CHAPTER VIIPRECIPITATION

Alto-Cumulus Clouds.These clouds always occur in roundish fleecy masses or in elongated fleecy rolls, with blue sky between. A score of different types have been distinguished and named by certain cloud specialists. (Photographed by A. J. Weed.)

Cumulus.Cloud photography has become a special branch of photographic art, entailing not only the use of appropriate lenses and plates, but also of ray filters, or other special devices for sharpening the contrast between the cloud and the blue sky. Mr. Ellerman’s pictures, made on Mt. Wilson and elsewhere in California, stand in the front rank. (Photographed by F. Ellerman.)

One more species of fog requires mention here, viz., the dirty, foul-smelling “painter” of the Peruvian coast, which deposits on vessels lying in the harbor of Callao and elsewhere a slimy brown substance known as “Peruvian paint.” This substance comes from the ocean and is probably due to the decomposition of marine organisms. The “painter” prevails during the months December to April. According to a plausible hypothesis a change in the temperature of the water at that season, resulting from a periodical shift of ocean currents,kills vast quantities of plankton, the decay of which would give rise to the phenomena observed.

Mammato-Cumulus.A rather rare cloud form, associated with thunderstorms and tornadoes. It is known in Scotland as the “pocky” (i. e. baggy) cloud and in parts of England as “rain balls.” (Photographed by L. C. Twyford.)

Cumulo-Nimbus.This is the thundercloud. (Courtesy of the Naval Air Service.)

Clouds, though they are nothing more than masses of fog situated at some distance from the earth, are susceptible of a classification, according to shape and texture, that is not applicable to fog. Among the billions of human beings who, in all ages, have amused themselves by discovering pictures in the clouds it would be remarkable if a good many had not, from time to time, conceived the idea of reducing these pictures to a few general types. According to a note published a few years ago in the “Quarterly Journal” of the Royal Meteorological Society, there is some reason to believe that an elaborate classification of the clouds was in use among the ancient Hindus. A passage quoted from an Indian work of the fourth century B. C. says:

“Three are the clouds that continuously rain for seven days; eighty are they that pour minute drops; and sixty are they that appear with the sunshine.”

In the occidental world, however, we have no record of any attempt to classify the clouds prior to the year 1801, when the following classification was proposed by the French naturalist Lamarck:

In 1803 the English meteorologist Luke Howard published the system of classification that, with some additions and modifications, is now in general use. This system is based upon three fundamental forms; viz., fibrous or feathery clouds (cirrus), clouds withrounded tops (cumulus), and clouds arranged in horizontal sheets or layers (stratus). Intermediate forms are described by compounding the names of the primary types; e. g.,cirro-cumulus,cirro-stratus, etc. The rain cloud is callednimbus. Howard’s classification was quickly adopted in all countries. His definitions were translated into German by no less a personage than Goethe, who, in his enthusiasm over Howard’s achievement, wrote a poem about it, and also a separate poem about each of the principal types of cloud!

The Latin names that Howard gave to the clouds made his system immediately available for international use; and in nearly all of the many systems of cloud nomenclature that have since been proposed the excellent plan of using Latin names has been preserved. Very soon, however, after Howard’s classification appeared, a list of proposed English equivalents of his names was published in the “Encyclopædia Britannica”—which, nevertheless, did not change its name to “British Encyclopædia”—for the benefit of the unlettered majority, supposed to be incapable of using a few Latin terms that were, in fact, shorter and no more difficult to pronounce than their suggested English substitutes! A piquant sequel to this episode is that these superfluous English cloud names, “curl cloud,” “stackencloud,” “fall cloud,” “sondercloud,” “wane cloud,” and “twain cloud,” still survive in the dictionaries—and nowhere else. They are practically unknown to meteorologists, and were never adopted generally by the laity.

Of course some English names, which have been evolved and not deliberately invented, are applied to certain types of cloud in English-speaking countries; but the Latin names, comprised in the InternationalCloud Classification, should be learned by everybody. This classification, which has been adopted by the International Meteorological Committee and is used by all official weather services, is a little more detailed than Howard’s, upon which it is based; and there is a tendency to add new terms to it from time to time.

