CLOUDLAND
One of the things a tea kettle is good for is to provide, by means of
the little cloud seen at its nozzle and erroneously called “steam,” an
example of what happens when the invisible gas that is truly steam,
or water vapor, is cooled below its dew point in the free air. This
cloud has, however, been the starting point of a vast number of halfway
explanations. A generation or so ago physicists were content to say
that aqueous vapor turns to drops of water in the air merely on account
of being cooled. The question of how the drops get their start, or why
the moisture forms drops at all, does not seem to have troubled them.
One way in which air or any other gas is cooled is by expanding against
pressure. Some of the energy in the gas, originally manifesting itself
as heat, is applied to the work of expansion, and thus ceases to be
heat. Hence the temperature of the gas falls. Conversely, if a mass
of gas is compressed, the mere process of compression raises its
temperature. The heat produced in pumping up a bicycle tire is the
classic example of the latter fact. Heating by compression and cooling
by expansion are called, respectively, “dynamic heating” and “dynamic
cooling.” The processes thus described are of the utmost importance in
meteorology.
If air of average humidity is admitted to the receiver of an air pump
in the usual way and suddenly expanded by partial exhaustion, a cloud
of moisture is seen to form in the receiver. This moisture is condensed
and made visible by the dynamic cooling of the air. If, however,
after the receiver is exhausted air of the same humidity as before is
admitted through a filter of cotton wool, and is then similarly cooled
by expansion, no cloud will form. Evidently the filter has removed from
the air something that is essential to the process of condensation.
Perhaps it will occur to the reader that, in some obscure way, the
filter has prevented water vapor itself from entering the receiver.
There are several methods by which we can ascertain whether such is the
case. One of the simplest is to admit a little smoke to the receiver
before expanding the filtered air. In this case the cloud _does_ form,
showing that moisture is present, and also showing that smoke, though a
perfectly dry substance, aids the formation of the water drops.
Such experiments have led to the conclusion, now universally admitted,
that when water drops form in the atmosphere they always form around
“nuclei” of something that is not water. These nuclei are often
referred to as “dust particles,” but it is recognized that a vast
proportion of them are very much more minute than the dust that worries
housewives. They are largely beyond the power of the microscope, and
some of them, indeed, appear to be of molecular size, consisting of
molecules of hygroscopic gases, such as the oxides of sulphur and of
nitrogen.
Another important fact about water drops in the atmosphere has come
to light within the last half century. Since water is much heavier
than air, meteorologists of an earlier generation were puzzled by the
fact that the drops in clouds apparently float, instead of falling
to the ground. In the attempt to account for this supposed floating,
bygone authorities assumed that the drops were hollow “vesicles,” like
little bubbles. This assumption was eventually disproved by the optical
phenomena exhibited by the drops, as well as on other physical grounds.
Moreover, it is now known that a cloud never really floats, though the
rate at which its constituent particles fall with respect to the air is
generally very small, on account of the resistance they encounter. Thus
a very slight upward current usually suffices to maintain the altitude
of a cloud, or even to increase it. The speed with which a drop falls
increases with its size. Hence large drops may fall rapidly from great
heights all the way to the ground, constituting rain; but in a great
many cases such drops evaporate on falling into warmer air below the
cloud level, and thus the lower surface of the visible cloud remains at
about a constant height.
The drops in clouds and fog have often been measured, either by noting
their optical effects or by microscopic examination. Many are found to
be from 0.0006 to 0.0008 inch in diameter. The speed with which such
drops fall through still air can be calculated. A drop 0.0008 inch
in diameter falls at the rate of about half an inch a second, or 150
feet an hour. Even if a cloud consisting of such drops preserved its
integrity for an hour or more while sinking, its descent at this slow
rate would hardly be perceptible from the ground.
Some clouds consist of ice needles or tiny snowflakes. Apparently these
icy particles are produced directly in solid form, without passing
through the liquid stage. It is supposed that, like drops, they
require nuclei on which to condense, but this matter has not been fully
investigated. Another point that awaits elucidation is the fact that
the clouds that form in air much below the freezing point sometimes
consist of water and sometimes of ice. The common fleecy clouds of our
winter skies are composed of water drops, and such clouds also occur
in the polar regions. Dr. G. C. Simpson, when serving as meteorologist
of Scott’s antarctic expedition, observed fog consisting of liquid
water at a temperature of 29° below zero, Fahrenheit. Why such greatly
“undercooled” drops should sometimes occur in the atmosphere when at
other times, with higher temperatures, atmospheric moisture takes the
form of ice is not at all clear.
