THE CAUSE OF RAIN. 89 



THE CAUSE OF RAIK* 



By J. E. PLUMANDON, 



METEOROLOGIST AT THE OBSERVATORY OF THE PUY-DE-DOME. 



A GREAT many theories have been invented to explain the 

 •*—*- formation of rain, some of which are remarkable chiefly for 

 their absurdity or their complexity. Even most of the scientific 

 theories depend too largely on hypotheses and are not sufficiently 

 supported by facts. There are, however, some which are as a whole 

 established on authentic observations, and, although they are still 

 incomplete, they do not, like some of the speculations, contradict 

 facts that are observed every day. For more than thirty years I 

 have studied professionally and because I had a taste for it all the 

 atmospheric phenomena which came before me. Several times I 

 have been so fortunate as to witness, at Clermont, or on the top of 

 the Puy-de-D6me, the genesis or development of heavy showers, and 

 have fancied that I have detected some of the details or secrets of 

 their formation. In a pamphlet on this subject, which I published in 

 1885, I expounded the ideas whjch a large number of observations on 

 fog, drizzle, mist, rain, snow, sleet, and hail had suggested to me; 

 and by means of some of these ideas, the resultant of facts observed 

 hundreds of times, I hope to be able to explain the formation of 

 rain. 



First, I must say that heat, and especially moisture, do not vary 

 in the lower part of the atmosphere in the way it was long thought. 

 At extreme altitudes the temperature of the air is very low, but the 

 cold does not increase regularly as we rise, and the same is the case 

 with the moisture. In high ascensions, or while sailing almost hori- 

 zontal courses, aeronauts traverse atmospheric regions alternately 

 warm and cold, dry and moist. Such anomalies present themselves 

 even near the surface. There are between eighty and a hundred days 

 every year in which a higher temperature is registered for a greater or 

 less length of time on the Puy-de-D6me than at Clermont. Sometimes 

 the difference is very great. Thus, on the 26th of December, 1879, 

 the temperature was — 16° C. at Clermont, while on the summit of 

 the Puy-de-D6me the thermometer marked +5° C, showing a dif- 

 ference of 21° in favor of the top of the mountain. Differences 

 of temperature of this kind occur everywhere. The moisture of the 

 air varies in the same way through the atmosphere. In ascending 

 or descending a few hundred metres, the hygrometer may be observed 

 to pass from dryness to saturation. At the altitude of the Puy-de- 



* An address before the Society of Horticulture and Viticulture at Clermont-Ferrand. 



