected tree.

The final reliability of lightning protection will depend both on the correct choice of its materials, contacts and grounding, and on the stability of the tree itself. Only taking into account the features of the crown structure, radial growth, the location of the root system of the tree, it is possible to create a reliable lightning protection system that does not cause dangerous injuries to the tree, which means that it does not create unnecessary risks for people living nearby.

Thunderstorm - an atmospheric phenomenon in which electrical discharges occur inside the clouds or between the cloud and the earth's surface - lightning, accompanied by thunder. As a rule, a thunderstorm is formed in powerful cumulonimbus clouds and is associated with heavy rain, hail and squalls.

Thunderstorm is one of the most dangerous natural phenomena for humans: in terms of the number of recorded deaths, only floods lead to greater human losses.

Thunderstorm

At the same time, about one and a half thousand thunderstorms operate on Earth, the average intensity of discharges is estimated at 100 lightning per second. Thunderstorms are unevenly distributed over the surface of the planet.

Distribution of lightning discharges over the Earth's surface

There are approximately ten times less thunderstorms over the ocean than over the continents. About 78% of all lightning discharges are concentrated in the tropical and equatorial zone (from 30° north latitude to 30° south latitude). The maximum thunderstorm activity occurs in Central Africa. There are practically no thunderstorms in the polar regions of the Arctic and Antarctic and over the poles. The intensity of thunderstorms follows the sun: the maximum thunderstorms occur in the summer (in the middle latitudes) and in the daytime afternoon hours. The minimum recorded thunderstorms occur before sunrise. Thunderstorms are also affected by geographical features of the area: strong thunderstorm centers are located in the mountainous regions of the Himalayas and the Cordillera.

Development stages of a thundercloud

The necessary conditions for the formation of a thundercloud are the presence of conditions for the development of convection or another mechanism that creates ascending flows of moisture sufficient for the formation of precipitation, and the presence of a structure in which some of the cloud particles are in a liquid state, and some are in an icy state. Convection leading to the development of thunderstorms occurs in the following cases:

With uneven heating of the surface layer of air over a different underlying surface. For example, over the water surface and land due to differences in water and soil temperatures. Over large cities, the intensity of convection is much higher than in the vicinity of the city.

When warm air rises or is displaced by cold air at atmospheric fronts. Atmospheric convection at atmospheric fronts is much more intense and more frequent than during intramass convection. Often, frontal convection develops simultaneously with nimbostratus clouds and extensive precipitation, which masks the resulting cumulonimbus clouds.

When air rises in areas of mountain ranges. Even small elevations in the terrain lead to increased cloud formation (due to forced convection). High mountains create especially difficult conditions for the development of convection and almost always increase its frequency and intensity.

All thunderclouds, regardless of their type, successively go through the stages of a cumulus cloud, the stage of a mature thundercloud and the stage of decay.

Thundercloud classification

At one time, thunderstorms were classified according to where they were observed, such as localized, frontal, or orographic. It is now more common to classify thunderstorms according to the characteristics of the thunderstorms themselves, and these characteristics are mainly dependent on the meteorological environment in which the thunderstorm develops.

The main necessary condition for the formation of thunderclouds is the state of instability of the atmosphere, which forms updrafts. Depending on the magnitude and power of such flows, thunderclouds of various types are formed.

single cell cloud

Single-cell cumulonimbus clouds develop on days with weak winds in a low-gradient baric field. They are also called intramass or local thunderstorms. They consist of a convective cell with an upward flow in its central part. They can reach lightning and hail intensity and quickly collapse with precipitation. The dimensions of such a cloud are: transverse - 5-20 km, vertical - 8-12 km, life expectancy - about 30 minutes, sometimes - up to 1 hour. Serious weather changes after a thunderstorm do not occur.

The life cycle of a single cell cloud

A thunderstorm begins with a fine weather cumulus cloud (Cumulus humilis). Under favorable conditions, the resulting cumulus clouds grow rapidly both in the vertical and horizontal directions, while the ascending flows are located almost throughout the volume of the cloud and increase from 5 m/s to 15-20 m/s. Downstreams are very weak. Ambient air actively penetrates into the cloud due to mixing at the boundary and top of the cloud. The cloud passes into the Cumulus mediocris stage. The smallest water drops formed as a result of condensation in such a cloud merge into larger ones, which are carried away by powerful upward flows. The cloud is still homogeneous, consists of water droplets held by an ascending flow - precipitation does not fall. In the upper part of the cloud, when water particles enter the zone of negative temperatures, the drops gradually begin to turn into ice crystals. The cloud becomes a powerful cumulus cloud (Cumulus congestus). The mixed composition of the cloud leads to the enlargement of cloud elements and the creation of conditions for precipitation. Such a cloud is called a cumulonimbus cloud (Cumulonimbus) or a bald cumulonimbus cloud (Cumulonimbus calvus). Vertical flows in it reach 25 m/s, and the level of the summit reaches a height of 7–8 km.

Evaporating precipitation particles cool the surrounding air, which leads to a further increase in downdrafts. At the stage of maturity, both ascending and descending air currents are present in the cloud at the same time.

At the decay stage, the cloud is dominated by downdrafts, which gradually cover the entire cloud.

Multicell cluster thunderstorms

Scheme of a multi-cell thunderstorm structure

This is the most common type of thunderstorm associated with mesoscale (having a scale of 10 to 1000 km) disturbances. A multi-cell cluster consists of a group of thunderstorm cells moving as a unit, although each cell in the cluster is at a different stage in the development of a thundercloud. Mature thunderstorm cells are usually located in the central part of the cluster, while decaying cells are located on the leeward side of the cluster. They have transverse dimensions of 20-40 km, their tops often rise to the tropopause and penetrate into the stratosphere. Multi-celled cluster thunderstorms can produce hail, showers, and relatively weak squalls. Each individual cell in a multi-cell cluster is in a mature state for about 20 minutes; the multi-cell cluster itself can exist for several hours. This type of thunderstorm is usually more intense than a single cell thunderstorm, but much weaker than a supercell thunderstorm.

Multicell line thunderstorms (squall lines)

Multicell line thunderstorms are a line of thunderstorms with a long, well-developed gust front on the front front line. The squall line may be continuous or contain gaps. The approaching multicell line looks like a dark wall of clouds, usually covering the horizon from the western side (in the northern hemisphere). A large number of closely spaced ascending/descending air currents makes it possible to qualify this complex of thunderstorms as a multi-cell thunderstorm, although its thunderstorm structure differs sharply from a multi-cell cluster thunderstorm. Squall lines can produce large hail and intense downpours, but they are more commonly known as systems that create strong downdrafts. The squall line is similar in properties to a cold front, but is a local result of thunderstorm activity. Often a squall line occurs ahead of a cold front. On radar images, this system resembles a curved bow (bow echo). This phenomenon is typical for North America, in Europe and the European territory of Russia it is observed less frequently.

