[THE CAUSE OF RAIN.][12]
By J. E. PLUMANDON,
METEOROLOGIST AT THE OBSERVATORY OF THE PUY-DE-DÔME.
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-Dôme, 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 which 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 horizontal courses, aëronauts 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-Dôme 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-Dôme the thermometer marked +5° C., showing a difference 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-Dôme extreme moisture may succeed almost absolute dryness in a few instants, in a clear sky and without any change of wind.
The parts of the atmosphere included within the same limits of temperature or humidity therefore rarely form concentric or parallel layers. They constitute regions interlacing zones which the clouds, thick or scattered in groups, often mark to our eyes, showing us those which are saturated with the vapor of water. The distribution of these zones in space depends chiefly on the heat action of the sun, and upon preceding and present movements of the atmosphere. Now, if a certain increase of heat is adequate to melt a piece of ice and to transform the water of the fusion into vapor, in like manner a corresponding cooling may suffice to cause the vapor to return to the state of a liquid and then to that of ice. The processes in the atmosphere are not different, and all showers, results of the more or less extensive condensation of the vapor of water, may arise from the cooling of that vapor or of the water which it produces.
A certain volume of atmospheric air is capable of holding in suspension a quantity of water proportioned to the elevation of its temperature. But, for each determined temperature, there is a maximum which can not be exceeded without the excess of vapor returning to the liquid state. If, therefore, an atmospheric region is saturated with vapor, and its temperature falls, that region will give rain. Immense and superabundant causes for the cooling necessary to provoke rain exist in such an atmosphere as we have described. The cooling may take effect in three principal ways: first, by the radiation of different regions between one another and toward interplanetary space, the temperature of which is extremely low, as has been indicated by measurements made in high balloon ascensions; second, by the expansion which air rising in the atmosphere undergoes in being rarefied; and, third, by the mingling of masses of warm or moist air with cold or dry.
Cooling by mixture is the sufficient cause in the majority of cases; and this may be effected from above, by descent of the air from the upper regions; from below, by ascent, with the assistance of rising currents created by solar radiations; or, finally, in any and every direction under the influence of the winds and the general movements of the atmosphere. Furthermore, the cooling need not be very great in order to provoke rain under certain conditions of temperature and humidity of frequent occurrence.
Rain clouds very frequently descend a little below the altitude of the Puy-de-Dôme. It is, therefore, not difficult, in order to determine the degree of cooling necessary for the formation of rain, to take advantage of observations that have been made there. The hygrometer sometimes remains near saturation without there being precipitation of vapor; and, supposing that the temperature is near 3° or 4° C., which is about the mean temperature of the year, it will require a cooling of only one or two degrees centigrade at most for the air to be unable to hold all its vapor and for the excess of it to be transformed into rain. This is confirmed by experiment and observation.
I will mention a remarkable example illustrating this point. Not rarely, when the west wind is blowing violently on the top of the Puy-de-Dôme, an east wind, blowing opposite to it, prevails at Clermont. Then an eddy is formed behind the plateau and the chain of puys that runs from north to south, a little west of Clermont. This eddy gradually becomes a vast whirlwind with a horizontal axis, several leagues long, a few kilometres wide, and seven hundred or eight hundred metres high. It commonly gives rise to an abundant and continuous formation of black clouds, which appear in an instant along its length, following its intersection with the upper current. The phenomenon is frequent, and is sometimes produced under very interesting conditions, as on a certain day when the temperature at Clermont was five degrees above zero, centigrade, while the hygrometer indicated that the air contained seven tenths of the quantity of vapor required for saturation. Under such conditions the temperature on the Puy-de-Dôme would have only had to be a very little above the freezing point for the vapor of the horizontal eddy to be transformed into rain on meeting the upper current coming from the west. Now, on the top of the mountain the thermometer marked 4°C. below the freezing point. Hence, every time the lower east wind increased a little, this having the effect of carrying the vapor and the air of the lower regions a little higher, the black clouds could be seen developing with a recrudescence of intensity. A few instants afterward a torrential rain fell at Clermont.
In some cases—and such frequently occur in summer—the mingling of strata of air of different temperatures is effected by ascending currents. The sky is clear; the moist air in contact with the soil is warmed under the action of the sun, rises, and more or less quickly reaches a much colder stratum. Light mists are formed; they may frequently be seen rising and spreading out over the warmer or moister spots. On the flanks of the Puy-de-Dôme one may often find himself among ascending currents of this sort which succeed one another intermittingly when the air is calm, after a rain; they rise with a velocity of four or five metres at least per second.