It is obvious that if a discharge of lightning is not instantaneous, but has a sensible duration, the rotary movement of the camera, arranged as just mentioned, will spread out the image of the flash, on the photographic plate, into a more or less broad band or ribbon. Most photographs of ribbonlike streaks of lightning made with ordinary cameras are, in fact, due to accidental movements of the apparatus during exposure—such as an involuntary start of the operator, in case the camera is held in the hands—though a certain amount of spreading of the image is sometimes caused by what photographers call “halation.” Pictures taken with the revolving camera show that some flashes are practically instantaneous while others may last as long as half a second or more. Those of the latter class nearly always show several parallel streams of light, more or less distinctly separated by darker spaces. Each of these bright streams represents a separate discharge along the common path. As the speed with which the camera turns is known, it is possible to determine the intervals of time between the discharges of a multiple flash. These intervals may vary from a few thousandths to one or two-tenths of a second, while the duration of each of the consecutive discharges is probably not more than two or three hundred-thousandths of a second in most cases. Sometimes the path of the lightning flash is shifted by the wind while the picture is being taken. In one case this shift was estimated at 36 feet.
Photography is also applied to determining the distance of a lightning flash and hence the dimensions of any of its features. For this purpose a stereoscopic method is used, two cameras being mounted side by side and exposed at the same time. Sometimes one of the cameras is made to revolve, while the other remains stationary. The stationary camera will then show the relative positions of the flashes occurring during exposure, while the moving camera will indicate the times at which they occurred.
Streaks of “black lightning” and black borders of the white flashes, both often seen in photographs, are a trick of the camera and are due to what is called the “Clayden effect.” Some kinds of plates are much more susceptible to this effect than others. When a flash of lightning has registered its impression on such a plate, and, before the shutter is closed, another flash occurs, the general illumination of the field by light reflected from clouds, etc., often “reverses” the original image, and consequently it prints black.
“Sheet lightning” presents the appearance of a diffuse glow over the sky. When lightning of this character is seen playing about the horizon on summer evenings, in the absence of an audible thunderstorm, it is often called “heat lightning.” Most sheet lightning is probably a mere reflection of ordinary streak lightning below the horizon or hidden by clouds. Some authorities believe, however, that diffuse, silent discharges actually occur in the clouds. Balloonists claim to have encountered such discharges near at hand. An analogous phenomenon is the glowing of so-called “incandescent” or self-luminous clouds, to which several observers have called attention. A remarkable phenomenon of somewhat similar character has been reported by Dr. Knoche, late director of the Chilean meteorological service, who states that it occurs on a vast scale along the crest of the Andes during the warm season. The mountains seem to act as gigantic lightning rods, giving rise to more or less continuous diffuse discharges between themselves and the clouds, with occasional outbursts simulating the beams of a great searchlight. These displays are visible hundreds of miles out at sea. Something akin to this so-called “Andes lightning” has occasionally been reported from other mountainous regions, including the mountains of Virginia and North Carolina.
“Beaded” lightning and “rocket” lightning are as rare as they are interesting. The former resembles a string of glowing beads, while the latter is a form of streak lightning that shoots up into the air at about the apparent speed of a skyrocket.
“Ball lightning” takes the form of a fiery mass (not always globular), which generally moves quite deliberately through the air or along the ground, and in many cases disappears with a violent detonation. It occurs inside of buildings, as well as out of doors.
In order that a discharge of electricity may break through the resistance of the air along paths as long as those commonly observed, enormous differences of potential must exist in the atmosphere during thunderstorms. How such conditions arise has been the subject of an immense amount of speculation. The explanation now generally accepted was proposed in the year 1909 by the English physicist and meteorologist, Dr. George Simpson. This hypothesis is based upon the fact, well attested by laboratory experiments, that the breaking up of drops of water involves a separation of positive from negative electricity; in other words, the production of both positive and negative ions. In this process the drops become positively charged; i. e., they retain a greater number of positive than of negative ions, the latter being set free in the air. About three times as many negative as positive ions are thus released.
Now a thunderstorm is accompanied by strong upward movements of the air; so strong that small drops cannot fall to the ground, while large drops, which would be heavy enough to fall through such rising currents if they could retain their integrity, are broken up by the blast of air and carried aloft, where they tend to accumulate, recombine, and fall again. This process may be repeated many times, so that the positive charge of the drops is continually increasing, and at the same time negative ions are being set free and carried by the ascending air to the upper part of the clouds. Here they unite with the cloud particles and give them a strong negative charge. Thus eventually there is formed a heavily charged positive layer of cloud between a heavily charged negative layer above and the negatively charged earth beneath. When the differences of potential thus brought about become great enough, disruptive discharges of electricity will occur, and these may be either between the upper and lower layers of cloud or between the clouds and the earth, or, sometimes, between two different clouds.
Probably much the most frequent lightning flashes are those that occur within a single thundercloud and do not reach the earth. However, it will often happen that the negatively charged upper layer of cloud is either carried very high or drifted away by the wind, and then the discharges that occur will be chiefly between cloud and earth. Such conditions are likely to prevail in the case of cyclonic thunderstorms, in which there is often great difference in the direction and force of the winds at different levels. On the other hand, heat thunderstorms usually occur when the general winds are light at all levels, and it is probable that such storms are relatively free from cloud-to-earth discharges. We seem to have here an explanation of the paradox that tropical thunderstorms, which are nearly always of the noncyclonic type, though notoriously violent, are generally harmless.
It must not be inferred from what has been said above that the mystery of the lightning flash is now fully resolved. This is far from being the case. It is not at all clear how an electrical discharge can break down the resistance of the air along a path a mile or more in length, as commonly happens in the thunderstorm. It was formerly stated, on good authority, that the difference of potential required to produce such a flash would amount to upward of 5,000,000,000 volts. Certain facts have lately been adduced to show that such great differences of potential need not be assumed. Moving-camera photographs of the sparks produced by electric machines show that such sparks begin with small brush discharges which gradually ionize the air and thus build up a conductive path for the complete discharge. Something of this kind may occur in the atmosphere. Streaks of air already strongly ionized and more or less continuous sheets of rain would also help to provide conductors for a discharge. If lightning does build up its path somewhat gradually, the process might, in certain cases, be so slow as to account for the deliberate movement of rocket lightning, and also, perhaps, furnish a clue to the hitherto unsolved mystery of ball lightning. Humphreys has tentatively suggested that all genuine cases of ball lightning are “stalled thunderbolts”; i. e., lightning discharges that have come to a halt, or nearly so, in their progress through the air.