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.