It is a commonplace of conversation that for some months past the weather conditions have been abnormal, particularly in the matter of rainfall, in the battle-zones and elsewhere. Detailed data from regions close to the firing lines are not available; and we have only general statements of inclemency in so far as they affect military operations. But in districts not far away,—the British Isles, for instance,—the records of excessive raininess during the winter of 1914-15 and at subsequent times have not escaped comment; and others besides meteorologists are discussing the possibility of a connection between the heavy cannonading and the rainfall. The professional meteorologist is called upon to answer whether there is any rational explanation of what appears to be a marked departure from the usual sequence of weather conditions. Is it possible that the tremendous expenditure of ammunition—an expenditure which the layman may well regard as an experiment in concussion sufficiently vast to be decisive—has facilitated condensation and its later stage, precipitation? In concise terms, has the bombarding not only caused clouds but forced the clouds to send down rain?
It is conceivable that such could be the case; and stranger things have happened than the revelation through war of fresh progress in man's effort to comprehend and master the processes of Nature. And here, as is so often the case, Nature herself has suggested the relation, for we have all noticed that after an exceptionally near and heavy clap of thunder, the raindrops fall with a rush, as if the very tumult had shaken the clouds and caused the downpour. Later we shall see how this well-known phenomenon is to be interpreted.
Three separate lines of inquiry suggest themselves as throwing light on the problem. First, the underlying principles of the formation and flotation of a drop of rain; second, the causes of excessive rainfall in certain places at certain times; and third, the direct relation, if any, which exists between the use of high explosives and showery or rainy weather. To most of us the raindrop is an ordinary, commonplace drop of clean water, or rather it seems to be clean. It is one of the most common phenomena of everyday life, and most of us never stop to think that its life-history could be eventful; in fact we are sure that there can be nothing unusual about a drop of water falling through the air. On the contrary there is much that is wonderful in the wanderings of the little visitor; and the structure of each minute globe is in its way as marvelous as the structure the great nebula in Andromeda.
Probably no two raindrops are exactly alike. Photographs of snow crystals make it plain that no two flakes even in the same storm have identical shapes and structures. Raindrops are formed under somewhat similar conditions of strain, with forces more energetic, but never quite permanently balanced. Drops change incessantly, even those that seem to be quiescent. Many have made long journeys and undergone modification at every turn of the road; but large or small, each globule is a complex of ionic infinitesimals wrapped in a blanket of water vapor. It is an elastic blanket, beyond measure, and changes its size and sometimes its form, with every variation in pressure, temperature, and electrification. The process of wrapping the ions in the blanket of vapor still baffles science, although man has had recourse to certain small messengers, waves of light, wave-lengths little larger than a millionth of a millimeter, and sent these among the ions to do his bidding.
Generally speaking, a raindrop or any water-drop is an aggregation of hydrogen and oxygen atoms combining in the value of two to one. In a gram of hydrogen (that is, about fifteen grains), there are six million million million million atoms. But still smaller than atoms are these carriers of electric charge called electrons, oscillating many million times per second and as constantly colliding with one another. An English physicist who has worked much along these lines, once said that unless we had a better test for a man than we have for an unelectrified atom we could never detect that the earth was inhabited.
But larger than the electrons are certain foreign bodies called nuclei or centers of condensation; and if ever man succeeds in making rain artificially it will be by increasing the number of nuclei. In fact, the vortex guns or smoke-ring firers used in the grape growing regions of the West to dissipate hail have a certain scientific value in that they furnish nuclei at critical times. Notwithstanding the belief of the vineyard owners, however, the efficacy of these Steiger guns remains unproved. Without nuclei, condensation does not occur even when space is saturated with vapor. And here a word of caution, and a bit of information that rain-makers in general do not know. While textbooks speak of the capacity of air for vapor, they overlook the fact that it is space rather than air which contains the vapor, for air and water vapor are two separate entities and must be considered as such.
The man who has most studied the behavior of the nuclei and who therefore comes nearest to being a genuine rain-maker, though he would be surprised to hear himself so designated, is John Aitken of Edinburgh. His well-known dust-counter is a very practical means of studying the formation of fog or the first step in rain-making. His experiments show that the size of the nuclei or inorganic centers varies considerably; and also the number present at different times. In one of his papers Aitken speaks of the number of nuclei in a puff of smoke from a lighted cigarette as 4,000,000,000,000 per cubic centimeter. Now each of these little particles may serve as a foundation for a raindrop. On a much larger scale, the factory chimney as it belches forth its clouds of smoke is furnishing material for the building of raindrops; and, provided enough vapor is present and certain temperature changes occur, there is no uncertainty about the result.
Aitken, Barus, Wilson, Thomson, Langevin, Pollock, and other physicists have taught us a great deal about the building of a drop of water. There is no difficulty in making rain on a small scale. The moisture on the outside of an ice-water pitcher on a warm day proves how easily water-vapor may be condensed and dew or rain made. Nature makes rain by cooling a given volume of vapor. While there is no change in the weight of the individual atoms, the comparatively gross nuclei do change in size and weight because of physical changes, gravitational attraction, and probably electrical attraction and repulsion. These forces bring about cohesion, and a drop acquires sufficient weight to begin its downward movement against air resistance. The cooling of the vapor (and this is the effective agency) may be due to expansion, as when a stream of mixed air and vapor is carried up in a mighty cumulonimbus cloud or thunder-head; or the cooling may be due to radiation, or to contact and loss of heat by conduction, or, again, by mixing. This may throw interesting light on the many degrees of cloudiness, from the far-distant cirrus or feather to the towering cumulus, from the valley fog of dusk to the black-browed nimbus that precedes the cloudburst.