There are ten principal types of cloud in the International Classification, and the name of each type has an official abbreviation (a great convenience for those who record the clouds from day to day). The following definitions, translated from the French text of the “International Cloud Atlas,” have been published by the British Meteorological Office:

1.Cirrus(Ci.)—Detached clouds of delicate appearance, fibrous (threadlike) structure and featherlike form, generally white in color.

Cirrus clouds take the most varied shapes, such as isolated tufts of hair—i. e., thin filaments on a blue sky—branched filaments in feathery form, straight or curved filaments ending in tufts (calledcirrus uncinus), and others. Occasionally cirrus clouds are arranged in bands, which traverse part of the sky as arcs of great circles, and as an effect of perspective appear to converge at a point on the horizon, and at the opposite point also, if they are sufficiently extended. Cirro-stratus and cirro-cumulus also are sometimes similarly arranged in long bands. [Certain forms of cirrus are called “mares’ tails.” The long bands crossing the sky, as just described, are known as “polar bands” or “Noah’s ark.”]

2.Cirro-stratus(Ci.-St.)—A thin sheet of whitish cloud; sometimes covering the sky completelyand merely giving it a milky appearance; it is then called cirro-nebula, or cirrus haze; at other times presenting more or less distinctly a fibrous structure, like a tangled web.

This sheet often produces halos around the sun or moon.

3.Cirro-cumulus(Ci-Cu.) (Mackerel sky)—Small rounded masses or white flakes without shadows, or showing very slight shadow; arranged in groups and often in lines.

4.Alto-stratus(A.-St.)—A dense sheet of a gray or bluish color, sometimes forming a compact mass of dull gray color and fibrous structure.

At other times the sheet is thin, like the denser forms of cirro-stratus, and through it the sun and moon may be seen dimly gleaming as through ground glass. This form exhibits all stages of transition between alto-stratus and cirro-stratus, but, according to measurements, its normal altitude is about one-half that of cirro-stratus.

5.Alto-cumulus(A.-Cu.)—Larger rounded masses, white or grayish, partially shaded, arranged in groups or lines, and often so crowded together in the middle region that the cloudlets join.

A Cloud Banner Over Mt. Assiniboine, Canadian Rockies.(Photographed by Dr. C. D. Walcott.)

Cirrus(with a few patches of lower clouds in the foreground). This is cirrus, but not of the “mare’s tail” variety. There are many distinct types of cirrus, which have sometimes been given separate names. (Photographed at the Observatory of Trappes, France.)

The separate masses are generally larger and more compact (resembling strato-cumulus) in the middle region of the group, but the denseness of the layer varies and sometimes is so attenuated that the individual masses assume the appearance of sheets or thin flakes of considerable extent with hardly any shading. At the margin of the group they form smaller cloudlets resembling those of cirro-cumulus. The cloudlets often group themselves in parallel lines, arranged in one or more directions.

6.Strato-cumulus(St.-Cu.)—Large lumpy masses or rolls of dull gray cloud, frequently covering the whole sky, especially in winter.

Generally strato-cumulus presents the appearance of a gray layer broken up into irregular masses and having on the margin smaller masses grouped in flocks, like alto-cumulus. Sometimes this cloud form has the characteristic appearance of great rolls of cloud arranged in parallel lines close together (“roll cumulus”). The rolls themselves are dense and dark, but in the intervening spaces the cloud is much lighter and blue sky may sometimes be seen through them. Strato-cumulus may be distinguished from nimbus by its lumpy or rolling appearance, and by the fact that it does not tend to bring rain.

A Lenticular Cloud Over Mt. Rainier

7.Nimbus(Nb.)—A dense layer of dark, shapeless cloud with ragged edges from which steady rain or snow usually falls. If there are openings in the cloud an upper layer of cirro-stratus may almost invariably be seen through them.

If a layer of nimbus separates in strong wind into ragged cloud, or if small detached clouds are seen drifting underneath a large nimbus (the “scud” of sailors), either may be specified asfracto-nimbus(FR.-NB.).

8.Cumulus(Cu.) (Wool-pack cloud)—Thick cloud of which the upper surface is dome-shaped and exhibits protuberances, while the base is generally horizontal.

