Dissipating Fog
I
‘WHO commands the air, commands all!’ It is a far-flung dictum, at first blush seemingly extra-inclusive. Yet we have substituted but one word in that older, widely recognized aphorism, ‘They who command the sea, command all!’
This last phrase has been the basic belief of farseeing statesmen from the time of Themistocles to the middle period in the World War. Then it began to dawn on the minds of ministers, field marshals, and admirals that decisions might be obtained by air forces — decisions not less important and more significant than a fleet engagement or a stalemate in the trenches.
Historians who shall painstakingly unravel the causes leading up to the clash of arms in 1914 will find one of the prime provocatives in the systematic struggle for supremacy on the sea: the challenging attitude of the German Admiralty, and the dogged determination of the Mother of Colonies, on her tight little island, to brook no rival ruler of the waves. The historians of some later conflict will in all probability find that provocation came in a desire, not for sea supremacy, but for control of the air. This is the mare sine fine, this far-stretching ocean that flows everywhere, streaming so easily over all barriers, be they even mountain high, surging endlessly and at times relentlessly over cities, forests, plains, and watery wastes.
Man, after waiting for centuries, is about to come into his own. He has long walked the earth and sailed upon the sea, but now he traverses the air. There is no brag in his claim that every field has become an open port. No longer is he to be hampered by harbor, tide, and dock. And as for delay and danger, these are restricted to defective engines and adverse weather. The former are minimized by careful, constant inspection and test; the latter remains beyond man’s power to control. Yet even here our present foreknowledge of impending weather enables airmen to avoid dangerous zones. They may not with impunity defy the lightning, nor the tremendous rush of air in squall and tornado; but the route can be changed and the schedule altered in accord with the forecast.
There remains, however, one element of weather which now, as it has been in the past, is both bothersome and dangerous. It has always been a menace to travel by sea and land. It is fog.
If a landing field be obscured by fog, then it ceases to be an open port, and the airman’s boast is an empty one. The first requirement for safety in aviation is the existence of open flying fields. These must be easily discernible from the air, and, when found, free from any curtain of obscuring water vapor. But who shall presume to ‘stay the bottles of heaven,’ as cloud and fog are called in the book of Job? Who shall dissipate a fog? Hitherto it has been accepted as a necessary evil and an unpreventable menace. The sum total of disasters at sea due to fog is appalling; and, while less sensational, casualties on land are only too numerous.
One wonders why, when the struggle for sea supremacy was so keen, no one thought of attempting to devise means for dissipating fog, or, on the other hand, for making it. For surely guns are silenced when fogs prevail. Shooting at an enemy enshrouded in fog is a waste of ammunition, unless indeed the noise is expected to strike terror to the hearts of the adversary. But even in Oriental countries that day has long gone when the army which set off the loudest firecrackers and made the greatest din won the victory.
The smoke screen, in a way, is an attempt at fog-making. Its usage in naval operations developed during the war; but it has remained for the air forces of this country to tackle systematically the dissipation of fog.
What, then, is the texture of fog, this delicate stuff that a breath can almost blow away and yet so pervasive and potent that it is a sheer waste of effort to try to carry on when it asserts itself?
In the first place, there is no essential difference between fog and cloud. When water vapor is condensed by chilling and made visible near the ground, we call it fog; when the operation is carried on overhead, we call it cloud. A cloud is a lifted fog; a fog is a cloud resting on earth. Strictly speaking, there is this slight difference: to form cloud there is an upward movement, the ascension indicating excess heat over circumjacent air. Fogs frequently occur with marked anticyclonic conditions, characteristic of fair weather, with what is called an inversion of temperature, the low levels being colder than those above. Under such conditions we have groundradiation fogs.
The winter fogs of London, Paris, and New York are mostly anticyclonic ground-radiation fogs, the visible water vapor not extending upward beyond a hundred metres. The circulation is slow, at times nearly stagnant, and therefore considerably less energy need be expended in clearing a given area. The fogs of December 10 and 11, 1926, in Paris, denser even than a London ‘particular,’ were of this type. These differ in degree only from the fog of a late autumn morning overspreading the harbor of New York.
When fog is mixed with smoke and soot, and a curtain of black settles down over a community, the community has itself to thank for a dreary, dismal day. A short paragraph concerning London in the latest Britannica is significant: —
Greater London had, in 1926, a population of about 7,500,000 persons; but nothing has occurred in the years since 1910 to alter materially the story of its site, although under the heading of climate some diminution in the density of its fogs may be noted.