Sometimes Nature conducts a rainmaking experiment in very dramatic fashion, as when a volcano blows its head off. Thus, when Mont Pelée, Krakatoa, Asama Yama, Katmai, and even little Lassen were in eruption, there were produced the heavy rolling clouds, the lightning, the wind-rush, and the downpour. And not only is there direct rain-making close to the volcano: indirectly and at a distance eruptions cause rain, since the gases and fine ash or dust are carried far and wide by the winds, and, serving as nuclei, they increase the rainfall in countries far removed from the scene of outbreak. Someone will say, do not these facts prove that the claims of 'rain-makers' regarding explosions and rains are correct? The answer is, not quite. The explosive output and the atmospheric disturbance in the two cases are not comparable. For example, during one of the recent eruptions of Asama Yama, pressure disturbances were recorded on all the barographs in Japan; but the daily noon gun fired close to the Observatory in Tokyo never affects the instruments.
The idea that concussion alone produces rain, then, may be dismissed, as there is no removal or transportation of either water-vapor or nuclei by these compressional waves. And here we may explain the seeming relation of thunder-clap and rain-gush. There is probably marked electrical action facilitating the formation of big drops before, during, and after a flash of lightning. But the lightning, the beginning of the thunder, and the downward start of the raindrops, even if simultaneous, would appear to a person below as occurring one after the other, because of different speeds of propagation. We see the lightning as soon as it occurs because the velocity of light is 300,000 kilometers per second; we hear the thunder five or six seconds later, because the velocity of sound is only 0.33 kilometers per second, and we note the rain-gush still later because its velocity is, perhaps, only 0.03 kilometers. The rain may well have started before the flash occurred or the thunder began. It is also of interest to know that estimates have been made of the amount of energy represented in a thunder tone, if one may use this phrase for what is really a noise and not a tone. In nearly all loud thunder-claps there is one violent or shock wave, a sound wave that travels out in all directions from the path of discharge or core of incandescent air. Dr. Wilhelm Schmidt has shown us how the prolongation of the sound is largely a reflection, not so much from the clouds and sheets of falling rain, as from the 'interfaces' between atmospheric strata of different temperatures, largely by the action of wind. Thus the original sharp report becomes a prolonged roll. In a certain peal which he analyzed, the thunder lasted thirteen seconds.
A word or two is in order regarding the claims of those who insist that explosions, particularly gunfire, are accompanied by or cause rain. Edward Powers published a book in 1890 proving to his own satisfaction that the great battles of the Civil War were followed by heavy rain. A wider study of the facts does not bear out the statement. This volume, War and the Weather, led to an appropriation by Congress of the sum of $10,000 for experiments in producing rain by the use of high explosives. The writer witnessed some of these experiments, made under favorable conditions. There was no evidence of a causal relation between the detonations of the dynamite and the showers. Again in the course of a long residence in California he had occasion to follow closely the operations of certain much talked-of 'rain-makers.' Evidence of the production of rain directly or indirectly was lacking. An incident may be referred to here since it illustrates how popular opinion is formed and passes. During the course of a prolonged dry spell a meeting of prominent citizens of a certain town was held to consider the acceptance of an offer from a temporary resident to furnish enough explosives to produce rain. The visitor claimed that he had caused rain on his ranch in Texas by such means. While the meeting was in progress the long deferred rain began falling, and interest in the question waned. If the meeting had been held a day or two earlier and the explosives been used, credit for making the rain would naturally have gone to the visitor; and it would have been a difficult matter to convince the citizens that the test was not a valid one. It would be a rash man who would say that condensation and precipitation on a commercial scale are beyond human control; but certainly we lack conclusive evidence that any of man's efforts have produced rain in measurable amount. Finally, if the war is not the cause of the abnormal weather, what is? We do not know. The weather map tells only a part, and a very small part at that, of atmospheric motion; and it frequently misleads the forecaster. The writer speaks feelingly, for he has had the unique experience of forecasting the weather in Washington for all of the Eastern states, again at New Orleans for the Gulf section; and for many years at San Francisco for the Pacific and Inter-mountain states. Sometimes it has seemed to him that it was the valor of the forecaster rather than the value of the forecast which deserved commendation. But the time is coming when our information will be extended to all atmospheric levels available, and not limited to one,—that near the ground,—as at present. The newer meteorology, which may well be called aerography or the science of the structure of the air, will undoubtedly throw light on cloudiness and rain formation. At present we can only correlate the excessive rains and certain temperature departures over wide areas with displacements of the major pressure areas,—'hyperbars' and 'infrabars,' as they are termed. And we know, too, that excessive rains have occurred in previous years when there were no wars; and in all probability will occur again, regardless of the prevalence of gunfire.