ATMOSPHERIC ELECTRICITY: OPTICAL PHENOMENA
Atmospheric Electricity
Under ordinary conditions the electricity of the air is positive in relation to the ground and the oceans. Its potential does not vary greatly, being rather higher in winter than in summer—a change which might be considered normal. During rainfall or snowfall the potential usually is unsteady, varying rapidly between positive and negative. The changes are quiet; in ordinary cases they can be detected only by means of sensitive electrometers designed for the purpose.
Ether Waves.—From time to time there are sharp but slight variations in the electric potential both of the earth and of the air. The former are created by the “earth currents” which, in the time when the telegraph was operated by grounded battery circuits, were the bane of the telegrapher. The sharp variations of the atmospheric potential are known as “static waves,” or “ether waves”; they are the most common obstacle in radio-telegraphy and telephony.
The ether waves of atmospheric electricity apparently have little or no effect upon the activities of life; they also seem to be unimportant to meteorology, except as their increasing frequency may possibly indicate the approach of a thunder-storm.[1] Ether waves of the Hertzian type, caught at a distance of 50 miles, more or less, from the thunder-storm, may be compared to the ground swell of the sea formed by a distant storm.
Electrical Conditions of the Air.—Humidity may or may not 
Ether wave indicator, Meteorological Laboratory. The wire at the upper right of the spark gap leads to the aerial. The lower binding post of the condenser leads to the ground. The coherer is shown above the bell hammer. affect the potential of the air materially, but it affects the conductivity greatly. Dry air is a poor conductor; and dust particles, unable to discharge their load of electricity, are strongly repellant and remain suspended in the air, sometimes for several days. The desert simoon is followed by a condition which keeps the air at a high potential, with a highly electrified dust, for several days.
At the trading-posts in the Colorado and Mohave Deserts, after a simoon has passed, metal containers on wooden shelves become condensers of a considerable capacity. Horses’ manes and tails stare like fright wigs, and sparks crackle to any ground conductor that may be touched. At such times, strong earth currents may be detected, and their influence may be felt many miles distant.
In a case of this sort the high potential is local—that is, it is confined to the mass of dry desert air, and this mass of air is practically a great condenser which has been charged to a potential much higher than that of the air surrounding. In time—from twelve to forty-eight hours—the high electric charge disappears, and the potential sinks to normal. The question—“How can the air, which is composed mainly of gases, become a condenser and hold a charge of electricity?”—is not difficult to answer. The static charge of an electrified body practically is on the surface of the body. Every substance must possess surface and the molecules of the gases composing the air are not an exception; neither are the dust particles floating in the air; therefore they act as condensers, receiving and discharging electrons.
Just as water, by seeking its own level, acquires an even and uniform pressure, so the electricity of the air seeks an even and uniform potential. If a body of cold, dry and highly charged air flows into a region of low potential, or into one oppositely charged, an interchange, or flow of electricity, results. The interchange may be so quiet that it escapes notice;[2] on the other hand, it may be violent enough to produce strong electrical discharges.
The origin and source of atmospheric electricity is still a problem to be solved; so also is the origin of earth electricity. To the best of human knowledge, the earth is constantly giving off negative electricity, and receiving none in return, except that which is brought down by rain, or by snow, or by lightning strokes which pass from the clouds to the earth. The reason therefor is not known.
It has been found that a rainstorm carries to the earth about 3.5 times as much positive as negative electricity;[3] and that positively charged snow falls more frequently than that which is negatively charged. A reason therefor certainly exists, but it is not known. The breaking of large drops of water into spray is accompanied by the production both of positive and negative electricity. Conversely, when fine spray is charged with electricity, the spray immediately coalesces into very large drops of water.
Extra-terrestrial Influences in Atmospheric Electricity.—The fact that rapid movements in sun spots and similar disturbances in the photosphere, or envelope of the sun, are coincident with magnetic storms and earth currents leads to the belief that solar influences at times are factors in atmospheric electricity. It is not safe to infer, that because of this fact, the electricity of the earth and its atmosphere are derived from the sun. Practically all evidence is contrary to such an assumption; nevertheless, there seems no reason to doubt that high-frequency waves generated in the sun reach the earth.
