Atmospheric Electricity

ATMOSPHERIC ELECTRICITY, a concept that encompasses all electrical phenomena of natural origin observed in the Earth's atmosphere. 1. Electric field in the atmosphere. Numerous studies show that the Earth has a negative charge, while the adjacent layers of the atmosphere have a positive charge. The electric field in the atmosphere is directed vertically downward, so that the potential of the atmosphere increases with height above the Earth's surface. The ratio of the potential difference between two points in the atmosphere to the difference in their heights is called the potential gradient and serves to characterize the voltage (strength) of the electric field. Near the Earth's surface, the potential value averages 100 volts/meter. Points having equal potential are located on the so-called equipotential surface. These surfaces are parallel to the Earth's surface; but any protrusion above the Earth's surface (tower, tree) distorts the course of equipotential surfaces, which become denser near the top of the protrusion (due to this the field becomes stronger there) and diverge at its base. The potential of any point in the atmosphere does not remain constant but continuously changes. In clear, calm weather, these changes are regular; the potential gradient has a minimum around 4 AM and a maximum around noon. In summer, a maximum is also observed around 11 AM and a minimum before noon; however, they are expressed less sharply than the previous ones. In the annual course of the potential gradient, regularity is also observed: it has the highest values in winter and the lowest in early summer. Changes in temperature, pressure, and humidity of the air cause fluctuations in the gradient; however, the laws of these fluctuations have not yet been established. Under the influence of wind, the gradient undergoes very strong fluctuations, caused by differently charged layers of air passing through the point in space being studied. Dust raised from the ground and charged negatively (as well as smoke) causes huge fluctuations in the gradient (up to 100% per minute). Fog and atmospheric precipitation also have a great influence, under which the gradient takes even negative values. If there were no positive charges in the atmosphere, the gradient created by the negatively charged Earth would be constant. Experiments have shown that with height the gradient rapidly decreases, amounting to only 10 volts/meter at an altitude of 5,000 meters. This shows that there are positive charges in the lower layers of the atmosphere. The course of the gradient at high altitudes is much more constant than near the Earth's surface. But even there, periodicity in the daily and annual course is detected, obviously caused by cosmic reasons (possibly the position of the Earth relative to the Sun is of great importance). It can be calculated that the charge of 1 cubic cm of atmosphere (lying below 5,000 meters) is equal to 2.10^-9 electrostatic units. Above 5,000 meters there are almost no charges. 2. Radioactive phenomena and conductivity of the atmosphere. Studies of the Earth's crust have shown that the crust contains a significant amount of radioactive elements (Ra, Th, Ac, etc.); their decay products enter the atmosphere and ionize it. The air in the soil also turns out to be strongly ionized. With constant exchange of soil and atmospheric air, a significant amount of positively charged ions penetrate into the atmosphere, while negative ones are absorbed by the Earth's crust. Thus, the atmosphere always contains ionized molecules, and the number of positive ions exceeds the number of negative electrons; this circumstance determines the positive potential of the atmosphere. Ionized air is a conductor of electricity. Therefore, a charged body located in the atmosphere gradually loses its charge, which is neutralized by air ions charged with opposite electricity. Numerous experiments by Elster and Geitel have shown that at all points on the Earth's surface, dissipation of the negative charge occurs faster than the positive one (approximately 1.3-1.5 times). This is especially pronounced in the winter months, when the potential gradient has the highest values. The degree of dissipation, like the potential gradient, does not remain constant. In its annual course, a certain periodicity is observed (maximum in summer, minimum in winter); in the daily course, however, periodicity has not yet been established. Under the influence of the electric field of the atmosphere, ions are in constant motion: positive ones are attracted to the Earth, negative ones are repelled from it. Their motion creates a current called the conduction current; its magnitude is proportional to the potential gradient and the conductivity of the atmosphere. In addition, ions are carried by air currents, creating a convection current, the magnitude and direction of which depend on the strength and direction of the wind. In calm weather, the density of the convection current is approximately 10^-17 A/cm²; the density of the conduction current is roughly 10 times greater. The conductivity of the atmosphere under normal conditions is equal to 2.10^-* electrostatic units. Its annual course is opposite to the course of the gradient; its daily course