Supercell thunderstorms

Vertical and horizontal structure of a supercell cloud

A supercell is the most highly organized thundercloud. Supercell clouds are relatively rare, but pose the greatest threat to human health and life and property. A supercell cloud is similar to a single cell cloud in that both have the same updraft zone. The difference is that the size of the cell is huge: a diameter of about 50 km, a height of 10-15 km (often the upper boundary penetrates into the stratosphere) with a single semicircular anvil. The speed of the ascending flow in a supercell cloud is much higher than in other types of thunderclouds: up to 40–60 m/s. The main feature that distinguishes a supercell cloud from other types of clouds is the presence of rotation. A rotating updraft in a supercell cloud (called in radar terminology) mesocyclone), creates extreme weather events, such as a giant hail(more than 5 cm in diameter), heavy winds up to 40 m/s and strong destructive tornadoes. Environmental conditions are a major factor in the formation of a supercell cloud. A very strong convective instability of the air is needed. The air temperature near the ground (before a thunderstorm) should be +27 ... +30 and higher, but the main necessary condition is the wind of a variable direction, which causes rotation. Such conditions are achieved with wind shear in the middle troposphere. Precipitation formed in the updraft is carried along the upper level of the cloud by a strong flow into the downdraft zone. Thus, the zones of the ascending and descending flows are separated in space, which ensures the life of the cloud for a long period of time. There is usually light rain at the leading edge of a supercell cloud. Heavy rainfall occurs near the updraft zone, while the heaviest precipitation and large hail fall to the northeast of the main updraft zone. The most dangerous conditions occur close to the main updraft area (usually displaced to the rear of the thunderstorm).

Supercell (English) super and cell- cell) - a type of thunderstorm, characterized by the presence of a mesocyclone - a deep, strongly rotating updraft. For this reason, such storms are sometimes called rotating thunderstorms. Of the four types of thunderstorms according to Western classifications (supercell, squalline, multicell and singlecell), supercells are the least common and may pose the greatest danger. Supercells are often isolated from other thunderstorms and can have a front span of up to 32 kilometers.

Supercell at sunset

Supersells are often divided into three types: classic; low precipitation (LP); and high precipitation (HP). LP-type supercells tend to form in drier climates such as the highland valleys of the United States, while HP-type supercells are more common in wetter climates. Supercells can occur anywhere in the world if the weather conditions are right for them to form, but they are most common in the US Great Plains, an area known as the Tornado Valley. They can also be observed in the plains in Argentina, Uruguay and southern Brazil.

Physical characteristics of thunderclouds

Airborne and radar studies show that a single thunderstorm cell usually reaches a height of about 8-10 km and lives for about 30 minutes. An isolated thunderstorm usually consists of several cells in various stages of development and lasts on the order of an hour. Large thunderstorms can reach tens of kilometers in diameter, their peak can reach heights of over 18 km, and they can last for many hours.

Upstream and downstream

Updrafts and downdrafts in isolated thunderstorms typically have a diameter of 0.5 to 2.5 km and a height of 3 to 8 km. Sometimes the diameter of the updraft can reach 4 km. Near the surface of the earth, the streams usually increase in diameter, and the speed in them decreases compared to the streams located above. The characteristic speed of the updraft lies in the range from 5 to 10 m/s and reaches 20 m/s in the upper part of large thunderstorms. Research planes flying through a thundercloud at an altitude of 10,000 m record updraft speeds in excess of 30 m/s. The strongest updrafts are observed in organized thunderstorms.

Flurries

Before the August 2010 squall in Gatchina

In some thunderstorms, intense downdrafts develop, creating destructive winds on the surface of the earth. Depending on the size, such downstreams are called flurries or microstorms. A squall with a diameter of more than 4 km can create winds up to 60 m/s. Microsqualls are smaller, but create wind speeds up to 75 m/s. If the thunderstorm that generates the squall is formed from sufficiently warm and moist air, then the microsquall will be accompanied by intense rain showers. However, if the thunderstorm is formed from dry air, the precipitation may evaporate during the fall (airborne precipitation bands or virga) and the microsquall will be dry. Downdrafts are a serious hazard to aircraft, especially during takeoff or landing, as they create wind near the ground with sudden changes in speed and direction.

Vertical development

In general, an active convective cloud will rise until it loses its buoyancy. The loss of buoyancy is due to the load created by precipitation formed in the cloudy environment, or mixing with the surrounding dry cold air, or a combination of these two processes. Cloud growth can also be stopped by a blocking inversion layer, i.e. a layer where air temperature rises with height. Thunderclouds usually reach a height of about 10 km, but sometimes reach heights of more than 20 km. When the moisture content and instability of the atmosphere are high, then with favorable winds, the cloud can grow to the tropopause, the layer that separates the troposphere from the stratosphere. The tropopause is characterized by a temperature that remains approximately constant with increasing altitude and is known as a region of high stability. As soon as the updraft begins to approach the stratosphere, pretty soon the air at the top of the cloud becomes colder and heavier than the surrounding air, and the growth of the top stops. The height of the tropopause depends on the latitude of the area and on the season of the year. It varies from 8 km in the polar regions to 18 km and higher near the equator.

When a cumulus cloud reaches the blocking layer of the tropopause inversion, it begins to spread outward and forms the “anvil” characteristic of thunderclouds. Wind blowing at the height of the anvil usually blows cloud material in the direction of the wind.

Turbulence

An aircraft flying through a thundercloud (it is forbidden to fly into cumulonimbus clouds) usually gets into a turbulence that throws the plane up, down and sideways under the influence of turbulent cloud flows. Atmospheric turbulence creates a feeling of discomfort for the aircraft crew and passengers and causes undesirable stresses on the aircraft. Turbulence is measured in different units, but more often it is defined in units of g - free fall acceleration (1g = 9.8 m / s 2). A flurry of one g creates turbulence that is dangerous for aircraft. In the upper part of intense thunderstorms, vertical accelerations up to three g were registered.