The phenomenon known as the aurora borealis {aurora polaris), more commonly called “northern lights,” is most frequently observed during great disturbances in the sun’s photosphere. But it is by no means certain that the display, which is electrical, is due to solar causes. The belief that the aurora is of solar cause, however, is held by many physicists.
The height of the aurora above the earth does not vary much from 60 miles. It is rarely visible in the latitude of New Orleans, occasionally in the latitude of New York, and rather more frequently in the latitude of Quebec; its maximum frequency is in the latitude of Norway and the southern part of Alaska.
The time of frequency varies. At Hammerfest, Norway, it is not visible during the summer months, presumably because of daylight. In New York, the spring and fall months are the periods of greatest frequency. Records from 1764 show that auroras are much more frequent during the periods in which sun spots are most frequent; this is one reason why the aurora is thought to be due to solar influence.
The work of the observer is to watch carefully and to note faithfully whatever is visible. Information is desired concerning the position, direction and extent of the arch, if one appears—otherwise the position of the patch or patches of light. It is desirable to know whether the arch takes the form of a curtain, a luminous band, or a corona. It is also desirable to note whether the light occurs in rays with dark spaces between them, or is a diffuse illumination without definite outlines, or takes the form of dancing streaks of light, changing rapidly in color, form, and intensity. When possible, it is well to compare the aurora with illustrations in any known publication, especially with those in the Encyclopaedia Britannica.
Thunder-storms.—The phenomena of thunder-storms have been known ever since human beings peopled the earth. The cause or causes are still imperfectly known.
Thunder-storms derive their name from the reverberations and crashes of thunder following lightning discharges, which possess an intensity unknown except in nature. These discharges take place between cloud and earth, between earth and cloud, and between cloud and cloud. But the lightning discharges are not the cause of the storm; they are incidents merely in its progress; and except in intensity and volume the thunder does not differ from the snapping of an electric spark.
Several things take place in the formation of a
The movement of the wind in a thunder-storm; A, base of cumulo-nimbus cloud; B, ground level. A roll scud forms between the wind of updraught and that of a downdraught.
thunder-storm. A strong updraught of air and the shattering of rain- drops are among the features necessary to produce free electricity. The updraught of air is almost always a noticeable feature, and this takes place conspicuously in the cumulus thunder-head. Ordinarily the base of the cumulus cloud is less than I mile in height; but the updraught that precedes the thunder-storm, and is a potent cause of it, carries the cauliflower head of the cloud to a height of 4 or 5 miles. It is within this head that the potential electricity of the raindrops is changed to kinetic or free electricity.
Experiments have shown that a blast of air driven against drops of distilled water, with a force sufficient to blow them into spray, produces both positive and negative electricity—three times as many negative as positive electrons.[4] It has been found also that a velocity of 25 feet (8 meters) per second, or more, will cause the larger drops to be shattered and beaten into spray.[5] That is, if the drops falling in still air reach a velocity of 25 feet per second, they will be broken into smaller drops; or if the updraught exceeds 25 feet per second, the drops cannot fall against it; they will be shattered and carried upward until the velocity of the updraught is much reduced.
“Clearly,” Dr. Humphreys states, “the updraughts within a cumulus cloud frequently must break up, at about the same level, innumerable drops, which, through coalescence, have grown beyond the critical size and thereby according to Simpson’s experiments, produce electrical separation within the cloud itself. Under the choppy surges of a thunder-storm, the drops may undergo disruption and coalescence many times, and with each disruption a correspondingly increased electrical charge. Hence, once started, the electricity of a thunder-storm rapidly grows to a considerable maximum. After a time, the larger drops here and there reach places below which the updraught is slight; then they fall as positively charged rain. The negative electrons in the meantime are carried up into the higher part of the cumulus where they unite with the particles of cloud matter and thereby facilitate their coalescence into negatively charged drops. Hence the heavy rain of a thunder-storm should be positively charged—as almost always it is—and the gentler portions negatively charged—which frequently is the case.”
The falling rain—and also the hail which occasionally attends a thunder-storm—cools the air through which it passes and the cold air sinks to the earth with a considerable velocity. As it reaches the earth the down-rush plows underneath the warm, moist air in front of the storm, lifting it and thereby aiding the updraught. As the cold air spreads over the ground its velocity is great enough to raise clouds of loose dust that almost always precede the fall of rain.
