Popular Science Monthly/Volume 47/September 1895/Natural Rain-Makers
| NATURAL RAIN-MAKERS. |
By ALEXANDER McADIE.
THE efficiency of the clouds in lifting water will be brought home to us if we consider the rainfall over a garden fifty feet wide and one hundred feet in length. If one hundredth of an inch of rain occurs, about twenty-five gallons or two hundred and fifty pounds of water will have fallen. One inch of rain over the garden would mean twenty-five thousand pounds of water.
A rainfall of forty-five inches in a year is not an unusually large rainfall. New York city has a mean annual rainfall of 45·2 inches, the observations covering a period of twenty-two years. If this rain of a year fell in equal amounts each day, we would have for every acre of surface two thousand eight hundred gallons of water, or in avoirdupois nearly nine thousand tons of water to the square mile. Tipping Manhattan Island each evening and draining it would give two hundred thousand tons of water. In a year over seventy million tons of water are dropped on the roofs, sheds, and pavements of Manhattan Island.
It requires a powerful pump to lift water in such quantities and store it in reservoirs thousands of feet above us. And these reservoirs are remarkable; for they have no walls of rigid masonry, and they course across the sky at higher speed than man can travel. A locomotive can travel a mile in thirty-seven seconds, a fast yacht in about twice that time, and a swift torpedo boat in one hundred and ten seconds. The upper clouds move with an average velocity of a mile in thirty-six seconds, and have been observed moving as rapidly as a mile in eighteen seconds. Equally remarkable are the plastic walls of these aerial reservoirs. No courses of heavy stone and mortar are to be found; but in their stead drops of water so minute that a thousand of them side by side would not extend farther than one inch. If the temperature
Alto-cumulus.
was low during the building of the cloud, the water drops are changed into ice spicules and snowflakes.
From such reservoirs the rain falls as a rule harmlessly. A collapse, which rarely occurs, is known as a cloud-burst. Then, the deluge destroys life and property, sweeping all before it.
If we were able to control the valves and vents of this tremendous pump-reservoir, we could cause rain at will and shut off the downpour at pleasure. But hardly yet may we hope to master the rain. Rain-makers of our time bang and thrash the air, hoping to cause rain by concussion. They may well be compared to impatient children hammering on reservoir walls in a vain endeavor to make the water flow. Rain-control is a scientific possibility. Successful rain engineers will come in time, we venture to predict, from the ranks of those who study and clearly understand the physical processes of cloud formation.
Cloudland, for a realm so near us and so closely associated with our welfare, has been sadly slighted by man's genius. The ancients were surprisingly stupid in their views and discussions of air, wind, and clouds. The wisdom of Aristotle, filtered through the mind of his favorite pupil Theophrastus of Eresus, does not show to advantage in these subjects. Nor have the moderns achieved much that is worthy of detailed mention until a comparatively recent period.
Our cloud names date from the beginning of the century. At a meeting of the Askesian Society in 1802, a young chemist of Tottenham read an essay in which he proposed the terms stratus or sheet, cumulus or heap, and cirrus or feather for cloud names. One attempt at cloud classification had been made previously, but Howard's scheme was so superior that it at once received recognition. The essay was reprinted, translated, and officially adopted in all the great countries of the world. While Howard's name is known to all meteorologists, little has been handed down concerning the man himself. He is quaintly described on the title-page of his three-volumed Climate of London, as a Citizen of London, Honorary Citizen of Magdeburg, and Honorary Associate of the Art Societies of Hamburg and Leipsic. No less a person than Goethe was among those who were charmed by Luke Howard's work. A friendship sprang up, a long correspondence was carried on, and the poet sings of Howard as one worthy of all honor.
Within the past few years the leading countries of the world through their representatives on the International Meteorological Committee have decided to depose the Howardian nomenclature. The proposal was made four years ago at the Munich Conference, and at Upsala last year a new classification was formally approved. Some of the more prominent sponsors for the new system are Hildebrandsson, Köppen, Neumayer, and Rotch. Modern meteorology demands more than a record of the appearance of the cloud. It seeks the meaning of each formation. The cloud is primarily valuable not on account of its beauty but because it makes manifest atmospheric motions and conditions not otherwise noticeable. A striking illustration of the use which modern meteorology makes of the clouds is found in the storm of August 26 to 29, 1893. This is the storm more familiarly known as the Sea Islands storm, in which eleven hundred lives were lost. At a critical moment the telegraph lines were blown down and all reports were missing south of Savannah. It is said that the storm center was accurately located by the forecasting officials by means of the clouds at distant stations. Great progress has been made in the past five years in our knowledge of clouds. Two masters in physical science, von Helmholtz and Hertz, were brilliant cloud investigators. The former explained the formation of cloud billows; the latter devised a graphic method of following the adiabatic changes in moist air. The number of tiny solid particles in a cloud can even be counted. John Aitkin, of Edinburgh, has constructed a dust-counter delicate enough to do this. The dust nuclei in the smoky air of London, on the quiet shores of the Mediterranean, on Alpine peaks, or in the pure mists of the Scotch Highlands can be counted and
Fracto-nimbus. Advance Clouds of Thunderstorm.
their influence in the making of rain properly appreciated. Both in Europe and the United States meteorologists are studying clouds. At Berlin, Storlein, Upsala, and Blue Hill observers are daily determining cloud heights and velocities, and in the coming year forces will be massed and something akin to a systematic survey of cloudland attempted.