These clouds appear to be formed by ascensional movement of air in the daytime, which is almostalways observable. When the cloud and the sun are on opposite sides of the observer, the surfaces facing the observer are more brilliant than the margins of the protuberances. When, on the contrary, it is on the same side of the observer as the sun, it appears dark with bright edges. When the light falls sideways, as is usually the case, cumulus clouds show deep shadows. True cumulus has well-defined upper and lower margins; but one may sometimes see ragged clouds, like cumulus torn by strong wind, of which the detached portions are continually changing; to this form of cloud the name fracto-cumulus may be given.

9.Cumulo-nimbus(Cu.-Nb.)(The thundercloud)—Great masses of cloud rising in the form of mountains or towers or anvils, generally having a veil or screen of fibrous texture (“false cirrus”) at the top, and at its base a cloud mass similar to nimbus.

From the base local showers of rain or snow, occasionally of hail or graupel, usually fall. Sometimes the upper margins have the compact shape of cumulus, or form massive heaps round which floats delicate “false cirrus.” At other times the margins themselves are fringed with filaments similar to cirrus clouds. This last form is particularly common with spring showers. The front of a thunderstorm of wide extent is frequently in the form of a large low arch above a region of uniformly lighter sky.

10.Stratus(St.)—A uniform layer of cloud, like fog, but not lying on the ground.

The cloud layer of stratus is always very low. If it is divided into ragged masses in a wind or by mountain tops, it may be calledfracto-stratus. Thecomplete absence of detail of structure differentiates stratus from other aggregated forms of cloud.

We have given the foregoing official definitions and descriptions in full in order to aid the reader as much as possible, so far as verbal information goes, in learning to call the common clouds by their names. Good pictures are, of course, an essential part of this process, and apart from those that illustrate the present text, many collections of such pictures are easy of access. Some may be obtained free or at nominal cost from the Weather Bureau in Washington and from the Meteorological Office in London. The “International Cloud Atlas” (second edition, Paris, 1910) is now out of print, but may be consulted in libraries.

Of the clouds above enumerated, cirrus, cirro-cumulus, and cirro-stratus are the highest, and are always ice clouds. They consist in some cases of separate, minute crystals—a fine dust of ice—producing, according to the forms of the crystals, one or another of the various forms of halo around the sun and moon; while in other cases the crystals are aggregated in small snowflakes, so that the cloud is a real snowstorm in midair. The altitude of these clouds generally ranges from 4 to 8 miles. In the equatorial region their height is often 10 miles or more. The other main types of cloud are composed wholly or chiefly of water. Alto-cumulus and alto-stratus are clouds of medium altitude; strato-cumulus and nimbus are low clouds (generally not more than a mile high); while stratus, the lowest cloud of all, grades into fog, which commonly rests on the earth. Since cumulus and cumulo-nimbus are produced by the condensation of moisture from rising air currents, the height of their bases varies widelywith the temperature and humidity of the lower air; the average height is rather less than a mile. Their vertical extent, however, is much greater than that of the other cloud types. Cumulo-nimbus sometimes towers to a height of 4 or 5 miles above its base, and it is then commonly crowned with ice clouds, including a filmy “scarf cloud” draping the summit, and spreading wisps of so-called “false cirrus,” drawn out horizontally by the upper winds.

Besides the ten main classes of clouds, a few distinct minor varieties are recognized by all meteorologists. Among these is the “lenticular cloud”; an isolated small cloud, which frequently shows iridescence, and the shape of which has been compared to that of a lens or an almond. This cloud may remain stationary, or nearly so, but it really marks the position of a billow in a stream of air, the moisture condensing at one edge of the cloud and dissolving at the other. Another distinctive and rather rare form of cloud, seen chiefly in connection with thunderstorms, ismammato-cumulus, likewise known as “pocky cloud,” “festoon cloud,” “rain balls,” etc. It consists of numerous sacklike or udderlike protuberances, convex downward.

