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.

These fogs finally become stationary in a region of the same density with themselves. There they accumulate and form a cloud or a group of clouds that go on developing. When penetrated by the rays of the sun, which they almost wholly absorb, these clouds are warmed up again in the interior, and budding protuberances are seen, which are especially developed on the upper parts of the cloud. These protuberances are formed and grow so rapidly as to almost suggest the presence of a steam generator within every cloud. The external parts of the cloud, however, cool very soon by radiation, evaporation, or dissolution, but especially by their contact with the cold air, into which they continue going. Hence, when the vapors emitted by the cloud reach its periphery, they are cooled at once as if in a condenser; they then take on a rapid movement of descent, which is easily distinguished, and suffer condensation in their lower parts. As the surface of the cloud in contact with the cold air around it is considerable in proportion to that which receives the influence of the solar rays, the warm ascending currents slacken speed and are extinguished, because the cloudy mass, drawn on by the higher currents, removes from the place where it is formed, or because it stops the rays of the sun and prevents their reaching the ground. There results a more and more complete condensation, and the watery vapor is at last transformed into drops of rain. The condensation into rain is accelerated and augmented when the mass of cloud rises with great rapidity, especially when it enters abruptly into very cold atmospheric strata. A sudden mixture of the cloud with the air around it takes place then, and sudden and abundant rains result like those which are produced at the instant of thunderstorms.

The formation and mixture of masses of air of different temperatures are effected by ascending currents in zones of restricted extent, but sometimes very numerous. Local showers and thunderstorms are produced in this way. The phenomenon becomes much more important and at the same time extends over vast regions, when it is brought about by the aid of the wind and the larger movements of the atmosphere, and general rains result.

Babinet, in his Studies on the Sciences of Observation, explains the formation of rain by supposing that when the wind meets an obstacle, it ascends; the moving air cools in rarefying, and deposits its excess of vapor over saturation. This fact, when it occurs, should indeed contribute to the condensation of the vapor contained in the air; but it does not afford an adequate explanation of all rains; for, first, how can it rain on the vast oceans which present no obstacles to cause the air to ascend? It is necessary to suppose that internal movements of the atmosphere intervene in the production of rain.

Monk, Mason, de Saussure, and many others fix the prime condition for the formation of rain in the superposition of two beds of cloud. This assertion, although it is still repeated in a number of treatises on physics, is inexact. A single stratum of cloud—yes, a solitary cloud—has been seen, on the Puy-de-DÔme, 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 hundred to twelve hundred metres, covers all central France, and probably other countries. Only the high table-lands and mountains rise above this stratum of clouds over which the sun shines in a perfectly clear sky. Yet rain is found in such strata of clouds, however homogeneous 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 mountains.

The phenomenon may be observed under five aspects: First, we may find ourselves in a fog of greater or less thickness, the hygrometer indicating that the air is nearly saturated with vapor, without 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, everything enveloped in the cloud will be rapidly moistened. We are in the atmospheric stratum where the rain is beginning to form. Inhabitants of mountainous regions say at such times that there is a wet fog. At the top of the Puy-de-DÔme, 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 droplets, 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 another; 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 aËronaut, 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."

Rain drops, in fact, grow as they fall, whether by continuance of condensation, or by union with other drops. They should, therefore, be larger when they issue from the cloud in proportion as the region where drizzle is formed is higher above the base of the cloud. There is, however, a limit to the size they can attain, for the velocity of their fall increases with their mass, and they are divided by the resistance of the air.

The five aspects under which we have regarded the formation of rain are evidently five phases distinguished by our senses in the progressive transformation which the vapor of water undergoes in passing to the liquid state. It also sometimes happens that the condensation of the vapor in a cloud can only reach the first or second stage of the transformation without extending to the other stages. At other times it stops at the third phase, that of drizzling, which may then, as rain does, cross atmospheric regions below the cloud, and reach the ground, provided the base of the cloud is not too high and the air passed through is not too dry. In short, we may conclude that the formation of rain is due simply to variations in the temperature and moisture of the air. There is, however, another element, the intervention of which is indispensable, if not to reduce the vapor to water, at least to cause that water to fall in rain, or under the form of drops. This element is the atmospheric dust.