Thunderstorm movement

The speed and movement of a thundercloud depends on the direction of the earth, primarily by the interaction of the ascending and descending flows of the cloud with the carrier air flows in the middle layers of the atmosphere in which a thunderstorm develops. The speed of movement of an isolated thunderstorm is usually on the order of 20 km/h, but some thunderstorms move much faster. In extreme situations, a thundercloud can move at speeds of 65–80 km/h during the passage of active cold fronts. In most thunderstorms, as old thunderstorm cells dissipate, new thunderstorm cells emerge in succession. With a weak wind, an individual cell can travel a very short distance during its life, less than two kilometers; however, in larger thunderstorms, new cells are triggered by the downdraft flowing out of the mature cell, giving the impression of rapid movement that does not always match the direction of the wind. In large multicell thunderstorms, there is a pattern where a new cell forms to the right of the carrier airflow in the Northern Hemisphere and to the left of the carrier airflow in the Southern Hemisphere.

Energy

The energy that powers a thunderstorm is the latent heat released when water vapor condenses and forms cloud droplets. For every gram of water that condenses in the atmosphere, approximately 600 calories of heat are released. When the water droplets freeze at the top of the cloud, about 80 more calories per gram are released. The released latent thermal energy is partially converted into the kinetic energy of the upward flow. A rough estimate of the total energy of a thunderstorm can be made from the total amount of water that has precipitated from the cloud. Typical is an energy of the order of 100 million kilowatt-hours, which is roughly equivalent to a nuclear charge of 20 kilotons (although this energy is released in a much larger volume of space and over a much longer time). Large multi-celled thunderstorms can have 10 to 100 times more energy.

Downdrafts and squall fronts

Squall powerful thunderstorm front

Downdrafts in thunderstorms occur at altitudes where the air temperature is lower than the temperature in the surrounding space, and this stream becomes even colder when ice particles of precipitation begin to melt in it and cloud drops evaporate. The air in the downdraft is not only denser than the surrounding air, but it also carries a different horizontal angular momentum than the surrounding air. If a downdraft occurs, for example, at a height of 10 km, then it will reach the earth's surface with a horizontal speed that is noticeably greater than the wind speed near the earth. Near the ground, this air is carried forward before a thunderstorm at a speed greater than the speed of the entire cloud. That is why an observer on the ground will feel the approach of a thunderstorm along a stream of cold air even before the thundercloud is overhead. The downdraft propagating along the ground forms a zone with a depth of 500 meters to 2 km with a distinct difference between the cold air of the stream and the warm, moist air from which the thunderstorm is formed. The passage of such a squall front is easily determined by the increase in wind and a sudden drop in temperature. In five minutes, the air temperature can drop by 5°C or more. The squall forms a characteristic squall gate with a horizontal axis, a sharp drop in temperature, and a change in wind direction.

In extreme cases, the squall front created by the downdraft can reach speeds in excess of 50 m/s and cause damage to homes and crops. More often, severe squalls occur when an organized line of thunderstorms develops in high wind conditions at medium altitudes. At the same time, people may think that these destructions are caused by a tornado. If there are no witnesses who saw the characteristic funnel cloud of a tornado, then the cause of the destruction can be determined by the nature of the destruction caused by the wind. In tornadoes, destruction has a circular pattern, and a thunderstorm caused by a downdraft carries destruction mainly in one direction. The cold weather is usually followed by rain. In some cases, raindrops completely evaporate during the fall, resulting in a dry thunderstorm. In the opposite situation, typical for severe multi-cell and super-cell thunderstorms, there is heavy rain with hail, causing flash floods.

Tornadoes

A tornado is a strong small-scale eddy under thunderclouds with an approximately vertical but often curved axis. A pressure difference of 100–200 hPa is observed from the periphery to the center of the tornado. The wind speed in tornadoes can exceed 100 m/s, theoretically it can reach the speed of sound. In Russia, tornadoes occur relatively rarely, but they cause enormous damage. The highest frequency of tornadoes occurs in the south of the European part of Russia.

Livni

In small thunderstorms, the five-minute peak of intense precipitation can exceed 120 mm/hour, but the rest of the rain has an order of magnitude lower intensity. An average thunderstorm produces about 2,000 cubic meters of rain, but a large thunderstorm can produce ten times as much. Large organized thunderstorms associated with mesoscale convective systems can produce 10 to 1000 million cubic meters of precipitation.

Electrical structure of a thundercloud

Structure of charges in thunderclouds in different regions

The distribution and movement of electric charges in and around a thundercloud is a complex, continuously changing process. Nevertheless, it is possible to present a generalized picture of the distribution of electric charges at the cloud maturity stage. A positive dipole structure dominates, in which the positive charge is at the top of the cloud and the negative charge is below it inside the cloud. At the base of the cloud and below it, a lower positive charge is observed. Atmospheric ions, moving under the action of an electric field, form shielding layers at the cloud boundaries, masking the electrical structure of the cloud from an external observer. Measurements show that under various geographical conditions, the main negative charge of a thundercloud is located at altitudes with an ambient temperature of -5 to -17 °C. The greater the speed of the updraft in the cloud, the higher is the center of the negative charge. The space charge density is in the range of 1-10 C/km³. There is a significant proportion of thunderstorms with an inverse charge structure: - a negative charge in the upper part of the cloud and a positive charge in the inner part of the cloud, as well as with a complex structure with four or more zones of space charges of different polarity.

electrization mechanism

Many mechanisms have been proposed to explain the formation of the electrical structure of a thundercloud, and this area of ​​science is still an area of ​​active research. The main hypothesis is based on the fact that if larger and heavier cloud particles are predominantly negatively charged, and lighter small particles carry a positive charge, then the spatial separation of space charges occurs due to the fact that large particles fall at a higher speed than small cloud components. This mechanism is generally consistent with laboratory experiments that show strong charge transfer when particles of ice pellets (grains are porous particles of frozen water droplets) or hail particles interact with ice crystals in the presence of supercooled water droplets. The sign and magnitude of the charge transferred during the contacts depend on the temperature of the surrounding air and the water content of the cloud, but also on the size of the ice crystals, the velocity of the collision, and other factors. It is also possible the action of other mechanisms of electrification. When the magnitude of the volume electric charge accumulated in the cloud becomes large enough, a lightning discharge occurs between the areas charged with the opposite sign. A discharge can also occur between a cloud and the ground, a cloud and a neutral atmosphere, a cloud and the ionosphere. In a typical thunderstorm, two thirds to 100 percent of the discharges are intracloud discharges, intercloud discharges, or cloud-to-air discharges. The rest are cloud-to-ground discharges. In recent years, it has become clear that lightning can be artificially initiated in a cloud, which under normal conditions does not pass into the thunderstorm stage. In clouds that have zones of electrization and create electric fields, lightning can be initiated by mountains, high-rise buildings, aircraft or rockets that are in the zone of strong electric fields.

