Meteorologists have been in much perplexity over the naming and classification of the various deposits of atmospheric moisture known collectively as “precipitation.” The subject is one to which a good deal of attention has been paid in recent years, but it must be admitted that, even at the present time, the terminology of this group of atmospheric phenomena is not yet satisfactorily settled, either in English or in any other language. When, for example, a record of weather occurrences states that hail has fallen, this statement, unequivocal as it may seem to the layman, often raises a question in the mind of the meteorologist. For centuries people talked and wrote about hail before it occurred to men of science to inquire whether one and the same thing was always described under this name. The pursuit of this inquiry led to disconcerting results, one of them being the discovery that we do not now know, in many cases, what bygone weather observers meant when they made the entry “hail” in their records. There are at least three different kinds of icy lumps and pellets that fall from the sky, and they have all been called hail. What science now regards as true hail occurs only in connection with thunderstorms, and therefore chiefly in warm weather. It Although this decision of the Weather Bureau was arrived at only after an exhaustive overhauling of literature and much correspondence with philologists, scientific men, engineers, and others, it remains to be seen whether it will eventually prevail throughout the English-speaking world. In England “sleet” nearly always means a mixture of snow and rain. On the other hand, a great many Americans have been in the habit of applying this term This leads us to another difficulty. The icy coating just mentioned has, for some years, been called “glazed frost” by the British Meteorological Office, and the United States Weather Bureau now calls it “glaze.” It has likewise been called, even in scientific books, “silver thaw”; and an instance of its occurrence on a large scale is termed, both popularly and scientifically, an “ice storm.” To pursue this lamentable record of cross-purposes just a little further, it may be added that the expression “silver thaw,” besides being one of the aliases of glaze, or glazed frost, has been applied in various official British publications, until recently, to a very different rough or feathery deposit of ice from fog, now called by both the Meteorological Office and the Weather Bureau “rime.” Needless to say, when the scientific authorities are unable to agree about these terms, our dictionaries are sadly at sea in regard to them; so, altogether, the task of writing a chapter on precipitation is beset with verbal difficulties that would not be encountered in writing on many far more recondite subjects. Fortunately the name of the most important kind of precipitation—rain—is reasonably free from ambiguity. To be sure, opinions may differ as to whether a “Scotch mist” is a rain or a wet fog—and if one happened to have insured a lawn fÊte against rain at Lloyds’ the uncertainty on this point might lead to litigation—but, generally speaking, “Rain,” says Dr. Hellmann, “is the most widespread, most frequent, and most copious form in which the aqueous vapor of the atmosphere condenses. The area of its distribution embraces the whole surface of the earth, with the exception of the interior of Antarctica and probably of northern Greenland. The English and the Norwegian expeditions found no rain even at the edge of Antarctica. As the land rises inland to an altitude of about 2,800 meters about the South Pole, it may safely be assumed that only snow and no rain falls in the heart of Antarctica. At the North Pole, which lies in the midst of the sea, it probably rains at times; while on the high plateau of northern Greenland probably snow alone falls. As to its frequency, there are arid regions in which the average annual number of days with rain is less than one, while this number probably rises to 280 in some tropical districts. With the exceptions of the polar regions already mentioned, there are probably no regions where it never rains.” In its intensity rain varies all the way from the finest drizzle or the sprinkle of occasional drops up to the torrential downpours often known as “cloud-bursts.” Before citing instances of heavy rains, it may be well to remind the reader that an inch of rainfall is equivalent to 101 tons of water per acre, or 64,640 tons per square mile. In the county of Norfolk, England, in August, 1912, a single day’s rainfall brought down 670,720,000 tons of water—more than twice the volume of water contained in England’s largest lake, Windermere. Doubtless this record, for showers of similar extent and duration, The most remarkable example of a heavy brief shower was recorded at Porto Bello, on the Isthmus of Panama, May 1, 1908, when a fall of 2.47 inches in three minutes was registered by a self-recording rain gauge. An average heavy rainstorm in the eastern United States yields about this amount in twenty-four hours. At Baguio, in the Philippines, forty-six inches of rain fell from noon of July 14, 1911, to noon of the following day—probably a “world record” for twenty-four-hour rainfall. The corresponding record for the United States is 22.22 inches at Altapass, N.C., July 15–16, 1916. Statistics of what the meteorologist calls “excessive rainfall”—i. e., abnormally heavy rain during brief periods—have been collected over the greater part of the world for much more weighty reasons than to satisfy curiosity as to which showers were “record breakers.” Such data are indispensable to engineers in connection