The information might have been added that this diminution in density was brought about by the Coal Smoke Abatement Society, merged finally into a Committee on Atmospheric Pollution. It surprised even the Committee when, as a result of their investigations, it became clearly evident that the chief offender was the home fire and not, us everyone supposed, the tall chimney of the big factory. The kitchen range is the low-browed, blackfaced villain that steals sunlight. Seventeen million tons of coal are burned each year in London; but it is domestic smoke rather than the output from industrial plants that makes London’s famous aerial pea soup. A similar source makes purée de pois de Paris.
Slow-witted householders might well file a bill in equity for a portion of the dividends of illuminating companies, for they contribute the material from which the overhead screen is made, necessitating extra illumination. On a day with little wind the curtain hangs so low that noon is not very different from midnight. Dwellers in London not only grope about in the darkness, but breathe the mixture, imbibing, as it were, their own home brew. In ten hours a man can inhale 200,000,000,000 particles of a London ‘particular’; and thus the name becomes more appropriate than those who first coined it could have foreseen. These small Africans in every citizen’s air pile, if placed end to end, would make a string one hundred miles long. But who can regard the swallowing of sooty strings as a pleasant or even a profitable pastime?
The density of fog can be much further decreased by better methods of combustion, and by smokeless heating, such as by gas and electricity.
So far, the actual efforts to dissipate fog in our cities by discharges of electricity have not been commercially successful. Sir Oliver Lodge, some years ago, tried to do this on a small scale in Liverpool. The fog did disappear, but for various reasons the work was not repeated. An attempt was also made in San Francisco, in 1915, but here again the outfit was not on a scale commensurate with the requirements. The experiments to be mentioned later, conducted by the Army and Navy Air Services, indicate that even now, at terminal points and within definite limits, fog can be dissipated with admittedly inadequate apparatus.
II
This brings us to the problem of evaporation in the free air, and its converse, cloudy condensation. As set forth in the older classical physics, fog and cloud were easy problems in thermodynamics. Given so much water vapor, it will require so much thermal energy to evaporate it; or, conversely, rob the vapor of a definite quantity of heat, and so much fog results. The newer physics calls in the chemistry of colloid suspensions to determine the behavior of thin air cushions, liquid films, and delicate envelopes, subjected to strong electrical strain. The electrification of fog particles is of the utmost importance in the dissipation of fog, and we know very little about it.
Ten years ago any teacher of physics would have answered readily enough: calculate the temperature-pressure relations, and, knowing the heat of vaporization, 595 gramme calories, it is plain that the fuel required to dissipate a moderate fog over a fair-sized flying field will speedily force the owners into bankruptcy. Some have given it as 12 tons of coal an hour for a fog 50 feet deep, drifting slowly. Using electrical distribution, they run it up to 400 or 500 tons an hour. But this is according to the Golden Age of physics, when everything was known in thermodynamics, electricity, and light. There was in those days a luminiferous ether whose existence no one dared to question. Ah! what a change a few years have wrought. The whole structure of radiation of energy has been turned upside down and inside out.
When Planck promulgated the quantum theory, with its heterodoxical energy atoms jumping about in spasms, yet remaining multiples of a fundamental unit, the orthodox views of mass, matter, and energy were doomed. Many and far-spread are the teachers of physics who, abreast of their science ten years ago, now lag hopelessly.
The business of snaring electrons is a job for young men. Atomic lattices make the old professors dizzy; and even simple structures like helium and hydrogen produce mental anguish. Erstwhile leaders now peep about to find themselves dishonorable graves — as Cassius remarked to Brutus, when one Cæsar became proton and other leading citizens of Rome only electrons revolving round that central core.
III
What precedes is our justification for saying that, as in other matters, ancient good in physics and thermodynamics, so far as fog is concerned, has become uncouth and must not be taken too seriously. We have a lot to learn concerning the process of fogfracturing. We are not using economical methods, and it is only too manifest that as yet we have not found the true activation of a fog droplet. We can, however, and do cut channels in a cloud bank; and we can dissipate fog over restricted areas.