Zarnitsa - instantaneous flashes of light on the horizon during a distant thunderstorm.

During lightning, thunder peals are not heard due to the distance, but you can see flashes of lightning, the light of which is reflected from cumulonimbus clouds (mainly their tops). The phenomenon is observed in the dark, mainly after July 5, at the time of harvesting grain crops, so the lightning was timed by the people to the end of summer, the beginning of the harvest, and is sometimes called bakers.

snow storm

Scheme of the formation of a snow storm

A snow storm (also a snow storm) is a thunderstorm, a very rare meteorological phenomenon that occurs in the world 5-6 times a year. Instead of a heavy rainfall, heavy snow, freezing rain, or ice pellets fall. The term is used mainly in popular science and foreign literature (eng. thundersnow). In professional Russian meteorology, this term does not exist: in such cases, there is both a thunderstorm and heavy snow.

Cases of winter thunderstorms are noted in ancient Russian chronicles: thunderstorms in winter in 1383 (there was “a very terrible thunder and a whirlwind is strong”), in 1396 (in Moscow on December 25 “... there was thunder, and a cloud from the midday country”), in 1447 year (in Novgorod on November 13 "... at midnight terrible thunder and lightning is great"), in 1491 (in Pskov on January 2 they heard thunder).

Due to the complete unpredictability and huge power lightning(lightning discharges), they pose a potential hazard to numerous power facilities. Modern science has accumulated a large amount of theoretical information and practical data on lightning protection and lightning activity, and this allows solving serious problems related to lightning protection of industrial and civil energy infrastructure. This article discusses the physical nature of thunderstorms and behavior of lightning, the knowledge of which will be useful for arranging effective lightning protection and creating an integrated system for grounding electrical substations.

Nature lightning and storm clouds

In the warm season in the middle latitudes, during the movement of a cyclone, with sufficient humidity and strong ascending air currents, lightning discharges (lightning) often occur. The reason for this natural phenomenon lies in the huge concentration of atmospheric electricity (charged particles) in thunderclouds, in which, in the presence of ascending currents, negative and positive charges are separated with the accumulation of charged particles in different parts of the cloud. Today, there are several theories regarding atmospheric electricity and the electrification of thunderclouds, as the most important factors that have a direct impact on the design and creation of integrated lightning protection and grounding of power facilities.

According to modern concepts, the formation of charged particles in clouds is associated with the presence of an electric field near the Earth, which has a negative charge. Near the planet's surface, the electric field strength is 100 V/m. This value is almost the same everywhere, it does not depend on the time and place of measurements. The electric field of the Earth is due to the presence of free charged particles in the atmospheric air, which are in constant motion.

For example, in 1 cm3 of air there are more than 600 positively charged particles and the same number of negatively charged particles. With distance from the earth's surface in the air, the density of particles with a charge increases sharply. Near the ground, the electrical conductivity of air is negligible, but already at altitudes of more than 80 km, the electrical conductivity increases by a factor of 3,000,000,000 (!) and becomes equal to the conductivity of fresh water. If we draw analogies, then in the first approximation, our planet can be compared with a huge capacitor in the form of a ball.

In this case, the surface of the Earth and the air layer, concentrated at a height of eighty kilometers above the earth's surface, are taken as the plates. The part of the atmosphere 80 km thick, which has a low electrical conductivity, acts as an insulator. A voltage of up to 200 kV arises between the plates of a virtual capacitor, and the current strength can be up to 1,400 A. Such a capacitor has an incredible power - about 300,000 kW (!). In the electric field of the planet, at a height between 1 and 8 kilometers from the earth's surface, charged particles condense and thunderstorms occur, which worsen the electromagnetic environment and are a source of impulse noise in energy systems.

Thunderstorm phenomena are classified into frontal and thermal thunderstorms. On Fig. 1 shows a diagram of the appearance of a thermal thunderstorm. As a result of intense exposure to sunlight, the earth's surface warms up. Part of the thermal energy passes into the atmosphere and heats its lower layers. Warm air masses expand and rise higher. Already at an altitude of two kilometers, they reach an area of ​​low temperatures, where moisture condensation occurs and thunderclouds appear. These clouds are made up of microscopic water droplets that carry a charge. As a rule, thunderclouds form on hot summer days in the afternoon and are relatively small in size.

Frontal thunderstorms are formed under conditions when two air streams with different temperatures collide with their frontal parts. The flow of air with low temperature goes down, closer to the ground, and warm air masses rush up (Fig. 2). Thunderclouds form at altitudes with low temperatures where moist air condenses. Frontal thunderstorms can have a fairly large extent and cover a significant area.

At the same time, the background electromagnetic environment is noticeably distorted, inducing impulse noise in electrical networks. Such fronts move at a speed of 5 to 150 km/h and more. Unlike thermal thunderstorms, frontal thunderstorms are active almost around the clock and pose a serious danger to industrial facilities that are not equipped with a lightning protection system and effective grounding. During condensation in the electric field of cold air, polarized water drops are formed (Fig. 3): there is a positive charge in the lower part of the drops, and a negative charge in the upper part.

Due to the ascending air currents, the separation of water droplets occurs: smaller ones rise up, and large ones fall below. As the drop moves upward, the negatively charged part of the drop attracts positive charges and repels negative ones. As a result, the drop becomes positively charged. gradually collects a positive charge. Drops that fall down attract negative charges and become negatively charged as they fall.

The fission of charged particles in a thundercloud occurs similarly: positively charged particles accumulate in the upper layer, and negatively charged particles accumulate in the lower layer. A thundercloud is practically not a conductor, and for this reason charges are conserved for some time. If a stronger electric field of the cloud will have an effect on the "clear weather" electric field, then it will change its direction at the location (Fig. 4).

The distribution of charged particles in the cloud mass is extremely uneven:
at some points, the density has a maximum value, and at others - a small value. In the place of accumulation of a large number of charges, a strong electric field is formed with a critical strength of the order of 25-30 kV / cm, suitable conditions arise for the formation of lightning. Lightning lightning is like a spark observed in the gap between electrodes that conduct electricity well.

Atmospheric air ionization

Atmospheric air consists of a mixture of gases: nitrogen, oxygen, inert gases and water vapor. The atoms of these gases are combined into strong and stable bonds, forming molecules. Each atom is a nucleus of protons with a positive charge. Electrons with a negative charge ("electron cloud") revolve around the nucleus.

