Atmospheric Electricity


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atmospheric electricity

[¦at·mə¦sfir·ik i‚lek′tris·əd·ē]
(geophysics)
The electrical processes occurring in the lower atmosphere, including both the intense local electrification accompanying storms and the much weaker fair-weather electrical activity over the entire globe produced by the electrified storms continuously in progress.

Atmospheric Electricity

 

(1) The totality of electrical phenomena and processes in the atmosphere.

(2) The branch of atmospheric physics dealing with the electrical phenomena and electrical properties of the atmosphere. The many studies of atmospheric electricity include studies of the electric field, ionization, conductivity, and electrical currents in the atmosphere; space charge; cloud and precipitation charge; and thunderstorm charge. All the phenomena of atmospheric electricity are closely interrelated and their development is strongly influenced by meteorological factors such as clouds, precipitation, and snowstorms. Processes occurring in the troposphere and stratosph’ere are also usually included in the realm of atmospheric electricity.

The study of atmospheric electricity as a science was begun in the 18th century by the American scientist B. Franklin, who experimentally established the electrical nature of lightning, and by the Russian scientist M. V. Lomonosov, who wrote the first hypothesis explaining the electrification of thunderclouds. In the 20th century the conductive layers of the atmosphere, lying more than 60 to 100 km above the earth (the ionosphere and the earth’s magnetosphere), were discovered. The electrical nature of the aurora borealis was established, and a number of other phenomena were discovered, which led to the development of a number of sciences distinct from the study of atmospheric electricity. The development of space exploration made it possible to begin the study of electrical phenomena in the higher layers of the atmosphere by direct methods. Two basic contemporary theories of atmospheric electricity were created by the English scientist C. Wilson and the Soviet scientist Ia. I. Frenkel’. According to Wilson’s theory, the earth and ionosphere act as capacitor plates charged by thunderclouds. The difference in potentials arising between the plates leads to the creation of an electrical field in the atmosphere. According to Frenkel’, the electric field of the atmosphere is completely explained by electrical phenomena occurring in the troposphere, such as the polarization of clouds and their interaction with the earth, while the ionosphere plays no significant role in atmospheric electrical processes.

The atmospheric electricity in a given region depends on global and local factors. Regions in which there is no concentration of aerosols or source of strong ionization, and which are regarded as zones of “good” or “stable” weather, are regions in which global factors predominate. In zones of “unstable” weather (thunderstorms, dust storms, precipitation, and other atmospheric disturbances), local factors predominate.

Electric field of the atmosphere. In the troposphere, all clouds and precipitation, fog, and dust are usually electrically charged; even in a clear atmosphere, the electric field exists constantly. Studies in good-weather zones, begun in the 19th century, have shown that a standard stationary electric field with a charge E equal on the average to about 130 V/m exists near the surface of the earth. The earth has a negative charge equal to about 3 • 105 coulombs (C), but the atmosphere as a whole is charged positively. However, during precipitation and especially thunderstorms, snowstorms, dust storms, and the like, the intensity of the field can change sharply in direction and magnitude, sometimes reaching 1,000 V/m. E has the greatest value in the middle latitudes and decreases toward the poles and the equator. In zones of good weather, £ as a whole decreases with elevation—for example, over the oceans. Near the earth’s surface, in the so-called layer of turbulence—which is from 300 to 3,000 m thick and where aerosols accumulate— E can increase with altitude. Above the layer of turbulence, E decreases with altitude according to the exponential law and does not exceed a few V/m at an altitude of 10 km. This decrease in E comes about because the atmosphere contains positive space charges whose density likewise declines rapidly with increasing altitude. The difference in potential between the earth and the ionosphere is between 200 and 250 kV.

The charge E of the electric field changes with time. In addition to local diurnal and annual variations in E, diurnal and annual variations of E that are synchronous for all points—the so-called unitary variations—are also observed. Unitary variations are connected with a change in the electrical charge of the earth as a whole; local variations are connected with changes in the magnitude and altitude distribution of space electrical charges in the atmosphere of a given region.

Electric conductivity of the atmosphere. The electric state of the atmosphere is determined to a considerable degree by its electric conductivity λ created by ions in the atmosphere. The presence of ions in the atmosphere is what causes the loss of a charge in an isolated charged body upon contact with the air (a phenomenon discovered at the end of the 18th century by the French physicist C. Coulomb). The electric conductivity λ depends upon the number of ions contained in a unit of volume (their concentration) and the mobility of the ions. Light ions, which possess the greatest mobility (u > 10-5 m2 · sec-1 · V-1), make the most important contribution to λ.