As in the case of other storms, the latent heat set free by the condensation of moisture is the fuel of the thunder-storm,
Successive lightning flashes. Note the headed flashes and the scud of nimbus clouds in the center.
and the cause of the updraught. Rapid evaporation, on the other hand, together with the expansion of air in the updraught, is sufficient to account for the cold air, still further chilled by rain and hail, which finally culminates in the downrush.
Practically, the cumulus is the parent of the thunder-storm, and when it develops into the cumulo-nimbus stage it is essentially a thunder-storm. Even the apparently quiet cloud is always in motion within itself. Rising currents of moist air, chilled by its own expansion, cause condensation of the vapor into cloud matter. The coalescence of cloud matter into mist and droplets results in their fall to a lower level, where they are again vaporized; and the vapor, in turn, rises in the updraught. All this is constantly changing and disturbing the electric potential. When, however, the updraught is strong enough to shatter the drops into mist, the potential becomes so high that the violent discharges constitute the thunder-storm.
In other words, if the updraught is sufficiently strong to hurl the cloud matter to a height where condensation is very rapid, and also to shatter the falling rain-drops, the cumulus develops into a thunder-head at the top and a thunder-storm at the base.
Thunder.—The distance of the discharge may be found approximately by noting the interval between the flash and the thunder, allowing noo feet per second[6] for the velocity of the sound wave. In general, a nearby discharge is followed by an instantaneous report and this in itself indicates that the observer is in the danger zone. It also indicates a probability that the discharge passed between cloud and earth rather than between cloud and cloud. If there is no visible flash, it is likely that the discharge took place between cloud and cloud; and if no thunder follows a discharge, either the discharge occurred at a distance so great that the sound wave became inaudible, or else it was a silent “brush” discharge.
The long-drawn rolling of the thunder may be due to either or both of two causes. If the lightning is a flow or “streak” a mile or more in length, the sound from the farther part requires a proportionately longer time to reach the observer than that for the nearby part. Another factor also must be considered; what appears to be a single discharge may be an oscillatory discharge[7] which does not differ, except in intensity, from the undamped spark of a wireless transmitter, the several oscillations producing separate but interfering sets of sound waves. A more satisfactory theory makes the extreme and sudden heating of the air, with its moisture content practically an explosion with compression waves identical with those caused by instantaneous explosions. The reflection of sound also may be a factor in reverberation.[8]
Forms of Lightning.—The most common form of discharge is shown in the accompanying illustration. The discharge merely follows the line of least resistance. The zig-zag discharge, with sharp angles and saw-teeth points, once patronized by artists in order to give effect to their illustrative work, has never been discovered in photographs of lightning discharges. The most extraordinary effects of lightning are the dark flashes occasionally caught in photographs of lightning.
Sheet lightning is generally regarded as the reflection of distant flashes from the surface of clouds. On various occasions the exchange of electricity takes the form of a bluish glow between the earth and a low cloud. This form of discharge is rare; probably it does not differ from the brush-shaped discharge visible when a static generator is operated in the dark. The St. Elmo fire is a discharge of this sort. During its occurrence, the peaks of roofs, the limbs of trees, flag-poles, church spires, and weather vanes are tipped with coronal circles of electricity. The St. Elmo fire is of rare occurrence. It sometimes follows thunder-storms.
Ball lightning has been observed so many times that its existence seems to be established beyond doubt.[9] It has been explained as being due to a slowly moving point at which intense discharge is taking place; but this explanation is merely a possibility, not an established fact.
Occurrence of Thunder-storms.—Roughly speaking, the lower the latitude of moist regions, the greater the frequency of thunder-storms. In general, of two regions of the necessary warmth, one having moist air and the other dry air, the former is more likely to be visited by thunder-storms. They are more prevalent in the United States than in Europe; they are more prevalent in the southern part of the United States than in the northern part, so far as the region east of the Rocky Mountains is concerned.
According to A. J. Henry of the United States Weather Bureau, the regions of greatest frequency are in Florida, where thunder-storms occur 45 days in the year; in the central Mississippi Valley, where they occur 35 days in the year; and in the upper Missouri Valley, where the average is 30 days in the year. Thunder-storms rarely occur in the Pacific Coast states, but they are common in the Plateau Region.