with the building of sewers, reservoirs, and dams, and in flood-protection work. Sewers must be made large enough to carry off the “storm water” from the heaviest showers that ever occur in the locality; while, on the other hand, in the absence of rainfall statistics, much money might be wasted in making them larger than necessary. A great flood raises questions as to the intensity of the rainfall that caused it, and the frequency with which similar rains may be expected to occur in the drainage area concerned. The unprecedented floods in the Ohio Valley and adjacent regions in March, 1913, which caused losses amounting to $200,000,000, led to an exhaustive study of the records of storm rainfall in the eastern United States, carried The distribution of rainfall over the earth (using the word “rainfall” in the broad sense to include snow reduced to its water equivalent) is most conveniently described in terms of mean annual values. This element is very unevenly divided between different parts of the globe and between the different regions of every large country. The raininess or dryness of a climate is determined especially by the prevailing movement of moisture-bearing winds and the relief of the land, while a second important control is the location of a region with respect to storm tracks. The rainiest regions are found on the windward slopes of mountain ranges not far from the ocean, where the moist winds, forced by the mountains to ascend rapidly, cool dynamically and shed their moisture. Thus the southern slopes of the eastern Himalaya receive an enormous rainfall from the southwest monsoon, blowing from the Indian Ocean, and an abundant rainfall also prevails on the south slopes of the high mountains of eastern Tibet, while northern Tibet, in the lee of the mountains, is a desert. For more than half a century the little hill station of Cherrapunji, in Assam, at an altitude of 4,100 feet, has been credited with having the heaviest rainfall in the world. According to the latest official The heaviest average annual rainfall in the United States (not including Alaska) is about 130 inches in Tillamook County, Ore. Over most of our Atlantic seaboard States the rainfall ranges from forty to fifty inches. Extensive tracts in southern California and western Nevada have a rainfall of five inches or less. Any region with an annual rainfall of less than ten inches is normally a desert, though irrigation or “dry farming” methods may enable its inhabitants to practice agriculture. The process of rain formation is not well understood. As we have seen, the existence of nuclei in the air serves to explain why, when the conditions of temperature and humidity are right, moisture condenses in the tiny droplets that constitute clouds. The difficulty is to explain how, at certain times, quantities of drops are formed of a size large enough to carry them rapidly to the earth. The number of The speed with which drops fall through the air, which is only a fraction of an inch per second for the average cloud droplet, increases rapidly with the size of the drop up to a certain point, but for the drops that reach the earth as rain the speed of fall tends to become approximately uniform. Several investigators have measured the size of raindrops. One method of doing this is to allow the drops to fall into a shallow layer of fine, uncompacted flour. Each drop forms a little pellet of dough, which is found, by experiments with previously measured drops (produced for the purpose and dropped from various heights), to correspond very closely with the size of the drop. These pellets dry and harden, and can then be carefully measured, photographed, etc. Hundreds of samples of raindrops were thus measured by Mr. W.A. Bentley of Jericho, Vt., and the measurements were tabulated with reference to the kinds of clouds from which they fell, the distribution of large and small drops in the different parts of a storm, and other circumstances attending their fall. Drops of very different dimensions are found to fall at one time. The commonest sizes recorded by Bentley were from one-thirtieth to one-eighth of an inch in diameter; but many While rain is often the final product of snow that melts before it reaches the ground, snow is probably never formed from raindrops, but always condensed For ages mankind has admired the diversity of beautiful forms exhibited by snow crystals. Drawings of such crystals, and also of the frost tracery on window-panes, were made as early as the sixteenth century, by the learned Swedish historian Olaus Magnus, and many collections of similar drawings have been published since his time; but nowadays the combination of the camera and the microscope gives us a far greater wealth of information concerning these interesting objects. Bentley, whose study of raindrops we have just mentioned, has made and published photomicrographs of hundreds of different forms of snow and ice crystals, and several collections have been published in Europe. One of the facts revealed by the camera is that the perfectly regular forms of these crystals seen in drawings are comparatively uncommon in snow as it reaches the ground. A snow crystal is so fragile that it is easily mutilated by the wind and by contact with other crystals. In very calm weather and at the beginning of a snowstorm many single and perfect crystals are wafted gently to the ground, and their beauty is revealed when they fall on dark objects, especially if they are examined with a magnifying glass. In spite of their immense variety in detail, all perfect snow crystals and other ice crystals have six sides or principal rays. When secondary rays form they