We designate generally as atmospheric dust all the corpuscles which the atmospheric envelope of the earth holds in suspension; but distinctions should be made. Some dust occurs in the air fortuitously and for the moment, such as troubles us in dry weather when the wind is blowing. This is coarse, and so evident that we say "It is dusty," and soon falls by its weight to the ground. There is other dust which remains in the air almost permanently. It becomes visible to the eye when illuminated against a dark background, as when a sunbeam comes into a dark room. Other dust may be studied under a microscope of low power; and still other, and the largest proportion of that in the atmosphere, is so fine that it can not be distinguished, even with the most powerful instruments.

This extremely fine and light dust is disseminated to heights that may exceed fifteen or twenty or more miles. Cyclones, volcanic eruptions, and immense prairie fires are the principal causes of its production and expansion in the atmosphere. Mr. Aitken, a Scotch meteorologist, has made some remarkable experiments to demonstrate the existence of this dust. For that purpose he employed a very ingenious method, which permitted him to count all the particles, even those which could not be seen with a microscope. The principle of his method is as follows: If we fill a receiver with air that has been deprived of all its dust by passing it through a liquid, and saturate it with vapor, and then by cooling cause the vapor to condense, the resultant water is deposited directly. If the receiver is filled with air not cleared of its dust, the cooling of the mixture of air and vapor provokes first the formation of a fog that marks the presence of dust, because each particle of dust becomes a nucleus, a center of condensation, for the vapor. Finally, if the cooling is carried far enough, the water formed falls in very fine droplets, each one of which incloses a dust particle. Mr. Aitken has succeeded in counting these droplets, by introducing only a very small volume of dusty air into the receiver and finally filling it with absolutely pure air. He has thus found that the external air contains on the average 32,000 particles of dust per cubic centimetre after a rain of considerable duration, and 130,000 particles in fine weather. There are 1,860,000 particles in the same volume of air in the middle of a room, and 5,420,000 particles near the ceiling. The figures look fanciful, but they are exact, for they have been corroborated by numerous consistent experiments and agree with the determinations that have been made by other methods.

As to the formation of rain, it should be observed that absolutely pure air can not give either fog or drops of water when it is supersaturated with vapor. If there were no dust in the atmosphere we should have no clouds or rain. The sky would always be clear, and the sun would shine uninterruptedly as long as it was above the horizon. There would be no dawn or twilight, and day and night would succeed one another instantly, without transition. Atmospheric water would be deposited only when in contact with things, as in Aitken's experiments, very much as dew is deposited.

The causes of the formation of rain are evidently the same everywhere. The secondary conditions change only according to climates; but they vary so much that rains are distributed very unequally over the earth. According to Desanis, the quantity of vapor contained in a column of air as high as the atmosphere would give, in France, a layer of water about four centimetres thick. Few rain storms would furnish so much; but there are storms sometimes that give much more. On August 17, 1888, seven centimetres of water fell at Clermont in five hours; and September 12, 1875, the pluviometer measured ten centimetres for the whole day. Still more copious rains fall in some tropical countries; at Purneah, in India, eighty-nine centimetres have fallen in twenty-four hours.

Mr. John Murray has calculated, from the charts of Elias Loomis, that the quantity of rain falling every year over the whole earth would form a bed of water averaging nine hundred and seventy millimetres in depth.

When we consider the annual quantities of rain in particular regions or localities, we find the numbers exceedingly variable, and some of them surprising. Clermont receives 630 millimetres, and the mean of the fall in Europe is about the same. About one metre falls on the western coast of Iceland, two metres in Norway, 2.80 metres in Scotland, 4.60 metres at Vera Cruz, 5.20 metres at Buitenzorg, in the Dutch East Indies, 7.10 metres at MaranhÃo, Brazil, and 12.50 metres at Cherrapunji, in British India. On the other hand, it rarely rains in some regions of the globe north and south of the equator; as in the center of the Sahara and of Arabia, the plateau of eastern Persia and Beluchistan, the desert of Kalahari, and the desert of Atacama. The plains or pampas of the eastern slopes of the Andes, in about 23° south latitude, are likewise subject to extreme droughts, in one of which, lasting three years, three million head of cattle perished.—Translated for the Popular Science Monthly from Ciel et Terre.