any of these surfaces will become cooled below the dew-point, and
its moisture deposited on the surface in the form of dew. If this
chilled temperature be below 32° Fahr., the dew becomes frozen and is
called _hoar-frost_. The above two phenomena, though both expressed
in our language by the word dew, which perhaps helps to give rise to
a confusion of ideas on the subject, are not necessarily expressed
by the same word. For instance, in French, the first phenomenon--the
falling evening-dew--is expressed by the word _serein_; while the
latter--the dew seen in the morning gathered in drops by the leaves
of plants, or other cool surfaces--is expressed by the word _rosée_.
The merit of the discovery of the “Theory of Dew” has been commonly
ascribed to Dr. William Charles Wells, who published in 1814 his
_Essay on Dew_, which obtained great popularity. The merit should,
however, be divided between him and several others. M. Le Roi of
Montpellier, M. Pictet of Geneva, and especially Professor Alexander
Wilson of Glasgow, largely contributed by experiment and inducement
to its formation.
THE AURORA BOREALIS
--RICHARD A. PROCTOR
The aurora is one of those phenomena of nature which are
characterized by exceeding beauty, and sometimes by an imposing
grandeur, but are unaccompanied by any danger, and indeed, so far
as can be determined, by any influence whatever upon the conditions
which affect our well-being. Comparing the aurora with a phenomenon
akin to it in origin--lightning--we find in this respect the most
marked contrast. Both phenomena are caused by electrical discharges;
both are exceedingly beautiful. It is doubtful which is the more
imposing so far as visible effects are concerned. When the auroral
crown is fully formed, and the vault of heaven is covered with the
auroral banners, waving hither and thither silently, now fading from
view, anon glowing with more intense splendor, the mind is not less
impressed with a sense of the wondrous powers which surround us than
when, as the forked lightnings leap from the thundercloud, the whole
heavens glow with violet light, and then sink suddenly into darkness.
The solemn stillness of the auroral display is as impressive in its
kind as the crashing peal of the thunderbolt.
The reader is no doubt aware that auroras or polar streamers, as they
are sometimes called, are appearances seen not around the true poles
of the earth, but around the magnetic poles which lie very far away
from those geographical poles which our Arctic and Antarctic seamen
have in vain attempted to reach. The formation of auroral streamers
around the magnetic poles of the earth shows that these lights are
due to electrical discharges of electricity, which, though only
visible at night, take place in reality in the daytime also.
Remembering that the aurora is due to electrical discharges in the
upper regions of the air, it is interesting to learn what are the
appearances presented by the aurora at places where the auroral
arch is high above the horizon--these being, in fact, places nearly
_under_ the auroral arch. M. Ch. Martins, who observed a great number
of auroras in Spitzbergen in 1839, thus writes: “At times they are
simple diffused gleams or luminous patches; at others, quivering rays
of pure white which run across the sky, starting from the horizon
as if an invisible pencil were being drawn over the celestial vault;
at times it stops in its course, the incomplete rays do not reach
the zenith, but the aurora continues at some other point; a bouquet
of rays darts forth, spreads into a fan, then becomes pale, and
dies out. At other times long golden draperies float above the head
of the spectator, and take a thousand folds and undulations as if
agitated by the wind. They appear to be but at a slight elevation
in the atmosphere, and it seems strange that the rustling of the
folds as they double back on each other is not audible. Generally, a
luminous bow is seen in the north; a black segment separates it from
the horizon, the dark color forming a contrast with the pure white
or bright red of the bow, which darts forth rays, extends, becomes
divided, and soon presents the appearance of a luminous fan, which
fills the northern sky, and mounts nearly to the zenith, where the
rays, uniting, form a crown, which in its turn darts forth luminous
jets in all directions. The sky then looks like a cupola of fire; the
blue, the green, the yellow, the red, and the white vibrate in the
palpitating rays of the aurora. But this brilliant spectacle lasts
only a few minutes; the crown first ceases to emit luminous jets,
and then gradually dies out; a diffused light fills the sky; here
and there a few luminous patches, resembling light clouds, open and
close with incredible rapidity, like a heart that is beating fast.