The electric conductivity of the atmosphere is very small and can be compared to the conductivity of good insulators. Near the earth’s surface the average value of λ = (1–2) • 10-18-1 • m-1 and increases approximately exponentially with increasing altitude; at an altitude of about 30 km, λ reaches values almost 150 times greater than those at the earth’s surface. Higher up, conductivity increases still more, especially sharply at altitudes where the ionizing radiation of the sun penetrates and the ionosphere begins to form. The conductivity of the ionosphere is approximately 1012 times as great as that of the atmosphere near the earth’s surface.

The most important ionizers of the atmosphere are (1) cosmic rays acting throughout the atmosphere, (2) the radiation of radioactive matter present in the earth and in the air, and (3) ultraviolet and corpuscular radiation from the sun, whose ionizing effect becomes significant at altitudes above 50–60 km. The concentration of light ions increases with an increase in the intensity of ionization and a decrease in the concentration of particles in the atmosphere. Therefore, the concentration of light ions increases with altitude. This fact, combined with the increase in mobility of ions as the density of air decreases, explains the nature of the variations in λ and E that accompany variations in altitude.

Electric current in the atmosphere. The movement of ions under the influence of forces of the electric field creates in the atmosphere a vertical conduction current in = E λ, with an average density equal to about (2 - 3) • 10~12 A/m2. Thus, in good-weather zones the force of the current for the entire surface of the earth is about 1,800 A. The time during which the charge of the earth from the conduction currents of the atmosphere would decrease to l/e ≈ 0.37 from its original value is equal to ~500 sec. Since the earth’s charge does not show an average change, it is obvious that there are no “generators” of atmospheric electricity charging the earth. In addition to currents of conductivity, considerable electrical diffusion and convective currents flow through the atmosphere.

“Generators” of atmospheric electricity. Generators of atmospheric electricity in unstable-weather zones include dust storms and volcanic eruptions, snowstorms and the spraying of water from geysers and waterfalls, clouds and precipitation, and steam and smoke from industrial sources. In almost all the phenomena enumerated, electrification can be manifested quite violently: volcanic eruptions, dust storms, and even snowstorms can lead to the formation of lightning. However, clouds and precipitation still contribute the most to electrification of the atmosphere.

Electrification arises from a cloud as the cloud’s particles increase in size, the cloud thickens, and its precipitation intensifies. Thus, in stratus and stratocumulus clouds the density of space charges is p ≈ 3 • 10-12 C/km3, which is approximately ten times as great as their density in a clear atmosphere. In thunderclouds the value of p reaches 3 • 10-8 C/m3. Clouds may be charged positively in the upper part and negatively in the lower part, or vice versa, and may also have a predominant charge of one sign. The current density of precipitation falling to the earth from nim-bostratus clouds is ioc = 10-12 A/m2 and from storm clouds, ioc = 10-9 A/m2. The full force of a current flowing to the earth from one thundercloud is equal to about -(0.01 - 0.1) A in the temperate zones and up to -(0.5 - 1.0) A closer to the equator. The force of the currents flowing within these clouds is from ten to 100 times as great as the force of the currents reaching the earth. Thus, from an electrical standpoint a rainstorm is similar to a short-circuit generator.

Given high values of the electric field near the earth’s surface, on the order of 500–1,000 V/m, an electric discharge begins from upright pointed objects (grass, trees, masts, smokestacks, and the like), which sometimes becomes visible. This is the so-called St. Elmo’s fire, especially bright in the mountains and at sea. The corona currents that arise during snowstorms, heavy rains, and especially thunderstorms aid the exchange of charges between the earth and its atmosphere.

Thus, the electric field of the earth and the flow of current between the earth and its atmosphere in good-weather zones are maintained by processes in the zones of unstable weather. About 1,800 thunderstorms exist simultaneously on the earth; the total force of the current from them, which gives the earth a negative charge, reaches 1,000 A . Stratus clouds, less active than thunderclouds but covering about one-half of the earth’s surface, also contribute significantly to maintaining the earth’s electric field. The investigation of atmospheric electricity enables us to explain the nature of the processes leading to the enormous electrification of thunderclouds in order to predict and control them; to explain the role of electrical forces in the formation of clouds and precipitation; to decrease the electrification of planes and thereby increase the safety of flights; and to discover the secret of the formation of ball lightning.

REFERENCES

Frenkel’, La. I. Teoriia iavlenii atmosfernogo elektrichestva. Moscow-Leningrad, 1949.
Tverskoi, P. N. Atmosfernoe elektrichestvo. Leningrad, 1949.
Imianitov, 1. M. Pribory i metody dlia izucheniia elektrichestva atmosfery. Moscow, 1957.
Imianitov, I. M., and K. S. Shifrin. “Sovremennoe sostoianie issledovanii atmosfernogo elektrichestva.” Uspekhi fizicheskikh nauk, 1962, vol. 76, no. 4. p. 593.
Imianitov, 1. M., and E. V. Chubarina. Elektrichestvo svobodnoi atmosfery. Leningrad, 1965.

I. M. IMIANITOV

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