Poet, painter, and all of us have felt the keen delight of following the cloud transitions of a summer sky. All men in all lands are nephelolaters or cloud admirers—for the cloudscape gives all that the most varied landscape can offer. A generous sky knows no difference between the sons of earth, and spreads everywhere scenes of wondrous grace and color. Even the most commonplace cloud formation—fog—which on earth is often aggravating and trying to health and temper, becomes beautiful as soon as it leaves the earth.
A fog may be defined as a cloud viewed from within, and is therefore the first distinct cloud type. The next low type is the stratus or "raised fog," less than one thousand metres high. And here it may be noticed that in summer the earth pushes her cloud mantle away from her and draws it closer to her in winter. In other words, clouds are lower in winter than in summer. The highest cloud is the cirrus, with a mean elevation of nine thousand metres. The cirrus is a fine, featherlike cloud, and its neighbor, cirro-stratus, something like it, only more diffuse and lower. When a veil of cirro-stratus is drawn before the sun or moon, large halos forty-four and eighty degrees in diameter, with faint red on the inside or nearest the sun, and blue on the outside, appear. These are caused by the refraction of light by ice crystals. A lower cloud, alto-stratus, without causing halos may cause coronas or smaller circles of prismatic colors, about one fourth the diameter of halos. In coronæ the red is on the outside. The Brockenspecter is a particular kind of coronal cloud shadow. Midway between high and low clouds are the cirro-cumuli and alto-cumuli. These give perhaps the most beautiful of all cloud effects. The fairest meadows of earth seldom show such flocks grazing so leisurely and scattered so harmoniously. Cirro-cumuli are small, white, fleecy clouds, often arranged in rows, while the alto-cumuli are denser, larger, and less regular. Both types are like tranquil fleets upon a serene sea. "Their very motion is rest," as John Wilson said of them long ago. Trailing in lustrous glory before the midnight moon, they turn into silver bars and "streak the darkness radiantly." Of the low clouds, the stratocumuli and nimbi are most common: the former, large rolls of dark cloud, often covering the whole sky and of somewhat dreary aspect; the latter, nondescripts without definite form and with little gradation in color. The sky effects of both are as a rule somber and depressing, though there are times, especially if the sun be close to the horizon, when the nimbus gives the golden rain of Greek mythology, a downpour inexpressibly beautiful. The cumuli and cumulo-nimbi are the largest clouds in cloudland. The familiar "castles in air" are the turreted cumuli, thick clouds with domes and summits. The cumulo-nimbus, or towering thunder cloud, rises mountain high, and has peaks of snowy whiteness with a flat and frowning base. Its monstrous size can be better appreciated if we imagine Mont Blanc (14,134 feet high) lifted into the air and set down on top of Mount Washington (6,279 feet). This would make a medium-sized cumulonimbus. The thunder cloud is noteworthy in another respect, namely, that the water in it may be cooled below the freezing point and yet not frozen. A snowflake or ice crystal falling into it may suffice to start a sudden congelation, just as we may see ice needles dart in all directions when the chilled surface of a still pond is disturbed. We liken this monstrous cloud to a huge gun loaded and quiet, but with a trigger so delicately set that a falling snowflake would discharge it. The sudden puffs, gusts, and elongations of the thunder cloud may have their origin in this way. Again, there is every reason for believing that electricity plays an important part in the enlargement and subsequent history
Cumolo-Nimbus.
of this cloud. We have ourselves measured with sensitive quadrant electrometers the pull in volts experienced by the air between one of these clouds and the ground. The approach of the cloud can be foretold without seeing it and the sky mapped out roughly by the changes in the electrical potential caused by the passage of the cloud.
From what precedes it will be readily understood that cloud motion is not always a true exponent of air motion. Meteorologists know that it is not safe to obtain the motion of the air currents from the motion of the clouds, for the latter may move faster or more slowly, or even apparently stand still in the wind, as in the "table-cloth" cloud on Table Mountain at the Cape of Good Hope. In reality the cloud is changing rapidly, forming and dissolving at one and the same time.
In forecasting weather, clouds have, as we all know, special significance. They are the true robes and garments of earth. The poet sings of hills clad in verdure, the mantle of tender green that the Earth puts on in the spring, and the splendid hues of her autumnal dress; but the garment which protects old Earth the year round from extreme temperatures is the cloud layer. Where there is little cloudiness the range of temperature is large, and where there is much cloudiness the temperature is very even.
So, while the clouds delight us, they are also active for our welfare. In never-ending procession they move—ragged ranks of fracto-nimbi jostled by frowning cumuli, tatterdemalion scud leading an army of mighty nimbi, the baleful funnel cloud, hovering and ill-omened, rolling strato-cumuli that lie far out on the flank; thus they pass, while in the calm above appear the cirri dainty and lacelike, or curling wisps of laughing cirro-stratus.