They soon get pale in their turn, everything fades away and becomes
confused, the aurora seems to be in its death-throes; the stars,
which its light had obscured, shine with a renewed brightness; and
the long polar night, sombre and profound, again assumes its sway
over the icy solitudes of earth and ocean.”
The association between auroral phenomena and those of terrestrial
magnetism has long been placed beyond a doubt. Wargentin in 1750
first established the fact, which had been previously noted, however,
by Halley and Celsius. But the extension of the relation to phenomena
occurring outside the earth--very far away from the earth--belongs
to recent times. The first point to be noticed, as showing that the
aurora depends partly on extra-terrestrial circumstance, is the
fact that the frequency of its appearance varies greatly from time
to time. It is said that the aurora was hardly ever seen in England
during the Seventeenth Century, although the northern magnetic pole
was then much nearer to England than it is at present Halley states
that before the great aurora of 1716 none had been seen (or at least
recorded) in England for more than eighty years, and no remarkable
aurora since 1574. In the records of the Paris Academy of Sciences no
aurora is mentioned between 1666 and 1716. At Berlin one was recorded
in 1707 as a very unusual phenomenon; and the one seen at Bologna
in 1723 was described as the first which had ever been seen there.
Celsius, who described in 1733 no less than three hundred and sixteen
observations of the aurora in Sweden between 1706 and 1732, states
that the oldest inhabitants of Upsala considered the phenomenon as a
great rarity before 1716. Anderson of Hamburg states that in Iceland
the frequent occurrence of auroras between 1716 and 1732 was regarded
with great astonishment. In the Sixteenth Century, however, they had
been frequent.
Here then we seem to find the evidence of some cause external to
the earth as producing auroras, or at least as tending to make
their occurrence more or less frequent. The earth has remained
to all appearance unchanged in general respects during the last
three centuries, yet in the Sixteenth her magnetic poles have been
frequently surrounded by auroral streamers; during the Seventeenth
these streamers have been seldom seen; during the last two-thirds of
the Seventeenth Century auroras have again been frequent; and during
the Eighteenth Century they have occurred sometimes frequently during
several years in succession, at others very seldom.
Connected as auroras are with the phenomena of terrestrial magnetism,
we may expect to find some help in our inquiry from the study of
these phenomena. Now it appears certain that magnetic phenomena are
partly influenced by changes in the sun’s condition. We may well
believe that they are in the main due to the sun’s ordinary action,
but the peculiarities which affect them seem to depend on _changes_
in the sun’s action.
Many of my readers will doubtless remember the auroras of May 13,
1869, and October 24, 1870, both of which occurred when the sun’s
surface was marked by many spots, and both of which were accompanied
by remarkable disturbance of the earth’s magnetism.
It may, then, fairly be assumed that the occurrence of auroras
depends in some way, directly or indirectly, on the condition of the
sun. But what the real nature of that connection may be is not easily
determined.
Angström was the first to observe the spectrum of the aurora
borealis. He found that the greater part of the auroral light, as
observed in 1867, was of one color, yellow, but three faint bands of
green and greenish blue color were also seen. The aurora of April 15,
1869, was seen under very favorable conditions in America. Professor
Winlock, observing it at New York, found its spectrum to consist of
five bright lines, of which the brightest was the yellow line just
mentioned. One of the others seems to agree very nearly, if not
exactly, in position with a green line, which is the most conspicuous
feature of the spectrum of the solar corona. During the aurora of
October 6, 1869, Flögel noticed the strong yellow line and a faint
green band. Schmidt, on April 5, 1870, made a similar observation.
He saw the strong yellow line, and from it there extended toward the
violet end of the spectrum a faint greenish band, which, however, at
times showed three defined lines, fainter than the yellow line.
