THE CAUSE OF RAIN. 



93 



solitary cloud — has been seen, on the Puy-de-D6me, to produce rain 

 and lightning, with thunder. 



Frequently, under the influence of the centers of perturbation 

 which often exist south of the Alps, a vast sea of clouds, the upper 

 face of which does not exceed an altitude varying from seven hun- 

 dred to twelve hundred metres, covers all central France, and proba- 

 bly other countries. Only the high table-lands and mountains rise 

 above this stratum of clouds over which the sun shines in a perfectly 

 clear skv. Yet rain is found in such strata of clouds, however homo- 

 geneous they may be, and it rains in the regions they cover. I 

 have long been able to affirm this fact, important because it destroys 

 old errors elaborated in the isolation of the study, and to support it 

 with authentic proof. 



We may witness the formation of rain when we rise into the usual 

 region of the clouds, either in balloon ascents or by climbing moun- 

 tains. 



The phenomenon may be observed under five aspects: First, we 

 may find ourselves in a fog of greater or less thickness, the hy- 

 grometer indicating that the air is nearly saturated with vapor, with- 

 out one being able to detect the fall of the smallest liquid particle, 

 and without exterior objects being moistened. Second, while we can 

 not observe the fall of a single liquid drop, however small, every- 

 thing enveloped in the cloud will be rapidly moistened. We are in 

 the atmospheric stratum where the rain is beginning to form. In- 

 habitants of mountainous regions say at such times that there is a 

 wet fog. At the top of the Puy-de-D6me, when this condition lasts 

 for a day, we can collect three, four, or five millimetres of water. 

 Third, we may remark, in the fog, the fall of exceedingly fine drop- 

 lets, which we can hardly distinguish — it is drizzling. Fourth, the 

 rain is falling, while we are still in the fog; and, fifth, the rain is 

 falling and we are below the fog — that is, below the clouds. 



These five aspects may be present in the same cloud, when we 

 will find them in the order given in successive strata, one beneath an- 

 other; so that, entering such a cloud from the upper part, we may 

 traverse, in regular order, " dry " fog, wet fog, fog with drizzle, fog 

 with rain, and, as we leave the- cloud at the bottom, rain without 

 fog. Mr. Glaisher, the English scientific aeronaut, thus records his 

 experience in an ascension he made July 1, 1863: " We let ourselves 

 drop at eight hundred metres, and went into a fog which was dry for 

 the first thirty metres, but shortly afterward became moist. As we 

 descended, the fog seemed to become more charged with water, and 

 seemed very dark beneath us; at five hundred or six hundred metres 

 we heard the sound of the rain striking the trees, so violent was 

 the fall." 