are parallel with the adjacent The cohesive character of moist snow, which is utilized by the younger generation in the making of snowballs and snow men, enables this substance to assume naturally a variety of striking forms. Thus a strip of snow lying along a window ledge or the branch of a tree, will sometimes slip down in the middle and hang in festoon-shape, supported only at the ends, constituting a “snow garland.” Over a level or gently sloping surface of snow the wind occasionally rolls muff-shaped snowballs, which are known as “snow rollers.” Thousands of them are sometimes formed at once, and the largest may grow to the size of barrels. Huge overhanging caps of snow formed on tree stumps, posts, and the like have been aptly named “snow mushrooms” by Mr. Vaughan Cornish, who has described those that occur in great numbers in the Selkirk Mountains of western Canada. Perhaps the strangest of all the shapes assumed by snow is seen in the greatest perfection in the high Andes of tropical Argentina and Chile. Here are found innumerable pinnacles of snow or glacier ice, averaging from four to seven feet in height, though sometimes much higher. Viewed from a Snow has its economic aspects, comparable in importance to those of rain. The problem of snow-removal crops up every winter in our American cities, and is not always solved with brilliant success. In the larger cities of Europe snow is removed by spreading salt on the streets to reduce the snow to slush, which is then washed into the sewers with water, but this method does not seem to be generally applicable to the heavier snowfall of this country. The snow-removal conference held by a number of municipal engineers in Philadelphia in 1914 brought this difficult phase of street cleaning prominently before the engineering world, and it has been actively discussed in recent years in the technical journals. Snow presents a formidable problem in the operation of many railway lines, the solution of which takes the form of snow sleds, fences, plows of various types, flangers, gasoline torches for melting snow in switches, etc. The heaviest snowfall in the United States occurs in the high Sierra Nevada of California and in the A fall of snow under a cloudless sky is fairly common in the polar regions and is sometimes observed in calm and very cold weather in the temperate zones. Rain from a cloudless sky is a more doubtful phenomenon, of which only a few observations are recorded, most of them of early date. If such rain occurs, it may come from clouds that have passed beyond the horizon before the raindrops reach the earth. Probably the older reports of this phenomenon really relate to dew, which was once believed to fall from the sky. Of the three haillike forms of precipitation that we have mentioned above, true hail is much the most important, on account of the large size sometimes attained by hailstones and the damage that they are consequently able to cause. The maximum possible size of a hailstone cannot be positively stated, but stones larger than a man’s fist and weighing over a pound have several times been reported on good authority. During a hailstorm in Natal, on April 17, 1874, stones fell that weighed a pound and a half and passed through a corrugated iron roof as if it had been made of paper. Hailstones fourteen inches in circumference fell in New South Wales in February, 1847. At Cazorla, Spain, on June 15, 1829, houses were crushed under blocks of ice, some of which are said to have weighed four and one-half pounds. In October, 1844, a hailstorm at Cette, France, wrecked houses and sank vessels. Hail appears to be formed in the violent updraft of air at the front of a thunderstorm. In this turbulent region the hailstone, first frozen at a high level, probably makes several journeys alternately up and down, as it encounters stronger or weaker rising currents; at one time gathering a coating of snow aloft, and at another a coat of ice from the rain below, until finally, on account of its large size or on account of a weakening of the upward blast, it falls to the ground. A record of these ups and downs in the life of a hailstone is seen in the concentric layers of clear and snowlike ice of which it is composed. Although, from immemorial usage, we still speak conventionally of the “falling of the dew,” it has now been known for more than a century—especially since the publication of Wells’s “Essay of Dew” in 1814—that dew does not fall. The cooling of air below the dew point of its water vapor by contact with any cold object results in a deposit of visible moisture, which is liquid or frozen, according to whether the temperature is above or below the freezing point, respectively. This process is not exclusively nocturnal. It is observed by day in the Hoarfrost is often described as “frozen dew.” This expression is misleading, for, although dew-drops are sometimes frozen into little globules of ice, hoarfrost is more often condensed directly from atmospheric water vapor in the shape of ice crystals. “Glaze” and “rime”—to use the latest official designations of the two kinds of ice coating formed from water in the atmosphere—differ greatly in appearance, as a rule, though transition forms are sometimes found. Glaze is produced by the falling of rain on surfaces whose temperature is below freezing, and is typically smooth and transparent. Rime is a rough deposit formed from fog, the drops of which are “undercooled”—i. e., are below the freezing point—and turn to ice on coming in contact with solid objects. The most remarkable examples of rime are seen on mountains and in the polar regions. It occurs on the branches and leaves of |