is subject to very significant fluctuations that occur irregularly. With height, the value of conductivity increases, reaching at 6,000 m values 10 times greater than the conductivity values at the Earth's surface. It is very probable that this increase in conductivity is due to the ionizing effect of ultraviolet rays of the sun, which is stronger in the upper layers of the atmosphere than at the Earth's surface, since these rays are strongly absorbed by the air. Under the influence of illumination by sun rays, some minerals (feldspar, granite) release negative electrons (the photoelectric effect of Stoletov-Halvachs). Due to this, even stronger ionization occurs in the lower layers of the atmosphere. In addition to the factors mentioned, ionization of the atmosphere also occurs under the influence of radiation resembling X-rays but with a wavelength 1,000_o times shorter (from 0.0007 to 0.0004 Å), due to which its penetrating and ionizing effect is very great. Milliken discovered the effect of this radiation in water in a lake at a depth of 14 m. The absorbing capacity of the atmosphere during the experiment was equivalent to the absorption of a 7 m column of water; thus, the depth of penetration of the rays into water is 21 m, which is equivalent to 180 cm of lead (X-rays of the latest installations penetrate only through 2 cm of lead). The magnitude of this radiation does not depend at all on the weather, humidity, and other factors; most likely, it is caused by changes occurring in the nuclei of atoms of some substance located in outer space; this phenomenon, named cosmic rays, has been studied relatively little. Finally, atmospheric precipitation, which always has an electric charge, creates an additional convection current. The magnitude of this current is usually of the same order as that of the conduction current. During heavy downpours, however, it increases 1,000 times. Raindrops have both positive and negative charges; as recent studies have shown, the amount of positive charges is greater than negative ones. The potential of the drops reaches 30 volts, the charge 10^-4 electrostatic units (radius of the drop 0.2-2.5 mm). Snow is more electrified, which is probably explained by greater friction of it against air particles; the sign of the charge varies; graupel and hail are mostly positively charged. The electrification of clouds and precipitation is undeniable, but so far has not received a satisfactory explanation. 3. Visible discharges in the atmosphere. At very high values of the potential gradient, the electric field in the atmosphere imparts to the ions and electrons moving in it such significant speeds that, when colliding with air molecules, they destroy them, due to which the number of ions and electrons increases, and the air becomes very conductive to current. With sufficient ionization, an electrical discharge occurs, during which the air begins to glow. Two types of discharges are distinguished: silent and spark. In a silent discharge, electricity flows from a charged body if it is pointed at the end (lightning rod, ship mast), and a bluish glow of the surrounding air is observed and a slight crackling is heard; sometimes the glow takes the form of a brush (St. Elmo's fire). A spark discharge (linear lightning) is observed at very high gradient values; it consists of a series of discharges following one another rapidly; the duration of a single discharge reaches 0.001 sec., of the entire lightning - up to tenths of a second; the length of lightning reaches 50 km, the strength of the electric current at this time is very great (up to 20,000 A). Usually lightning has the appearance of a glowing line and is directed from a cloud to the Earth or another cloud; in the spectrum of lightning, the lines of N are brightly expressed, the lines of H, O, and noble gases are less distinct. A flat lightning is observed less frequently, which is a discharge over the entire part of a cloud. Due to the rapid and strong heating of the layers of air through which lightning passes, strong air oscillations arise, perceived by us as sound (thunder).

Very rarely, lightning takes the form of a fireball several decimeters in diameter, which moves quite slowly through the air and finally bursts with a crack (ball lightning). The nature of ball lightning remains mysterious to this day. Finally, occasionally a beaded lightning is observed; in this case, a whole series of separate glowing points is visible simultaneously; beaded lightning is probably a transitional stage between ball lightning and ordinary lightning. In addition to the phenomena mentioned, the aurora undoubtedly has an electrical character; the cause of its occurrence has not yet been clarified; it is assumed that they are caused by cathode rays of the sun upon collision with gas particles in the uppermost layers of the atmosphere. So far, no theory has been created that would cover all electrical phenomena occurring in the atmosphere. But, apparently, the most acceptable explanation for the existence of an electric field in the atmosphere is the action of radioactive elements of the earth's crust (Ebert's theory), which create a negative charge on the Earth and saturate the lower layers of the atmosphere with positive charges, n. M.