Latent in every gramme atom of hydrogen and of oxygen are tremendous stores of energy. There may be in a single fog droplet energy enough, if let loose, to run a hundred automobiles for a year. Ah — to be able to unlock that energy! The newer physics teaches that mass is interchangeable with energy; and so even the edge of a fog droplet might, if converted, furnish a vast supply of heat. The physicists of to-day are hiking down a trail which leads ultimately to the liberation of atomic energy. The quest is on; the finish may not come for years; but the steady increase in knowledge will undoubtedly lead to methods that can be used advantageously in dissipating fog.
At present it is feasible to dissipate morning mist and fog over lowlands — the ground-radiation fog referred to above — by a combination of mechanical and electrical agencies. The backwash of an airplane, always regarded as a source of danger to the unwary, and of no special importance, has become an effective agency for scattering dust and clearing cotton fields of miasmatic mists. Dr. Coad, entomologist at Tallulah, Louisiana, going forth in an airplane to spray the cotton bolls with calcium arsenate and thinking only about his bugs, discovered that he had at his control a means of dissipating fog below. One thinks of Saul going forth to seek his father’s asses and finding a kingdom.
At the aircraft factory in Philadelphia, the Bureau of Aeronautics has been carrying on for some time experiments on the dissipation of fog with a view of devising suitable apparatus for use on flying fields. A large compartment is filled with man-made fog. This is readily done by vaporizing water in high-pressure nozzles and scattering spray and vapor into the enclosed space. After a minute or two the chamber is filled with dense fog. Fog can be produced, it is well known, by any cooling process — whether due to sudden decrease of pressure and expansion or to a chilling by radiation, or by contact, or by advection. The fog thus formed does not disappear; and test electric lights are so dimmed as to be practically of no light value. Then a dozen jets of highly electrified water are sprayed into the room. Almost instantly the fog fades out, and the lights glow with their usual brilliancy. The water has a potential of 75,000 volts. The charge on each water particle is of the order of .000005 electrostatic unit.
The entire demonstration is a noteworthy modification of the well-known laboratory experiment of clearing a jar of smoke by the passage of an electric spark.
A second line of experimentation promising well has been carried out under the supervision of Warren and Bancroft, backed by the Army and Navy. Here sand of fine mesh, carried aloft by airplanes, is discharged by a blast from an electrified chute. Thus far the voltages have been moderate, and far below the desired values. When the sand is scattered on the top surface of a cloud, the effect is marked, and that portion of the cloud disappears. There seems to be little doubt but that, with improved apparatus, dramatic results will follow. We seem to be in a fair way to succeed with the practical problem of clearing fog from flying fields, thus making good the airman’s boast that every harbor is an open port.
IV
It may be that ultimately the more difficult problem of artificially producing rain will be successfully solved. Seemingly it is far in the distance.
Congress once interested itself in rain-making, and appropriated $10,000 to prove that high explosives could produce rain. The result put a quietus on a widely held belief that rain always followed big battles. Across the Potomac, not far from where the Unknown Soldier now rests, on one memorable muggy night in August 1891, half a ton of rackarock — trade name for dynamite — was exploded at intervals. The writer well remembers that night of misery and noise. Years later he had to listen to the barking of Big Bertha, and he has no hesitation in awarding the palm to the rain-makers as champion distributors of the ‘blows of sound.’ There was constant uncertainty as to what a Big Bertha noise would do next time, yet the Parisians remained calm. The good people of Washington, whose chandeliers crashed down in the small hours of the night, were not noticeably calm. The band of rain-making sleep-robbers discreetly fled to Texas and thus escaped lynching. Later they fled even from Texas.
There is no scientific basis for associating sound waves with coalescence of fog particles. Benjamin Franklin, in 1749, thought that there was. He wrote extensively on rain-making, holding that the ‘concussion or jerk given to the air, contributed to shake down water.’ The ingenious and illustrious Philadelphia philosopher on this occasion was wrong. We know that concussion waves do not shake down raindrops from the sky.
Finally some friendly Thomas will inquire, ‘What about rain-stopping?’ Lightly he will ask, ‘If you can dissipate fog, can you not stop an ill-timed downpour?’ That problem physicists have yet to tackle. Undoubtedly rainstoppers are needed! Even more than rain-makers! Long, long ago there was a flood. Doubtless Noah’s wife, sons, and daughters-in-law annoyed the Ark’s skipper by asking all too frequently, ‘When will it stop raining?' As there were no morning papers with official forecasts, it is hard to see how he could hope to answer truthfully.