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.