It was not till the magnificent aurora of October 24-25, 1870,
that any red lines were seen in the spectrum of an aurora. On that
occasion the background of the auroral light was ruddy, and on the
ruddy background there were seen three deep red streamers very well
defined. The ruddy streamers, on the night of October 25, converged
toward the auroral crown, which was on that occasion singularly well
seen. Förster of Berlin failed to see any red line or band despite
the marked ruddiness of the auroral light. But Capron at Guildford
saw a faint line in the red part of the spectrum; and Elger at
Bedford observed a red band in the light of the red streamers, the
band disappearing, however, when the spectroscope was directed on the
white rays of the aurora.
As yet the auroral spectrum has not been interpreted. The reason
probably is, that the conditions under which the light of the aurora
as of the corona is formed are not such as have been or perhaps can
be attained or even approached in laboratory experiments.
CLOUDS
--D. WILSON BARKER
Those who are professionally engaged in the scientific work of
weather bureaus recognize the importance of accurate observations
of cloud forms and nature, and much good work has been done in this
connection in recent years by scientific observers in England,
Australia, and the United States; but as a popular study, nephology
is almost entirely overlooked, and this notwithstanding the fact
that, perhaps, no branch of knowledge offers greater facility and
ease of acquisition. Each cloud has its history fraught with meaning;
its open secret is writ on its face, and may be read by any one who
will, give himself a little trouble, nor need he go deeply into the
study in order to make observations interesting to himself, and
perhaps of great use in the furthering and perfecting of weather
lore. To the ancients, the sky was doubtless an object of constant
remark and interest, and possibly their intuitive knowledge of
weather forecasting was much more accurate than ours. The dwellers
in our modern cities see little of the sky, clouds have no interest
for them beyond the personal consideration as to the advisability
of taking out an umbrella or not. But farmers, fishermen, sailors,
and others following open-air avocations are dependent on the
weather, and to be wise in its forecast is of importance to them.
To these, especially, cloud study should appeal; it can not fail to
be profitable to them in their personal work, and they have all the
opportunity, if the will be there, to forward the general knowledge
of the subject by careful painstaking observations, which they may
transmit to those scientifically engaged in dealing with weather
laws, and thus assist in the elucidation of questions on which we are
at present but very imperfectly informed.
In this article the broad distinctions of clouds will be dealt with.
There are two well-defined types--Stratus and Cumulus--so distinct
in actual appearance and in physical formation that they may be
taken as the basis of classification. Sometimes both types appear to
merge into each other, in which case no variety of classification
suffices to describe them satisfactorily, as any one who has studied
cloud-forms must allow. “Stratus” is a sheet-like formation of
cloud. “Cumulus” is recognizable by its heaped-up appearance and
vertical thickness. Numerous varieties of cloud-forms may be observed
graduating from one of these types to the other, but when an
observer can clearly distinguish Stratus from Cumulus he has already
acquired valuable knowledge.
The presence of either type of cloud alone indicates a more or
less set condition of the atmosphere, and generally foretells a
continuance of the existing weather. The simultaneous presence of
both types indicates a coming change, the gradation of Stratus
into Cumulus foreboding worse weather, and of Cumulus into Stratus
heralding good. Again, as we shall show later on, the vertical
thickening of the stratiform clouds is a distinctly bad indication.
Up to quite recently, Luke Howard’s division of clouds, formulated
in 1802, held first place; even now it is in constant use, for
though attempts have been made at a more scientific classification,
all of them, with the single exception of that proposed by the
late Rev. Clement Ley, can only be termed make-shifts. Mr. Ley’s
classification, unfortunately, is long, and not well adapted to the
use of any but professional investigators, or enthusiasts with ample
time on their hands. There exists a so-called “international” system
of cloud nomenclature, but, for all that, each country has its own
especial system, with the result that vast collections of cloud
statistics are of little value as helps to a classification, and are
useful only as records of clouds present at certain times.
Clouds owe their existence to two causes:
1. Through the passing of warm, moist air into colder, when, owing to
condensation, a certain proportion of the moisture becomes visible in
the form of a cloud.