In quantitative terms, the charge of the nucleus and the total charge of the electrons are equal to each other. During ionization, electrons leave the atom (molecule). In the process of atmospheric ionization, 2 charged particles are formed: a positive ion (a nucleus with electrons) and a negative ion (a free electron). Like many physical phenomena, ionization requires a certain amount of energy, called air ionization energy.

When a sufficient voltage arises in the air layer formed by 2 conductive electrodes, then all free charged particles, under the influence of the electric field strength, begin to move in an orderly manner. The mass of an electron is many times (10,000 ... 100,000 times) less than the mass of the nucleus. As a result, when a free electron moves in the electric field of the air layer, the speed of this charged particle is much greater than the speed of the nucleus. Having a significant momentum, the electron easily detaches new electrons from the molecules, thereby making the ionization more intense. This phenomenon is called impact ionization (Fig. 5).

However, not in every collision, an electron is detached from a molecule. In some cases, electrons move to unstable orbits far from the nucleus. Such electrons receive part of the energy from the colliding electron, which leads to the excitation of the molecule (Fig. 6.).

The "life" period of an excited molecule is only 10-10 seconds, after which the electron returns to its former, more energy-stable orbit.

When the electron returns to a stable orbit, the excited molecule emits a photon. The photon, in turn, under certain conditions, can ionize other molecules. This process has been called photoionization (Fig. 7). There are also other sources of photoionization: high-energy cosmic rays, ultraviolet light waves, radioactive radiation, etc. (Fig. 8).

As a rule, ionization of air molecules occurs at high temperatures. As the temperature rises, air molecules and free electrons involved in thermal (chaotic) motion acquire higher energy and more often collide with each other. The result of such collisions is the ionization of air, called thermal ionization. However, reverse processes can also occur, when charged particles neutralize their own charges (recombination). In the process of recombination, intense emission of photons is noted.

Formation of streamers and corona discharge

When the electric field strength increases to critical values ​​in the air gap between the charged plates, impact ionization can develop, which is a frequent cause of high-frequency impulse noise. Its essence is as follows: after ionization by an electron of one molecule, two free electrons and one positive ion appear. Subsequent collisions lead to the appearance of 4 free electrons and 3 ions with a positive charge.

Thus, ionization takes on an avalanche-like character, which is accompanied by the formation of a huge amount of free electrons and positive ions (Fig. 9 and 10). Positive ions accumulate near the negative electrode, and negatively charged electrons move to the positive electrode.

In the process of ionization, free electrons acquire greater mobility than ions, so the latter can be conditionally considered immobile particles. When electrons pass to the positive electrode, the remaining positive charges have a strong influence on the state of the electric field, thereby leading to an increase in its strength. A large number of photons accelerates the ionization of the air near the anode and contributes to the emergence of secondary electrons (Fig. 11), which are sources of repeated avalanches (Fig. 12).

The resulting secondary avalanches move towards the anode, where the positive charge is concentrated. Free electrons break through the positive space charge, leading to the formation of a rather narrow channel (streamer) in which the plasma is located. Due to the excellent conductivity, the streamer "lengthens" the anode, while the process of formation of avalanches of free electrons is accelerated and there is a further increase in the electric field strength (Fig. 13 and 14), moving towards the head of the streamer. Additional electrons mix with positive ions, again leading to the formation of plasma, due to which the streamer channel lengthens.

Rice. 13. An increase in the electric field strength is accompanied by an increase in photoionization and generates new avalanches of charged particles

After filling the free gap with the streamer, the spark stage of the discharge begins (Fig. 15), which is characterized by super-powerful thermal ionization of the space and ultraconductivity of the plasma channel.

The described streamer formation process is valid for small gaps characterized by a uniform electric field. However, according to their shape, all electric fields are divided into homogeneous, slightly inhomogeneous and sharply inhomogeneous:

• Within a uniform electric field, the intensity along the lines of force is characterized by a constant value. As an example, the electric field in the middle part of a flat type capacitor.
• In a weakly inhomogeneous field, the intensity values ​​measured along the lines of force differ by no more than 2 ... 3 times; such a field is considered to be weakly inhomogeneous. For example, an electric field between 2 spherical arresters or an electric field that occurs between the sheath of a shielded cable and its core.
• An electric field is called sharply inhomogeneous if it is characterized by significant jumps in strength, which leads to a serious deterioration in the electromagnetic environment. In industrial electrical installations, as a rule, electric fields have a sharply inhomogeneous shape, which requires checking devices for electromagnetic compatibility.

In a sharply inhomogeneous field, ionization processes are collected near the positive or negative electrode. Therefore, the discharge cannot reach the spark stage, and in this case the charge is formed in the form of a corona ("corona discharge"). With a further increase in the electric field strength, streamers are formed in the air gap and a spark discharge occurs. So, if the gap length is one meter, then a spark discharge occurs at a field strength of about 10 kV/cm.

Leader form of lightning discharge

With the dimensions of the air gap being several meters, the streamers being formed do not have sufficient conductivity for the development of a full-fledged discharge. As the streamer moves, a lightning discharge is formed, which takes on a leader form. The part of the channel, called the leader, is filled with thermally ionized particles. In the leader channel, a significant amount of charged particles is concentrated, the density of which is much higher than the average for the streamer. This property provides good conditions for the formation of a streamer and its transformation into a leader.

Rice. Fig. 16. The process of streamer movement and the emergence of a negative leader (AB is the initial avalanche; CD is the formed streamer).

On Fig. 16 shows a classic scheme for the emergence of a negative leader. The flow of free electrons moves from the cathode to the anode. The hatched cones show the formed electron avalanches, and the trajectories of the emitted photons are shown as wavy lines. In each avalanche, electron collisions ionize the air, and the resulting photons further ionize other air molecules. Ionization takes on a massive character and numerous avalanches merge into one channel. The speed of photons is 3*108 m/s, and the speed of freely moving electrons in the frontal part of the avalanche is 1.5*105 m/s.

The development of a streamer is faster than the progress of an avalanche of electrons. On Fig. 16 shows that during the passage of the first avalanche distance AB, a streamer channel with ultraconductivity along the entire length is formed on segment CD. A standard streamer moves at an average speed of 106-107 m/s. If free electrons have a sufficiently high concentration, intense thermal ionization occurs in the streamer channel, which leads to the appearance of a leader, a linear structure with a plasma component.

During the movement of the leader, new streamers are formed in its end part, which later also pass into the leader. On Fig. Figure 17 shows the development of a negative leader in an air gap with an inhomogeneous electric field: the leader moves along the streamer channel (Fig. 17a); after the transformation of the streamer channel into the leader is completed, new avalanches appear.