Practically all the violent thunder-storms of the United States occur in the warm months. By far the greater number occur in June, July and August, during the hottest part of the day. There is also a period of minor frequency between 9 o’clock at night and midnight. Over the sea, however, the period of frequency is apt to be in the early morning, before daylight. Occasionally the updraught of the ordinary cyclone may produce a thunder-storm; the thunder-storms of winter are of this sort and they are rarely severe.
Pressure Waves.—The accompanying barogram, recorded at the Mount Vernon Meteorological Laboratory, illustrates pretty clearly the progress of a thunder-storm. The barometer had fallen steadily for more than twelve hours preceding the storm; and this continued until well along in the afternoon. The slight rise of the barometer in the morning is the diurnal pressure wave. The jump in pressure in the afternoon is the characteristic “thunder-storm nose” which usually is found on barograph records of thunder-storms. An expert observer does not need to refer to his daily reports to find the records of thunder-storms; the barograms show them in most instances. The rise in pressure occurs when the descending wind lifts the warmer air above it. A second “nose” appears about
Forecasting Thunder-storms.—From the nature of the case, the general forecasts made by the Weather Bureau cannot designate the loci of possible thunder-storms, because the general forecasts are made too far ahead, and also because such storms are local.
The meteorologist in charge of the local station is able to forecast more definitely; and, where stations not far apart are fortunately situated, the formation of thunder-storms may be indicated with a fair probability of verification. With warm, moist air on the south side of a low, thunder-storms may be expected; and if one has formed, its path may be predicted with reasonable exactness. In the hands of a trained observer a barograph is a most useful aid. With the aid of the daily weather map, the local conditions of temperature and humidity, and the barogram, at least two hours’ notice may be given.
The layman also may forewarn himself with a reasonable degree, if not of certainty, at the least, of probability. An aneroid barometer, if watched closely, may be serviceable; unless intelligently used it is of doubtful service to any but a trained observer. Nevertheless, there are indications that should warn even a casual observer who bears in mind that the thunder-storms disastrous to crops occur mainly in June, July, and August, and also that almost always they occur between mid-afternoon and sunset.
Warm and moist air is necessary to the formation of a thunder-storm; moderately quiet air is also necessary. A thunder-storm is not likely to form where a stiff wind is blowing. Cumulus clouds may be regarded with suspicion; indeed the cumulus is the thunder-storm factory; and when it develops into a cumulo-nimbus, the thunderstorm is probably at hand.
If the air of a warm, moist summer afternoon becomes still and oppressive and if cumulus clouds increase in size, a thunder-storm is very likely to follow; and if a nearby cumulus expands vertically into a thunder-head the storm is pretty certain to follow, somewhere or other in the vicinity. The thunder-head may be visible every where within a radius of 25 miles, but the storm path may be a narrow strip not more than 30 miles in length. The path of the thunder-storm, like that of the tornado, is determined by the circulation of the cyclone in which it is formed. Its forward movement, except in the extreme southern part of the United States, is from a westerly to an easterly direction.
Safeguards Against Lightning.—The destructive effects of lightning in the United States are chiefly loss of life and loss from fire. Loss of life occurs usually when lightning strikes trees under which people and animals have taken shelter. Trees are the objects most frequently struck. Wooden buildings when struck are apt to take fire instantly, but cases are on record which show that wet shingles and weather boards may be ripped off without further damage. Among structures, oil tanks stand first in the likelihood of destruction by lightning. Church spires and large barns are frequently struck, and isolated buildings are regarded as a far greater risk than city buildings; indeed, in the compactly built areas of a city the risk from lightning stroke is negligible.
Lightning rods afford the best protection against lightning. J. Warren Smith of the United States Weather Bureau found that in many thousand insurance risks, the destruction of rodded buildings was negligible. Other authorities regard the safety afforded by lightning rods at from 90 per cent to 97 per cent. The Bureau of Standards[10] points out the necessity of connecting all exposed metal surfaces such as metal roofs, gutters and tanks with the lightning rods. Sir Oliver Lodge recommends iron in preference to copper as a material for lightning rods for the reason that its greater resistance tends to damp the oscillations of a discharge, practically converting them into a one-way current.