2. Through changes occurring in the atmosphere as it rises
into higher regions of atmosphere, where decrease in pressure
and expansion and consequent loss of heat take place and cause
condensation of moisture.
The first process may be described as the condensation formation of
clouds, and the second as the adiabatic formation of clouds. As a
matter of fact, no hard and fast line separates these two operations;
they act in unison, and the combination of vertical and horizontal
currents goes to make up the diversity of forms which clouds assume.
In settled states of the atmosphere, Stratus clouds are common, or
the sky may be clear. In unsettled conditions, Cumulus or Heap clouds
are formed.
We shall now describe a few familiar forms of cloud, giving them
simple names and endeavoring to compare them with other nomenclatures.
Of Cumulus clouds there are five well-defined varieties.
_Rain Cumulus_, of which there are two sub-varieties:
(_a_) Shower-cumulus, when rain falls from the cloud without
increment of wind. The edges of this cloud are not cirrus-topped.
(_b_) Squall-cumulus, when the rain is accompanied by wind, or by
wind with hail and snow falling from this cloud.
In these cases the Cumulus cloud is generally much serrated, having
a cirriform edging. In some cases this cirriform edging extends far
over the sky and forms halos, particularly at the rear.
Two rarer varieties of Cumulus are:
_Pillar-cumulus_, generally noticed over the calm belts of the
ocean, and distinguishable by its slender forms, which rise to great
altitudes.
_Roll-cumulus_ generally accompanies strong winds, particularly polar
west winds, which succeed cyclonic disturbances. Here we have the
ordinary Cumulus cloud so blown along by the wind as to assume the
roll formation from which it is named.
A still rarer form of Cumulus appears in scattered patches over the
sky, and is indicative of an electrical state of the atmosphere.
Cumulus clouds form at a low altitude, but they frequently tower
upward to great heights.
It should be noticed that in these clouds the fine weather form is of
soft, smooth outline, and has a quiet appearance.
_Stratus Clouds_ may be divided into four varieties as follows:
1. _Fog_, so well known as not to need description. It is, in fact, a
Stratus cloud resting on the earth’s surface.
2. _Stratus_, a cloud sheet which covers the whole sky at a
moderate elevation. Here and there the cloud is thin, and under
surfaces appear as parallel lines all round the horizon. This is
the characteristic cloud of anti-cyclonic, or dry, fine weather
conditions. It may continue to cover the sky for several days in
succession.
3. _High Stratus_, including all the varying forms of Cirro-cumulus
from the mackerel skies to the Cirro-macula of Clement Ley. Many
beautiful varieties of this cloud of minute cumuliform appearance
are caused by the changes taking place in the atmosphere. We notice
waves, wavelets, stipplings, and flecks. To it are due the coronas
sometimes seen round the sun, as also iridescent clouds occasionally
noticed in the same vicinity. The wave-like appearance of the clouds
is due to the passage of a more rapidly moving air current over a
slower one, or of a wave current crossing a motionless portion of the
air. When two air currents pass over one another at an angle, the
particles of clouds tend to fall into different shapes, hence our
mackerel skies. But this cloud, although beautiful, is essentially
one of warning, more especially when the flecks are of a thin, scaly
appearance (resembling the scales of certain fishes so closely that
I have called it the scale cloud). Sometimes these detached flecks
appear in lines, and very striking is the effect produced.
4. _Cirrus._--The highest form of cloud and the most important as a
factor in the science of weather forecasting. Cirrus, ordinarily,
appears as wisps and feather pieces scattered over the sky, and its
significance is then of no import.
When, however, this cloud takes the form of lines parallel to the
horizon, or of lines appearing to radiate as wheel-spokes from any
one part of the horizon, it should be carefully noted as indicative
of approaching weather. Its movement and propagating transition
should be observed. This cloud is composed of ice-dust or crystals.
When a cyclonic disturbance is about to pass over an observer, Cirrus
generally appears first in parallel lines, or at a radiant point;
the threads gradually increase and interlace until a complete sheet
of Cirro-stratus covers the sky, causing a halo. The cloud further
thickens, the halo disappears, all becomes overcast, and rain comes
on. The cloud is now known as Nimbus, and after it has endured some
time, the wind shifts, the Nimbus clears off, and it is succeeded by
a polar west wind.