When a stream of moist air is forced to ascend in passing over a mountain its moisture is often condensed by the process of dynamic cooling, already explained, and a “cloud cap” is seen over the summit. In local weather lore such caps are generally regarded as a sign of rain. These clouds attached to mountains were called “parasitic clouds,” by writers of a century ago, who proposed some naïve explanations of them. Occasionally a “cloud banner” streams far to the leeward of the mountain. One of the most famous and striking of cloud capsis the “tablecloth” that spreads over Table Mountain, near Cape Town, when a moist wind blows in from the sea. Sometimes the local topography causes the wind that has swept up over a mountain to form a second “standing” wave to the leeward of the summit, and this may also be marked by a cloud, which, like the cloud cap, presents a delusive appearance of permanence, while it is, in reality, in constant process of formation on the windward side and dissipation on the leeward. The two clouds thus formed, one over the summit and the other to the leeward, are often seen at Table Mountain, and are further exemplified in the celebrated “helm and bar” of Crossfell, in the English Lake District.

In the case of a wind blowing athwart a ridge or mountain range, a bank of cloud may extend along the whole crest, as in the “foehn wall” that appears along Alpine heights when the foehn wind is blowing.

Some day meteorology will be taught in art schools, for the same reason that anatomy now is. When that blissful day arrives painters will probably show us skies less at odds with nature than those that deface the work of artists of all degrees of celebrity, including the “old masters.”

Meteorologistshave 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. Itconsists of balls or irregular lumps, each of which, on examination, is found to have an opaque snowlike center, surrounded by ice, which is often in alternately clear and opaque layers. The second class of icy particles takes the form of miniature snowballs, about the size of large shot or small peas. It falls in cold weather, often in conjunction with ordinary snow. Because it readily crumbles, English-speaking meteorologists have called it “soft hail”; but this name is inappropriate for the two cogent reasons that, though friable, it is not soft, and that it is not hail; hence this term is now giving way to the German name “graupel” (in whichau=owin “growl”). Lastly, little pellets or angular particles of clear ice sometimes fall in cold weather. These frozen drops, though fairly common, have, until recently, enjoyed the distinction of being anonymous, so far as the scientific world was concerned, while the general public called them various things, including “hail.” A few British authorities have tentatively styled this form of precipitation “ice rain,” a name which has, however, been otherwise applied. Finally, in the year 1916, the United States Weather Bureau took the bull by the horns and decreed that such ice particles should be called “sleet.”

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 termto the coating of smooth ice, due to rain in cold weather, which often breaks down the branches of trees, lays low miles of wires, and incidentally produces one of the most beautiful spectacles of American winters.

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,“it rains or it does not rain,” as we are told in the books on logic.

“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,has often been surpassed in other countries, including our own, anda fortioriwithin the tropics.

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 inchesin three minuteswas 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, carriedout by the engineers of the Miami Conservancy District. Their report on this subject (published at Dayton, Ohio, in 1917) is probably the most elaborate discussion of the kind hitherto prepared for any part of the world. Of the 2,641 storms investigated, seventy-eight, which covered areas of 500 square miles or more, were found to have had a rainfall amounting to at least 20 per cent of the normal rainfall for a year.

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 officialrecord, its average annual precipitation is 426 inches. Recently it has come to light that Cherrapunji has a serious rival in the Hawaiian Islands. A fragmentary record totaling 90 months, between 1911 and 1921, kept on Mount Waialeale (altitude 5,075 feet), in the island of Kauai, showed an average of 455 inches per annum, and there is reason to believe that a longer record will give an even higher average for this place. The spot in question, which has been described as “Uncle Sam’s dampest corner,” is so difficult of access that it can be reached only after a three-day trip on foot over mountain trails. Hence the United States Geological Survey has installed here a huge rain gauge—said to be the largest in the world—capable of holding an entire year’s rainfall, so that measurements need be made only once a year.

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 ofnuclei is so great that, as Humphreys has pointed out, even if all the water vapor in a volume of humid air was condensed upon them, the size of each drop would remain very small. He has suggested that, in a column of rising air, the small drops formed at the base of a cloud filter out most of the nuclei, so that at greater heights there are relatively few of the latter, which can, therefore, gather sufficient water about them to form drops of “falling” size.

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 manydrops too minute to form casts were estimated to be less than a hundredth of an inch in diameter, while the largest drops observed had a diameter of a quarter and even a third of an inch. This range of size corresponds to a range in the rate of fall from about five feet a second for the smallest drops up to about twenty-five feet a second for the largest. The maximum size of raindrops is limited by the fact that very large drops are broken up in their fall through the air. Theoretically, the limiting size is somewhat less than the largest sizes found by Bentley.