There are several ways in which the free air may be cooled to the
point at which condensation occurs. The commonest is dynamic cooling,
due to the rise of a mass of moist air and its expansion under the
reduced pressure that prevails at the higher levels. This process is
beautifully illustrated in the formation of the roundish masses of
fleecy cloud known as _cumulus_, on a warm summer day. Each of these
clouds marks the summit of a column of air that is rising after having
been heated at the surface of the earth. When the process goes on
very actively, the cloud may tower up to enormous heights, forming a
thundercloud. Some clouds are formed by the mixing of air of different
temperatures. Fog, which is merely cloud at the surface of the earth,
is often formed by the cooling of the air in contact with cold land or
water. The persistent fogs of the Newfoundland Banks are due to the
passage of warm moist air from the Gulf Stream region over the cold
Labrador Current. On the other hand, a cold wind blowing over warm
water will also often produce a fog by lowering the temperature of
the moist air overlying the water. A common cause of land fog is the
cooling of the air adjacent to the ground in consequence of nocturnal
radiation. The moister the air, the more readily fog forms, and hence
the frequent formation of fog by night along rivers and over marshes
and damp valleys.
Town fogs, such as the famous “London particular” and the fogs of
Lyons, usually consist partly of smoke. Dense fogs of this sort occur
when the conditions of the atmosphere are such as to cause the smoke
to hang low over the city, instead of being dispersed. These fogs
constitute a serious economic problem. Thus it is estimated that they
cost the people of London upwards of half a million dollars a year, due
to extra lighting, damage to vehicles, loss of business, etc. Since
marine fog is also a source of enormous loss, through causing delays
and accidents, and since fog along air routes is the greatest of all
obstacles to successful aerial navigation, it is no wonder that much
ingenuity has been devoted to the attempt to disperse fog artificially.
Electric discharges have been successfully used for this purpose on a
small scale.
The depth of a fog may be anything from inches to miles. Measurements
made by the United States Coast Guard during the international ice
patrol of the North Atlantic show that the fogs on the Newfoundland
Banks are very commonly so shallow that the mastheads of vessels rise
above them, though in some cases they were found, from observations
with kites, to be from 2,500 to 3,000 feet thick. Observations on the
mountains of the California coast show that the upper level of fog
in that region rarely exceeds 4,000 feet. On the other hand, aviators
flying between London and Paris have encountered fog more than 10,000
feet deep.
The United States Weather Bureau classifies a fog as “dense” if it
hides objects at a distance of 1,000 feet; otherwise it is described as
“light.” British meteorologists record fogs on a scale of five degrees.
During the ice patrol of the _Seneca_ in 1915 samples of foggy air were
examined for the purpose of calculating the amount of water and the
number of drops they contained per unit volume, as well as the size
of the drops. A block of dense fog 3 feet wide, 6 feet high, and 100
feet long was found to contain less than one-seventh of a glassful of
water, distributed in 60,000,000,000 drops. During the densest fog of
the voyage the diameter of the fog particles averaged 0.0004 inch; just
about the limit of visibility with the naked eye.
In spite of the extremely attenuated state of the water in fogs, as
indicated by these figures, the moisture they deposit on terrestrial
objects is great enough to be of considerable agricultural importance
in some parts of the world. Thus along the coast of Peru, where the
rainfall is negligible (though not, as often stated, nonexistent),
a wet fog known as the “garúa” suffices to maintain a luxuriant
vegetation during several months of each year.
There are frozen fogs as well as frozen clouds. The “frost smoke”
that rises over the Norwegian fjords and over ice-free spots in the
polar seas is generally composed of icy particles or snowflakes. An
ice fog that sometimes forms in mountain valleys in the western United
States is known as the “pogonip”--a name derived from the Shoshonean
language. This fog often appears very suddenly, even in the brightest
weather. The minute needles of ice of which it consists are said to be
extremely injurious to the lungs. There are tales of a whole tribe of
Indians perishing from its effects. Whatever truth there may be in such
stories, it is greatly dreaded by both the Indians and the whites. The
mountains of Nevada appear to be the favorite home of the pogonip.
What meteorologists call “dry fog” is a haze of dust or smoke,
sometimes very dense. We have already described the prevalence of
this turbid state of the atmosphere following volcanic eruptions, the
burning of forests and moors, and desert dust storms. Under the head
of dry fog many writers include a sort of heat haze, which does not
necessarily involve the suspension of either solid or liquid matter in
the air, but is due to the mixing of local air currents of different
densities, especially when evaporation is proceeding rapidly from moist
ground under strong sunshine. The _callina_ of Spain and the _qobar_ of
the upper Nile region are probably due partly to this cause, and partly
to dust.
[Illustration: 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._)]
[Illustration: 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.
[Illustration: 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._)]
[Illustration: 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:
Nuages en balayures (cloud sweepings),
Nuages en barres (clouds in bars),
Nuages pommelés (dappled clouds),
Nuages groupés (grouped or piled clouds),
Nuages en voile (veil clouds),
Nuages attroupés ou moutonnés (clouds in flocks).
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 with rounded
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 called _nimbus_. 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
International Cloud 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 (called
_cirrus 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 completely and 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._
[Illustration: A CLOUD BANNER OVER MT. ASSINIBOINE, CANADIAN ROCKIES.
(_Photographed by Dr. C. D. Walcott._)]
[Illustration: 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.
[Illustration: Copyright O. P. Anderson
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 as _fracto-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 almost always 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 called
_fracto-stratus_. The complete 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 widely
with 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, is _mammato-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
caps is 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.”