In addition to these forms of clouds, we may often notice,
particularly during high winds, fragments of clouds hurrying across
the sky. These are known as “scud”; they are generally pieces carried
off by the winds from the main bodies of clouds.
Occasionally two forms of cloud are present at the same time. This
is ordinarily taken as a case of Cumulus and Stratus, and has become
known as Cumulo-stratus; but, if observed in the zenith, it may
readily be noted that the two forms of cloud are distinct, and they
had better be dealt with separately. The appearance of Cumulo-stratus
is an effect of perspective.
Clouds float at varying altitudes, according to the latitude and
elevation of the ground; the vertical temperature and adiabatic
gradients determining the level at which the vapor becomes visible as
cloud. It is desirable in all cloud observations, that note should
be made of the approximate relative altitudes of clouds and of their
velocity of motion. This is particularly desirable when dealing with
the stratiform clouds, whether as ordinary Cirro-cumulus or as very
high Cirro-macula.
The beautiful coloring of clouds results from the breaking up of
light beams in passing through them or along their edges. This
phenomenon is caused by diffraction, and to it is due our lovely
sunrises and sunsets. When the sun is high in the heavens, the light
is white, but as the orb nears the horizon, and its rays pass through
thicker layers of atmosphere, the smaller light waves get gradually
cut off, until the sun sinks as a red ball below the horizon. The
largest waves of light produce the red rays and the after glow
which are so beautiful. Sunrise and sunset effects are matters of
much interest, but are of too complicated a nature to be fully gone
into here; we must, however, notice them briefly, because of their
importance in weather forecasts. Soft sunset colors indicate fine
settled weather; fiery brilliant hues denote change to stormy or wet
weather.
Other color effects in clouds are due to phenomena, known as halos
and coronas. Halos appear as rings round the sun and moon; they
are caused by the shining of the orb through very high Stratus or
Cirrus clouds, and have a diameter of 42°. Sometimes shades of
color, resembling those of a rainbow, are visible--red appears on
the inside and blue on the outside. These rings of color are due
to the reflection and refraction of light passing through the fine
ice crystals of which high Stratus or Cirrus clouds are composed.
Occasionally a complicated series of beautifully colored rings is
noticeable. Generally speaking, these rings are due to the thinness
of the high cloud through which the light is passing. Still more
curious arrangements of halos sometimes occur.
Coronæ are broader rings seen quite close to the sun or moon, and are
due to the shining of light through the edges of loose Cumulus or
Stratus clouds. They have red on the outside and blue on the inside
of the ring; the colors are, generally, easily distinguishable.
The more brilliant hues occasionally seen, as has been said, in
the vicinity of the sun and moon, would appear to be incomplete
sections of circles intermediate in size between coronæ and halos. An
interested observer will be well repaid if he chooses to study more
closely the many curious optical phenomena connected with clouds, but
it would be beyond the scope and object of this paper to go into them
more fully here.
Whoever wishes to be weatherwise, and who has time to study the
weather charts published daily, may easily acquire such knowledge
of local characteristics as will enable him to forecast fairly
accurately. Cirrus clouds, as a rule, are reliable guides; they
form, as we have said, in parallel threads, from the position and
movements of which forecasts may be made. Should the threads appear
on, and parallel to, the west horizon, and moving from a northerly
point, a depression is approaching from the west, but, although
causing some bad weather, it will probably pass to the north of
the observer. Should the lines appear parallel to the southwest or
south-southwest horizon, and be moving from a northwesterly point,
the depression will very likely pass over the observer and occasion
very bad weather. These are two of many possible prognostics. Weather
forecasting is much helped by a study of the daily weather charts.
Again, weather is often very local, and to predict with fair accuracy
a knowledge of local conditions is necessary.
WINDS
--WILLIAM HUGHES