Rice. 17. Scheme of formation and development of a negative leader over a long period.

Electron avalanches move throughout the air gap (Fig. 17b) and a new streamer is formed (Fig. 17c). As a rule, streamers move along random trajectories. With such a formation of a lightning discharge in extended air gaps, even at low electric field strengths (from 1,000 to 2,000 V/cm), the leader quickly travels considerable distances.

When the leader reaches the opposite electrode, the leader stage of the lightning discharge ends and the stage of the reverse (main) discharge begins. In this case, an electromagnetic wave propagates from the earth's surface through the leader channel, due to which the leader's potential decreases to zero. Thus, a superconducting channel is formed between the electrodes, through which a lightning discharge passes.

Stages of development of a lightning discharge

The conditions for the occurrence of lightning are formed in that part of the thundercloud, where the accumulation of charged particles and the electric field strength have reached threshold values. At this point, impact ionization develops and electron avalanches are formed, then, under the influence of photo- and thermal ionization, streamers appear, which turn into leaders.

a - visual display; b - current characteristic.

The length of lightning is from hundreds of meters and can reach up to several kilometers (the average length of a lightning discharge is 5 km). Thanks to the leader type of development, lightning is able to travel considerable distances within a fraction of a second. The human eye sees lightning as a continuous line of one or more bright bands of white, light pink, or bright blue. In fact, a lightning discharge is several impulses that include two stages: a leader and a reverse discharge stage.

On Fig. 18 shows the time sweep of lightning impulses, which shows the discharge of the leader stage of the first impulse developing in the form of steps. On average, the step line is fifty meters, and the delay between adjacent steps reaches 30-90 µs. The average propagation speed of the leader is 105...106 m/s.

The stepwise form of leader development is explained by the fact that some time is required for the formation of a leading streamer (a pause between steps). Subsequent pulses move along the ionized channel and have a pronounced arrow-shaped leader stage. After the leader reaches the 1st pulse of the earth's surface, an ionized channel appears, along which the charge moves. At this moment, the 2nd stage of the lightning discharge (reverse discharge) begins.

The main discharge is visible in the form of a continuous bright line piercing the space between thunderclouds and the earth (linear lightning). After the main discharge reaches the cloud, the glow of the plasma channel decreases. This phase is called afterglow. In one lightning discharge, up to twenty repeated impulses are noted, and the duration of the discharge itself reaches 1 or more seconds.

In four out of ten cases, there is a multiple lightning discharge, which is the cause of impulse noise in power networks. On average, 3 ... 4 impulses are noted. The nature of repeated pulses is related to the gradual influx of the remaining charges in the thundercloud to the plasma channel.

Selective action of a lightning discharge

When the leader channel is just beginning to develop, the electric field strength in its head is determined by the volume of the leader's charge and the accumulations of bulk charged particles under the thundercloud. The priority direction of the discharge depends on the maximum electric field strengths. At a considerable height, this direction is determined only by the leader's channel (Fig. 19).

When the leader channel of a lightning discharge moves towards the earth's surface, its electric field is distorted by the field of the earth and massive ground-based power facilities. The maximum intensity values ​​and the direction of propagation of the lightning leader are determined by both its own charges and charges concentrated on the ground, as well as on artificial structures (Fig. 20).

The height H of the leader's head above the earth's surface, at which a significant effect on the electric field of the leader of charge fields accumulated in a significant amount on the ground and at power facilities, which can change the direction of the leader's movement, is called the lightning discharge orientation height.
The more electric charges are in the leader channel, the higher the change in the trajectory of the lightning movement can occur.

Figure 21 shows the movement of the main discharge from the earth's surface to the thundercloud and the propagation of the leader towards the earth (flat surface).

When a lightning discharge moves towards a high-rise ground structure (power transmission tower or tower) towards the leader discharge propagating from a thundercloud to the earth's surface, a counter leader develops from the ground support (Fig. 22.). In this case, the main discharge occurs at the point of connection of the leaders and moves in both directions.

Rice. 22. Development of the leader stage (top) and the main discharge stage (bottom) when a lightning discharge strikes a metal support

The process of lightning formation shows that the specific location of the lightning strike is determined at the leader stage. If there is a high-rise ground structure directly under the thundercloud (for example, a television tower or a power line pylon), then the emerging leader will move towards the ground along the shortest path, that is, towards the leader, which extends upward from the ground structure.

Based on practical experience, it can be concluded that most often lightning strikes those power facilities that have efficient grounding and conduct electricity well. With equal height, lightning strikes the object that has better grounding and high electrical conductivity. At different heights of power facilities and if the ground next to them also has a different resistivity, lightning may strike a lower facility located on the ground with better conductivity (Fig. 23).

Rice. 23. Selective susceptibility of lightning discharges: soil with high electrical conductivity (a); soil with reduced conductivity (b).

This fact can be explained by the fact that during the development of the leader stage, conduction currents flow along a path with increased conductivity, therefore, in some areas, there is a concentration of charges related to the leader. As a result, the influence of the electric field of charges on the earth's surface on the electric field of the emerging leader increases. This explains the selectivity of lightning. As a rule, soil areas and ground-based artificial structures with high conductivity are most often affected. In practice, it has been established that on high-voltage power lines, lightning strikes no more than a third of the supports located in strictly defined places.

The theory of selective damage by lightning discharges of terrestrial objects has found practical confirmation in the arrangement of lightning protection and grounding of power facilities of electrical substations. Those areas that are characterized by low conductivity were much less likely to be struck by lightning. On fig. 24 shows the electric field between the ground and a thundercloud before a lightning strike.

With a gradual change in the intensity of the electric field of a thundercloud, the conductivity of the soil provides a balance in the number of charges when the electric field of the cloud changes. During a lightning discharge, the field strength changes so rapidly that, due to the low conductivity of the soil, there is no time to redistribute the charges. The concentration of charges in separate places leads to an increase in the electric field strength between the characteristic places and the thundercloud (Fig. 25), so the lightning discharge selectively strikes these places.

This clearly confirms the theory of lightning discharge selectivity, according to which, under similar conditions, lightning always falls into those places where there is an increased electrical conductivity of the soil.

The main parameters of lightning

The following parameters are used to characterize lightning currents:

• The maximum value of the lightning current impulse.
• The degree of steepness of the lightning current front.
• The duration of the front of the current pulse.
• Full pulse duration.

