lightning

a giant electric spark discharge in the atmosphere; it is usually manifested in a bright flash of light and an accompanying clap of thunder. The electric nature of lightning was discovered by the American physicist B. Franklin, who initiated experiments to draw electricity from a thundercloud.

Lightning occurs most frequently in cumulonimbus clouds, in which case they are called thunderclouds. It sometimes originates in nimbostratus clouds, and also during volcanic eruptions, tornadoes, and dust storms.

The most common lightning discharges are linear; they are among the electrodeless discharges, since they originate in accumulations of charged particles. This fact determines several properties, not yet fully understood, that distinguish lightning from interelectrode discharges. Lightning discharges are never shorter than several hundred meters, they originate in electric fields that are much weaker than the fields of interelectrode discharges, the collection of charges carried by lightning takes place within a few thousandths of a second, and the charges are collected from tens of thousands of small particles that are well insulated from one another and are contained within a volume of several cubic kilometers.

The most thoroughly studied process of lightning evolution is that of discharges in thunderclouds; such discharges can occur from cloud to cloud or from cloud to earth. For lightning to strike, an electric field whose intensity is sufficient to initiate an electric discharge (of the order of 1 megavolt per meter [MV/m]) must be generated in a volume that is relatively small but is above a certain critical size; at the same time, there must be an electric field in a substantial portion of the cloud whose intensity is sufficienty to sustain the discharge after its initiation (of the order of 0.1–0.2 MV/m). Within a lightning discharge the electrical energy of the cloud is converted to thermal energy.

The evolution of cloud-to-earth lightning discharge consists of several stages. During the first stage, collision ionization begins in the zone where the electric field attains a critical value. The ionization is initiated by the free electrons that are always present in small quantities in the air. Under the action of an electric field, the electrons acquire significant velocity, directed toward the earth and, upon collision with atoms of air, ionize them. Thus, electron avalanches are generated that are transformed into filaments of electric discharges, called streamers, which are high-conductivity channels and which flow together and form a bright, thermally ionized high-conductivity channel, called the stepped leader stroke. The advance of the stepped leader toward the surface of the earth proceeds in stages of several dozen meters at speeds of the order of 5 × 10⁷ m/sec. After such a step the motion of the leader is interrupted for several dozen microseconds, and its luminescence weakens markedly. During the next stage the leader again advances several dozen meters. As this happens, a bright glow envelops all steps already passed, and the interruption of motion and decrease in luminescence recur. These processes are repeated as the leader advances toward the earth at an average speed of 2 × 10⁵ m/sec. During this advance the intensity of the field at the end of the leader increases. As a result of the action of the field, objects protruding from the surface of the earth emit a response streamer that joins the leader. This peculiarity of lightning is used in lightning arresters In the final stage the reverse, or main, stroke progresses along the channel ionized by the leader. This discharge is characterized by currents of tens of thousands to hundreds of thousands of amperes and is considerably brighter than the leader. It advances at high velocities, at first up to about 10⁸ m/sec and later diminishing to about 10⁷ m/sec. The temperature of the channel during the main discharge can exceed 25,000°C. The lightning channel is 1–10 km long and several centimeters in diameter. After the passage of the current pulse, the ionization and luminescence of the channel grow weaker. In the final stage the lightning current may have a duration of hundredths or even tenths of a second, and its magnitude may run to hundreds or thousands of amperes. Such lightning strokes are called protracted strokes; they often cause fires.

The main stroke often discharges only part of a cloud. Charges located at higher altitudes can give rise to a new leader (a dart leader), which moves continuously at an average speed of about 10⁶ m/sec. Its brightness is close to that of a stepped leader. When such a dart leader reaches the surface of the earth, a second main stroke follows that is similar to the first. A lightning stroke usually includes several discharges, but there may be as many as several dozen. The duration of a multiple lightning stroke may exceed 1 sec. Displacement of the channel of multiple lightning by wind creates ribbon lightning (a luminescent band).

Cloud-to-cloud lightning discharges usually include only the leader stages; their length is from about 1 km to 150 km. The proportion of cloud-to-cloud lightning strokes increases with decreasing distance from the equator (from 0.5 in the temperate latitudes to 0.9 in equatorial regions).

The passage of lightning is accompanied by changes in electric and magnetic fields and by radio radiation (called atmospherics). The probability of a terrestrial object being hit by a lightning stroke increases with the height of the object and the electric conductivity of the soil (on the surface or at some depth); the action of a lightning rod is based on these factors. If an electric field sufficiently strong to support a discharge but not strong enough to initiate it exists in a cloud, the discharge can be initiated by a long metal cable or by an airplane, particularly if such an object carries a strong electric charge. Thus, lightning is sometimes “provoked” in nimbostratus clouds and large cumulus clouds.

Ball lightning is a special form of lightning; it is a luminescent spheroid with high specific energy. It often originates after a stroke of linear lightning. Ball lightning exists for a time ranging from seconds to minutes; its disappearance can be accompanied by a destructive explosion. The nature of ball lightning is not yet understood. Lightning, whether linear or ball-shaped, can cause severe injury and death of humans.

Lightning strokes can cause destruction as a result of their thermal and electrodynamic effects, and also because of certain dangerous consequences of electromagnetic and luminous radiation. Lightning strokes are most damaging to terrestrial objects that do not have a high-conductivity path between the point of strike and the ground. Narrow channels are formed in the material as a result of electric breakdown, and the lightning current rushes into them. Since a very high temperature is created in the channels, a part of the material is intensly evaporated, with an explosion. This leads to rupture or splitting of the object and ignition of its combustible components. Large potential differences (and, hence, electrical discharges) may also arise among individual objects in a structure. Such discharges can also cause fires and injuries from electric current. Structures that sustain direct lightning strokes are frequently taller than surrounding buildings—for example, nonmetallic smokestacks, towers, and firehouses, and also isolated structures in open country. Very tall objects (television towers and captive balloons) sometimes sustain lightning strokes at points located far below their highest point. This phenomenon is associated with the effect on the lightning’s path of space charges generated by the object.

Direct lightning strokes on overhead communications lines with wooden poles are particularly dangerous. When the atmospheric voltage surge of large amplitude enters the line, it propagates along the conductors and can cause electric discharges from conductors and electrical apparatus (loudspeakers, telephone equipment, switches, and so on) to ground and other objects, leading to damage, fires, and injuries to humans. Direct lightning strokes to high-voltage power lines cause electric discharges from conductor to ground or from conductor to conductor. Such discharges often become electric arcs because of the effects of the operating voltage and may cause short circuits and shutdowns of transmission lines. If an atmospheric surge passes from a line into power-plant and substation equipment, it can cause breakdown of insulation and damage to apparatus and machines. A direct lightning stroke on an airplane can cause damage to its structural components, disrupt operation of its radio and navigational equipment, and blind or directly strike the crew. If a direct lightning stroke hits a tree, the discharge can injure persons nearby; there is also a danger from the voltage arising near the tree as the current is drained from the tree to the ground.