PRECIPITATION
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 consists 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 which _au_
= _ow_ in “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 term to 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, and _a fortiori_ within 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 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 out 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 official
record, 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 of nuclei 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 many drops 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.
[Illustration: 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 condensed directly 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 adjacent primary 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.
[Illustration: 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 a distance, they bear an uncanny resemblance to throngs of
white-robed human beings, and they have thus acquired the Spanish name
of _nieve de los penitentes_ (“snow of the penitents”). In the abridged
form _nieve penitente_ this 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 of
_nieve penitente_, are produced in hot, dry summer weather among the
glacier fields of Mount Rainier.
[Illustration: 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
a _totalisateur_, 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 the Cascade 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
the familiar “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 of trees, 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.