The duration of the lightning current pulse is the time required for the reverse discharge to pass the distance between the earth and the thundercloud (20...100 µs). The front of the lightning current pulse in this case is in the range from 1.5 to 10 µs.

The average duration of the lightning discharge current pulse has a value equal to 50 μs. This value is the standard value for the lightning current impulse when testing the dielectric strength of shielded cables: they must withstand direct lightning strikes and maintain the integrity of the insulation. To test the insulation strength when exposed to lightning voltage impulses (tests are regulated by GOST 1516.2-76), a standard impulse of lightning voltage currents is adopted, shown in Fig. 26 (for the convenience of calculations, the actual front is reduced to an equivalent oblique front).

On the vertical axis of the surge overvoltage sweep at a level equal to 0.3 Umax and 0.9 Umax, control points are marked, connected by a straight line. The intersection of this straight line with the time axis and with the horizontal straight line tangent to Umax makes it possible to determine the pulse duration Tf. The standard lightning impulse has a value of 1.2/50: where Tf=1.2 µs, Ti=50 µs (total pulse duration).

Another important characteristic of a lightning impulse is the rate of rise of the voltage current at the pulse front (front slope, A * μs). Table 1 shows the main parameters of lightning discharges for flat terrain. In the mountains, there is a decrease in the amplitude of oscillations of lightning currents (almost two times) in comparison with the values ​​for the plains. This is explained by the fact that the mountains are closer to the clouds, therefore, in mountainous areas, lightning occurs at a much lower density of charged particles in thunderclouds, which leads to a decrease in the amplitude values ​​of lightning currents.

According to the table, when lightning strikes high-voltage power transmission towers, huge currents are generated - more than 200 kA. However, such lightning discharges that cause significant currents are extremely rare: currents over 100 kA occur in no more than 2% of the total number of lightning discharges, and currents over 150 kA occur in less than 0.5% of cases. The probabilistic distribution of the amplitude values ​​of the lightning currents depending on the amplitude values ​​of the currents is shown in Fig. 27. About 40% of all lightning discharges have currents that do not exceed 20 kA.

Rice. 28. Curves of probability distribution (in %) of the steepness of the front of the lightning current pulse. Curve 1 - for flat areas; curve 2 is for mountain conditions.

The level of impulse noise and overvoltages that appear at power facilities depends on the actual steepness of the front of the pulsed current of a lightning discharge. The degree of steepness varies over a wide range and has a weak correlation with the amplitude values ​​of lightning currents. On fig. 28 shows a picture of the probability distribution of the level of steepness of the frontal impulse of the lightning current on the plain (curve 1) and in the mountains (curve 2).

Impact of lightning currents

During the passage of lightning currents through various objects, the latter are subjected to mechanical, electromagnetic and thermal influences.
Significant heat generation can destroy metal conductors of small cross sections (for example, fuse links or telegraph wires). To determine the critical value of the lightning current Im (kA), at which the conductor melts or even evaporates, the following formula is used

k - specific coefficient depending on the conductor material (copper 300...330, aluminum 200...230, steel 115...440).
Q is the cross section of the conductor, mm2;
tm is the duration of the lightning current pulse, µs.

The smallest section of the conductor (lightning rod), which guarantees its safety during a lightning discharge into a power facility, is 28 mm2. At maximum current values, a steel conductor of the same cross section heats up to hundreds of degrees in a matter of microseconds, but retains its integrity. When exposed to a lightning channel on metal parts, they can melt to a depth of 3-4 mm. Breaks of individual wires at lightning protection cables on power lines often occur due to overburning by a lightning discharge at the points of contact between the lightning channel and the cable.

For this reason, steel lightning rods have significant sections: lightning protection cables must be at least 35 mm2 in cross section, and rod lightning rods must be at least 100 mm2. Explosions and fires can occur when a lightning channel impacts combustible and flammable materials (wood, straw, fuels and lubricants, gaseous fuels, etc.). The mechanical effect of the current of lightning discharges is manifested in the destruction of wooden, brick and stone structures, in which there is no lightning protection and full-fledged grounding.

The splitting of wooden power transmission poles is explained by the fact that the lightning current, moving through the internal structure of the wood, generates an abundant release of water vapor, which breaks the wood fibers with its pressure. In rainy weather, wood splitting is less than in dry weather. Since wet wood is characterized by better conductivity, therefore, the lightning current passes mainly along the surface of the wood, without causing significant damage to wooden structures.

During a lightning discharge, pieces of wood up to three centimeters thick and up to five centimeters wide often break out of wooden poles, and in some cases lightning splits racks and traverses of poles that are not equipped with grounding in half. In this case, the metal elements of the insulators (bolts and hooks) fly out of their places and fall to the ground. Once a lightning strike was so strong that a huge poplar about 30 m high turned into a pile of small chips.

Passing through narrow cracks and small openings, lightning discharges produce significant damage. For example, lightning currents easily deform tubular arresters installed on power lines. Even classical dielectrics (stone and brick) are subjected to destructive effects of powerful discharges. The electrostatic forces of the impact nature that the remaining charges have easily destroy thick-walled brick and stone buildings.

During the stage of the main lightning discharge near the place of its strike in the conductors and metal structures of energy facilities, impulse pickups and overvoltages occur, which, passing through the grounding of energy facilities, create high-frequency impulse noise and a significant voltage drop, reaching 1,000 or more kV. Lightning discharges can occur not only between thunderclouds and the ground, but also between individual clouds. Such lightning is completely safe for personnel and equipment of power facilities. At the same time, lightning discharges reaching the ground pose a serious danger to people and technical devices.

Thunderstorm activity on the territory of the Russian Federation

In different parts of our country, the intensity of thunderstorm activity has significant differences. In the northern regions, the weakest thunderstorm activity is observed. When moving south, there is an increase in thunderstorm activity, which is characterized by the number of days in a year when there were thunderstorms. The average duration of thunderstorms for one thunderstorm day on the territory of the Russian Federation is from 1.5 to 2 hours. Thunderstorm activity for any point of the Russian Federation is established according to special meteorological maps of thunderstorm activity, which are compiled on the basis of data from long-term observations of meteorological stations (Fig. 29).

Interesting facts about lightning:

• In areas where thunderstorm activity is 30 hours per year, on average, there is 1 lightning strike per square kilometer of the earth's surface in two years.
• Every second, the surface of our planet experiences over a hundred lightning strikes.

Thunderstorm - what is it? Where do the lightnings that cut through the whole sky and the menacing peals of thunder come from? Thunderstorm is a natural phenomenon. Lightning, called lightning, can form inside clouds (cumulonimbus), or between and clouds. They are usually accompanied by thunder. Lightning is associated with heavy rains, heavy winds, and often with hail.