CASTS OF RAINDROPS FROM A THUNDERSHOWERCollected and Photographed by W. A. Bentley

CASTS OF RAINDROPS FROM A THUNDERSHOWER

Collected and Photographed by W. A. Bentley

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 condenseddirectly from water vapor. The finest snow consists of separate ice crystals, while snowflakes of larger sizes are always made up of several crystals partly melted together. The flakes on rare occasions attain a diameter of three or four inches, and larger sizes have been reported.

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 adjacentprimary rays. There are two principal forms of ice crystal—the tabular and the columnar. Sometimes the two forms are combined; a column or rod of hexagonal section will have at one or both ends a hexagonal plate. Both the size and the shape of snow crystals depend to some extent upon the temperature of the air. The smallest crystals form in the coldest weather. Star-shaped crystals are most abundant when the temperature is not far below freezing, while at lower temperatures there is a preponderance of hexagonal plates.

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.

An Ice Storm at Philadelphia, December, 1914.The branches are broken by heavy deposits of glaze. The photograph in the upper left corner, by Dr. David Fairchild, shows a glaze-incased twig. (Photographs from U. S. Weather Bureau.)

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 adistance, they bear an uncanny resemblance to throngs of white-robed human beings, and they have thus acquired the Spanish name ofnieve de los penitentes(“snow of the penitents”). In the abridged formnieve penitentethis name is now applied to more or less similar formations in other mountainous regions. Fine examples are found in the Himalaya, and one of the Himalayan peaks has been named Mount Nieves Penitentes. The origin of these pinnacles has been the subject of much discussion. Sunshine and wind both appear to take part in their formation. Some remarkable “snow honeycombs,” approaching the form ofnieve penitente, are produced in hot, dry summer weather among the glacier fields of Mount Rainier.

THE NIEVE PENITENTE IN THE ARGENTINE ANDES(Photograph by Dr. Juan Keidel.)

THE NIEVE PENITENTE IN THE ARGENTINE ANDES

(Photograph by Dr. Juan Keidel.)

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.

Economically, snow is perhaps most important in its effects on water supply, and this is true especially of mountain snowfields, the melting of which feeds adjacent streams. There are great areas in our Western States where the water required for irrigation is obtained almost entirely from melting snow. The mountain slopes constitute natural reservoirs, from which the moisture that falls in the winter as snow is gradually fed through the spring and summer to the surrounding country. In these regions extensive “snow surveys” are sometimes made in the early spring, in order to ascertain the total amount of water available. Professor J. E. Church, of the University of Nevada, was one of the pioneers of this idea, and both he and the experts of the Weather Bureau have developed ingenious apparatus and methods for making rapid estimates of the snowfall and its equivalent volume of water lying over a given area. The snow surveyor travels over the watershed, often on skis or snowshoes, cutting sections of snow with a cylindrical “snow sampler” and weighing them with a small spring balance. The Weather Bureau also maintains in the Western mountains a number of special stations at which daily measurements of snowfall are made for the benefit of irrigation projects. The use of “snow bins” and other forms of gauge for holding an entire winter’s snowfall—thus obviating the necessity of frequent measurement—has not proved very satisfactory in this country. An analogous device, known in French as atotalisateur, is, however, very extensively used in the Swiss and Italian Alps.

The heaviest snowfall in the United States occurs in the high Sierra Nevada of California and in theCascade Range of Washington and Oregon. At places in both of these regions more than sixty-five feet of snow has fallen in a single winter. The snow sometimes lies twenty-five feet deep on the ground, burying one-story houses to the eaves.

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.In the state of Bihar, India, October 5, 1893, hail covered the ground to a depth of four to six feet; six persons were buried beneath it and perished, and hundreds of cattle were killed. In the Moradabad district of India, May 1, 1888, about 250 people were killed by hail. The velocity attainable by falling hailstones is perhaps most strikingly shown by the fact that, even when falling obliquely, they have been known to pierce a pane of glass with a clear round hole, like a bullet hole, leaving the rest of the pane intact.