Atmospheric Optical Phenomena
A ray of light may pass through a solid, such as glass; a liquid, such as water; or a gas, such as oxygen, nitrogen or water vapor—that is, the air—without much apparent loss. Such substances are transparent. In passing through different substances the ray is likely to be bent out of its course, as is apparent when a stick is thrust obliquely into a body of water. The bending of the ray is called refraction. Or if it is turned back, as when it impinges upon a mirror, it is said to be reflected.
A ray of light impinging upon a piece of black cloth is said to be absorbed. If only a part of the ray is absorbed, the rest being reflected, the parts of the ray reflected produce the sensation of color.
If a ray of light is passed through a wedge-shaped prism the component parts of the ray are unequally bent or refracted, and reach the eye in a series of colors. Red is the least refracted; violet suffers the greatest refraction. A ray of white light, therefore, is not of a “bundle” of wave-lengths of the same magnitude, but a bundle of an infinite number of rays of different wave-lengths.
In passing by the edge of an opaque body, or in passing through a very narrow slit, a ray of light is deflected slightly, and alternate fringes of light and dark bands are produced. The deflection and interference constitute diffraction, and diffraction is also a factor in giving various color tints to the sky.
The various atmospheric optical effects of the sky are produced mainly by refraction, reflection, diffraction, and absorption of light by the constituents of the air. The color of the sky itself is due to the irregular scattering and dispersion of light as the sun’s rays glance from the gaseous molecules and minute dust motes of the air. The most common incidents of atmospheric optical phenomena are coronas, halos, rainbows and mirages.
Corona.—A corona consists of a ring—sometimes several rings—rarely more than 4 degrees of arc measurement in diameter, surrounding the sun or, more commonly, the moon. The corona is a case of diffraction, the deflection of rays passing by water droplets.[11] The inner border of the ring is brownish-red. Within the ring is a bluish-white surface, the aureole. If spectrum colors other than the red are observable, they follow each other in order from violet to red, reversing the order of halo colors. This sequence of color sometimes is repeated several times in the case of the corona but not with the halo.
Halo.—The most common form of halo is the ring around the sun or moon. It has a radius of about 22 degrees of a great circle. At times, however, the halo is a complex arrangement of concentric tangential and independent arcs of circles. The
Lunar halo observed by Gen. A. W. Greely at Fort Conger.
As a rule, the various circles, with the exception of the halo circle, are only partly visible; and in many cases the unusual arcs seem to have no connection with the halo. Many interesting illustrations of complex halo circles have been published. Usually these have the circles of 22 degrees, 46 degrees, and the mock-sun circle in common; otherwise they are unlike.
Occasionally a vertical column of sheen extends above and below the sun—perhaps more frequently the moon; it is known popularly as the pillar of light. Rather infrequently a horizontal bar of sheen may be seen forming the popularly named “heavenly cross.”[12] Sun pillars, varying in color from white to red are occasionally seen at sunset or at sunrise. Patches of color occasionally are observed in cirrus and cirro-stratus clouds at a considerable angular distance from the sun. They may be due to causes similar to those which produce halos, but the causes are not known.
Cirrus or cirro-stratus clouds, or ice mist, in front of the sun or the moon are necessary to the production of halos. Some of the ice crystals are tabular; others are columnar and prismatic in shape. It is thought that both reflection and refraction of light are involved, each depending on the character of the crystals. Spectrum colors which abound in halo phenomena are explainable as a result of refraction; white-light surfaces may be due to reflection.
Rainbow.—The rainbow against a dark gray background of cloud is one of the most beautiful objects in nature. It may be seen as a full circle against spray thrown into the air, or against a mist. The rainbow of the summer shower consists of a bright arc near the horizon and usually a fainter arc above. The radius of the bright, or primary bow is about 42 degrees of arc; that of the upper or secondary bow is not far from 52 degrees of arc.
The rainbow is best observed when the sun is not more than 45 degrees above the horizon; it forms in the side of the sky opposite the sun. On rare occasions a tertiary bow may be seen between the observer and the sun.
The colors of the rainbow vary in intensity and in quality. Red is always in evidence outside the primary and inside the secondary bow; orange, yellow and green are commonly though faintly observable; blue is sometimes seen; but violet is rarely if ever observable. The strength and the sequence of the colors depends mainly on the size of the drops, but partly on their distance and the number of them.