Activity

A thunderstorm is one of the most dangerous. People struck by lightning survive only in isolated cases.

At the same time, approximately 1,500 thunderstorms operate on the planet. The intensity of the discharges is estimated at a hundred lightning per second.

The distribution of thunderstorms on Earth is uneven. For example, there are 10 times more of them over the continents than over the ocean. Most (78%) of lightning discharges are concentrated in the equatorial and tropical zones. Thunderstorms are especially frequent in Central Africa. But the polar regions (Antarctica, the Arctic) and lightning poles are practically invisible. The intensity of a thunderstorm, it turns out, is associated with a heavenly body. In middle latitudes, its peak occurs in the afternoon (daytime) hours, in the summer. But the minimum was registered before sunrise. Geographic features are also important. The most powerful thunderstorm centers are in the Cordillera and the Himalayas (mountainous regions). The annual number of "stormy days" is also different in Russia. In Murmansk, for example, there are only four, in Arkhangelsk - fifteen, Kaliningrad - eighteen, St. Petersburg - 16, in Moscow - 24, Bryansk - 28, Voronezh - 26, Rostov - 31, Sochi - 50, Samara - 25, Kazan and Yekaterinburg - 28, Ufa - 31, Novosibirsk - 20, Barnaul - 32, Chita - 27, Irkutsk and Yakutsk - 12, Blagoveshchensk - 28, Vladivostok - 13, Khabarovsk - 25, Yuzhno-Sakhalinsk - 7, Petropavlovsk-Kamchatsky - 1.

Thunderstorm development

How does it go? formed only under certain conditions. The presence of ascending moisture flows is obligatory, while there must be a structure where one fraction of the particles is in an icy state, the other in a liquid state. Convection, which will lead to the development of a thunderstorm, will occur in several cases.

Uneven heating of surface layers. For example, over water with a significant temperature difference. Over large cities, thunderstorm intensity will be somewhat stronger than in the surrounding area.

When cold air displaces warm air. The frontal convention often develops simultaneously with oblique and nimbostratus clouds (clouds).

When air rises in mountain ranges. Even small elevations can lead to increased cloud formations. This is forced convection.

Any thundercloud, regardless of its type, necessarily goes through three stages: cumulus, maturity, and decay.

Classification

Thunderstorms were classified for some time only at the place of observation. They were divided, for example, into spelling, local, frontal. Thunderstorms are now classified according to characteristics that depend on the meteorological environment in which they develop. formed due to the instability of the atmosphere. For the creation of thunderclouds, this is the main condition. The characteristics of such flows are very important. Depending on their power and size, various types of thunderclouds are formed, respectively. How are they divided?

1. Cumulonimbus single-cell, (local or intramass). Have hail or thunderstorm activity. Transverse dimensions from 5 to 20 km, vertical - from 8 to 12 km. Such a cloud "lives" up to an hour. After a thunderstorm, the weather practically does not change.

2. Multicell cluster. Here the scale is more impressive - up to 1000 km. A multi-cell cluster covers a group of thunderstorm cells that are at different stages of formation and development and at the same time form a single whole. How are they arranged? Mature thunderstorm cells are located in the center, while decaying ones can be up to 40 km across. Cluster multi-cell thunderstorms “give” gusts of wind (heavy, but not strong), downpour, hail. The existence of one mature cell is limited to half an hour, but the cluster itself can “live” for several hours.

3. Lines of squalls. These are also multicell thunderstorms. They are also called linear. They can be either solid or with gaps. Wind gusts are longer here (on the leading front). The multicell line appears as a dark wall of clouds when approached. The number of streams (both upstream and downstream) is quite large here. That is why such a complex of thunderstorms is classified as multi-cell, although the thunderstorm structure is different. The squall line is capable of producing intense downpour and large hail, but is more often “limited” by strong downdrafts. It often passes ahead of a cold front. In the pictures, such a system has the shape of a curved bow.

4. Supercell thunderstorms. Such thunderstorms are rare. They are especially dangerous for property and human life. The cloud of this system is similar to the single-cell cloud, since both differ in one upstream zone. But they have different sizes. Supercell cloud - huge - close to 50 km in radius, height - up to 15 km. Its boundaries may lie in the stratosphere. The shape resembles a single semicircular anvil. The speed of ascending streams is much higher (up to 60 m/s). A characteristic feature is the presence of rotation. It is this that creates dangerous, extreme phenomena (large hail (more than 5 cm), destructive tornadoes). The main factor for the formation of such a cloud is the environmental conditions. We are talking about a very strong convention with a temperature of +27 and a wind with a variable direction. Such conditions arise during wind shear in the troposphere. Formed in the updrafts, precipitation is transferred to the downdraft zone, which ensures a long life for the cloud. Precipitation is unevenly distributed. Showers are near the updraft, and hail is closer to the northeast. The rear of the thunderstorm may shift. Then the most dangerous zone will be near the main updraft.

There is also the concept of "dry thunderstorm". This phenomenon is quite rare, characteristic of the monsoons. With such a thunderstorm, there is no precipitation (they simply do not reach, evaporating as a result of exposure to high temperature).

Movement speed

In an isolated thunderstorm, it is about 20 km / h, sometimes faster. If cold fronts are active, the speed can be 80 km/h. In many thunderstorms, old thunderstorm cells are replaced by new ones. Each of them covers a relatively short distance (about two kilometers), but in the aggregate the distance increases.

electrization mechanism

Where do lightning come from? around the clouds and within them are constantly moving. This process is rather complicated. It is easiest to imagine how electric charges work in mature clouds. The dipole positive structure dominates in them. How is it distributed? The positive charge is placed at the top, and the negative charge is placed below it, inside the cloud. According to the main hypothesis (this area of ​​science can still be considered little explored), heavier and larger particles are negatively charged, while small and light ones have a positive charge. The former fall faster than the latter. This becomes the reason for the spatial separation of space charges. This mechanism is confirmed by laboratory experiments. Particles of ice pellets or hail can have a strong charge transfer. The magnitude and sign will depend on the water content of the cloud, the air (ambient) temperature, and the collision velocity (the main factors). The influence of other mechanisms cannot be excluded. Discharges occur between the earth and the cloud (or the neutral atmosphere or the ionosphere). It is at this moment that we observe flashes dissecting the sky. Or lightning. This process is accompanied by loud peals (thunder).

Thunderstorm is a complex process. It can take many decades, and perhaps even centuries, to study it.