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 thefamiliar “sweating” of ice pitchers and also in the appearance of moisture on pavements, stone walls, and the like, in places shaded from the sun. At night the rapid cooling of the earth by radiation, especially (but not, as often stated, exclusively) under a clear sky and in still weather, favors this condensation of moisture, in the liquid form, as dew, or in the frozen form, as hoarfrost. The deposit occurs most copiously on objects that lose heat rapidly by radiation and gain it but slowly by conduction. Water vapor exhaled from the tissues of plants and from the soil undoubtedly contributes its quota to the moisture available for condensation, but this hardly seems to be a reason for asserting, as some writers have done, that dew comes mainly from the earth rather than the air.

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 oftrees, and on the corners and edges of upright objects, rather than on horizontal surfaces. In drifting fog it grows most rapidly if not entirely on the windward side of objects—i. e., it builds up against the wind. On Ben Nevis it has been observed to grow at a rate of more than an inch an hour. Trees, posts, telegraph poles, and the like are thus eventually changed to shapeless masses of rough or feathery ice.

Thestudy of the movements of the atmosphere constitutes a rather formidable branch of science known asdynamic meteorology. This subject has engaged the attention of a number of able physicists—though far too few—and has begun to assume the character of an exact science, but is still fruitful of unverified hypotheses.

We shall have only a little to say here about the theories and hypotheses relating to atmospheric circulation. They are at present, to a notable degree, in process of revision. Important modifications in them have resulted from the revelations of upper-air research, as well as from progress in other fields of inquiry. There are, however, a few fundamental matters that we must not ignore. We shall start with the solar heat that keeps the atmospheric machinery in motion.

Of the heat that comes to us from the sun, it is estimated that more than one-third is reflected by clouds and the earth, or scattered by dust and air molecules, and thus passes back into space without having had any effect in heating the atmosphere. Part of the remainder heats the atmosphere directly, and the rest indirectly, after first heating the underlying land and water. In both cases, certain atmospheric gases—notably water vapor—absorb a great deal more heat than others.

The first step in the production of a wind is a difference in temperature between two parts of the earth’s surface, and hence of the overlying air. Such contrasts of temperature always exist, both locally and on a large scale. The high sun of the equatorial regions heats the earth much more strongly than the low sun of high latitudes; a water surface has a more equable temperature than an adjacent land surface; a stretch of bare earth is warmer by day and colder by night than a neighboring tract covered with vegetation; and so on. Differences in atmospheric temperature produce differences in pressure, which gravity tends to adjust by setting up a circulation.

The exact manner in which this circulation is begun and maintained is not yet perfectly clear, and current ideas on the subject are difficult to put into brief language. Meteorological writers now lay less stress than formerly upon the lateral spreading, at high levels, of air that has been heated and expanded at the earth’s surface, and the inward flowing of the lower air toward the heated area. There is, we know, an initial impulse that tends to drive air from a region of high pressure toward a region of low pressure; but the actual movement of the air is another matter. The “life history” of an air current is found to be a very devious affair.

The important fact, for practical purposes, is that air does not flow in a straight line from the place where the pressure is high to that where it is low. As soon as it begins to flow it curves from the straight path, in accordance with Ferrel’s Law, which is thus stated:

“In whatever direction a body moves on the surface of the earth, there is a force arising from theearth’s rotation that deflects it to the right in the northern hemisphere, and to the left in the southern hemisphere.”

This law applies to all bodies moving freely over the earth, and not merely to the winds.

At the earth’s surface, if the atmospheric pressure is measured simultaneously at various places by means of barometers, we can get a clear picture of the horizontal distribution of pressure by drawing on a map lines, calledisobars, through places at which the pressure is identical. The isobars reveal the presence of extensive areas over which the pressure is above the average and of others over which it is below the average. If, at the same time, we chart the flow of air by indicating the direction of the winds at various points, we shall notice that the air shows a strong tendency to travelaroundthese areas; and if we could observe its course a thousand feet or so above the earth we should find the tendency even more pronounced at that level.