Each observer sees his own rainbow, and each rainbow is
Refraction of light passing through rain-drops.
practically a series of hollow concentric cones, the vertex being at the eye of the observer. The rainbow moves forward, backward or sideways as the observer moves. A shower in one part of the sky and sunshine in another, the observer being between, are requisite for rainbow formation; and this condition, in most parts of the world, is confined to summer showers.
Mirage.—Owing to changes in temperature the density of the air varies almost constantly at different heights. Rays of light passing through air of varying density are bent differently with each change of density. An observer looking at a distant object sees the object with distorted outlines. An elliptical sun at sunset is very common; and sometimes one sees it with greatly distorted outlines.
When a layer of air rests quietly on another the plane of contact, if below the observer, reflects the sky in much the same manner as does a body of water. An object at this plane is seen both upright and inverted, thereby forming a mirage. If the plane of contact is materially above the eye of the observer the inversion occurs in the air. Occasionally inverted images of the shipping in the harbor are formed.[13]
The looming of objects—that is, bringing to sight objects that normally are below the horizon—is clearly a case of refraction. The rays of light which should pass above the observer are bent within reach of his vision.
According to legends a fairy named Morgana hovered around and about the southern coasts of Italy. This sprite used her powers romantically rather than maliciously to change the commonplace shoreline across the straits of Messina to a most wonderful landscape of turreted embattlements and castellate fortifications. Hence the name, Fata Morgana. The phenomenon apparently is produced by a horizontal layer of air, denser at the center than at its surface. It therefore becomes practically a cylindrical lens which magnifies in a vertical but not in a horizontal direction. It may be considered as a form of looming.
It is probable that the Brocken spectre is produced in part by a mass of air which acts as a lens. The traditional “heiligenschein,” or halo, is an example of diffraction, however.
- ↑ Ether waves are made audible by means of the mineral detectors formerly used by radio-telegraphers, and by the use of the various devices known as audions. The passage of an ether wave caught by the antennae gives a distinctive hissing sound in the telephone. A strong wave illuminates a Geissler tube placed in the circuit. A more striking result may be obtained by using a coherer and a relay with one or two dry cells. The passage of the ether wave from the antennae to the ground electrifies the filings in the coherer to the extent that a battery circuit is formed, which closes the relay. The closing of the relay may be used to close a secondary bell circuit, or to communicate any other desired signal. A drop of clean mercury between two iron plugs within a glass tube makes an excellent coherer, when placed in a circuit. Lighting companies sometimes make use of such "storm indicators" to guide them in generating the additional current made necessary by the darkness accompanying summer storms.
- ↑ The interchange, no matter how quiet, will operate the apparatus described on p. 108.
- ↑ The records of Dr. C. G. Simpson, London Meteorological Office.
- ↑ C. G. Simpson, London Meteorological Office.
- ↑ P. E. A. Lenard.
- ↑ The rate varies slightly with temperature and density of the air.
- ↑ The oscillatory discharge is regarded as doubtful by some meteorologists. At all events, in traversing a conductor of moderate resistance it is damped practically to a current of unidirectional character.
- ↑ The electrolytic decomposition of water vapor and its recomposition in the form of successive explosions also has been suggested.
- ↑ Mr. George Reeder and his assistant Mr. Seaton of the Weather Bureau Station, University of Missouri, describe an instance of ball lightning, as “a pale red, slightly corrugated ball, apparently about 2 inches in diameter, moving across a space of about 6 feet between the telephone and a window. The ball seemed to float as a liquid bubble does, though it seemed solid. It kept a fairly straight line for the window; it rolled over the window sill and disappeared—not into the outer air, but by flickering out as a bubble does. There was no explosion or sound of any kind except a click of the telephone; there was no odor nor mark of any kind on the window sill.”
- ↑ Bulletin 56.
- ↑ In a foggy atmosphere, an observer with his back to the sun sometimes sees a dim, colored ring surrounding his shadow which is cast upon the fog. This phenomenon, known as a “glory halo,” is probably a corona.
- ↑ This effect may be produced by looking at the moon through a piece of polished copper screen-netting held at a distance of 20 feet. It is an effect of diffraction.
- ↑ Many physicists hold that the inversion in such instances is due to refraction. In one popular textbook of physics a diagram graphically describes the method of refraction; but the diagram illustrates reflection. The mirage is considered in detail in Chapter XII.