Another important law, springing in part from Ferrel’s Law and describing the movements of the air around areas of high and low pressure, is called Buys Ballot’s Law. One way of stating this law is as follows:

“If you stand with your back to the wind, in the northern hemisphere, the barometer will be lower on your left than on your right. The reverse is true in the southern hemisphere.”

The reader, whether he lives in the United States or any other civilized country, will have no difficulty in securing documentary evidence of the correctness of Buys Ballot’s Law in the shape of the daily weather maps issued by the various meteorological services. On a weather map showing conditionsanywhere in the northern hemisphere it will be found that the winds (which are indicated by little arrows), though subject to a good many local variations, have a general tendency to blow in the direction followed by the hands of a clock (“clockwise”) around an area of high pressure, and in the opposite direction (“counterclockwise”) around an area of low pressure. It will likewise be noticed that, in general, the winds, instead of blowing along the isobars, are strongly inclined inward in the case of a low-pressure area and outward in the case of a high-pressure area. Lastly, if the map indicates the force of the winds at different places, it will be seen that winds are strongest where the isobars are close together and weakest where they are far apart.

It is a matter of much interest to aeronauts that the force of the wind generally increases with altitude, and that, in the lower flying levels, the winds are little, if any, inclined to the isobars.

The spacing of the isobars is called thebarometric gradient. One of the conventional ways of expressing a gradient numerically is to state the horizontal difference of pressure, in hundredths of an inch, for an interval of fifteen nautical miles; but meteorologists as a rule merely describe gradients as “steep,” “gentle,” “moderate,” etc., without indicating their numerical values.

If the great difference in temperature between the equatorial and polar regions were the only factor in the control of atmospheric circulation, there would be a strong barometric gradient between these regions and there would result a simple circulation of winds, blowing poleward from the equator aloft and equatorward from the poles at the earth’s surface. The former tendency of writers on meteorologyand physical geography was to regard such a circulation as a substantial fact, though modified by the effects of the earth’s rotation and various local causes. Thus the idea has prevailed of a wholesale, direct exchange of air between the poles and the equator. Nowadays we can hardly maintain this idea, because we see that, on account of the great deflections they undergo, the main drifts of air are approximately along parallels of latitude and not along meridians of longitude. Within the tropics the general drift of the lower air is from the east (and near the equator this drift prevails up to a great height); in middle latitudes it is from the west; and in the circumpolar regions it is again from the east. Air from the equator presumably does find its way to high latitudes, andvice versa, but neither rapidly nor directly.

Perhaps the dominant feature of the whole circulation is the banking up of the air in so-called high-pressure belts at about latitude 30° North and South. From these “belts”—which are really broken up into separate areas of high pressure, and which shift north and south to a certain extent with the seasons—blow the northeast and southeasttrade winds, in the full development of which, found only over the Atlantic and the eastern Pacific, we have the most remarkable “permanent winds” of the globe.

Between the trade wind belts lies the equatorial region of low pressure, known as the “doldrums.” This is, in general, a region of light and variable winds, heavy rains, and thunderstorms.

In the temperate zones of both hemispheres, on the poleward side of the high-pressure belts above mentioned, the general drift of the atmosphere near the earth’s surface is from west to east. In thesouth temperate zone there is a very strong preponderance of west winds, especially over the vast oceanic tract of the “roaring forties,” where blow the boisterous “brave west winds,” well known to sailors. The corresponding belt of the northern hemisphere, which includes all but the southernmost part of the United States, is described as a region of “prevailing westerly winds,” but it is also a region of storm tracks, and hence, as local episodes in the general movement of the atmosphere from west to east, there are constant shifts of the wind to all points of the compass, for reasons that will presently be explained.

On the poleward borders of the two belts of “prevailing westerlies,” a little outside the Arctic and Antarctic Circles, there are zones of low pressure. That of the southern hemisphere is a continuous girdle around the earth, and has the lowest pressures found anywhere in the world. The corresponding subarctic zone, while fairly continuous in summer, is broken up in winter by the formation of high-pressure areas over Siberia and northern Canada.

The permanent ice sheets of Greenland and Antarctica are regions of high pressure, with calm air in the interior and strong outblowing winds at the borders. Furious blizzards prevail at the margin of Antarctica.

The table on the opposite page will serve as a recapitulation of the facts above stated.

Back to Index Next