ATMOSPHERIC OPTICS
When we look up into the sky on a cloudless day we behold a continuous
canopy, the prevailing color of which is blue. This canopy is a veil
that hides the starry hosts beyond, and its presence seems, at first
sight, incompatible with the fact that the air is a transparent medium.
We see the stars by night through the same intervening atmosphere.
Why are they cut off from our sight by day? The answer to this
question can, perhaps, best be made plain by a simple experiment.
Place a lighted candle behind a sheet hung across a room not otherwise
illuminated. The flame of the candle will be distinctly visible through
the sheet. Next, let the room be brightly lighted, say with electric
light or daylight. The candle can now no longer be seen through the
sheet, owing to the bright illumination of the latter as compared with
the feeble light of the candle.
In the atmosphere the counterpart of our sheet is a layer, several
miles in depth, of minute particles, which by day are lighted up by
the sun. Some of the particles are tiny dust motes, others are fine
droplets of water or bits of ice, and the rest are the molecules of the
atmospheric gases themselves. It is the light that comes to us from
these particles that makes our eyes insensitive to the fainter light of
the stars, and makes the sky itself a visible luminous vault.
Next, why is the clear sky generally blue, rather than some other
color? To answer this question, we must recall the fact that sunlight
is made up of ether waves of many different sizes. In combination,
these waves produce upon our eyes the sensation of white light. When
they are separated, as by passing through a prism, the smallest
waves--or, in more technical language, the vibrations of shortest wave
length--register the sensation of violet, and the largest or longest
waves that of red. The whole sequence of colors runs in the order
violet, indigo, blue, green, yellow, orange and red (easily fixed
in the memory by means of the word VIBGYOR, formed from the initial
letters of these words).
Now the passage of sunlight through the atmosphere is obstructed to
a certain extent, not only by suspended dust particles, but also by
the molecules of the air. Let us consider, first, the effect of air
molecules and of the finest dust particles, not much above molecular
size. These tiny objects have different effects on light waves of
different lengths. The longest waves are little disturbed by them, just
as ordinary waves in water are little affected by a floating cork, for
instance. The shortest waves are so small in proportion to the size
of the obstacles that they are diffused or scattered by them, as a
tiny ripple in water might be broken up by a floating cork. It is this
diffuse light, of short wave length, that gives the sky its color. A
large part of the violet and indigo light is lost by further scattering
before it reaches the earth, leaving a preponderance of blue in the sky
as we see it. When the air contains a considerable amount of suspended
particles larger than those above considered--whether in the form of
solid dust or crystals of ice or tiny droplets of water--light of all
wave lengths is reflected by them, and the sky looks white or grayish.
On account of the action of atmospheric particles in filtering out the
shorter light waves, as just described, sunlight becomes relatively
rich in red and orange in passing through the air. When the sun is
high, the path of the sunbeams to the earth is short, and the color
of their light is but little affected. Near the time of sunrise and
sunset, however, sunlight comes to us through a much greater extent
of air, and the filtering process is much more effective. Hence the
sunshine is both enfeebled and reddened when the sun is near the
horizon. The diffuse light of the sky around the sun is filtered in the
same manner, and therefore is commonly red when the sun is low.
A gray sunset sky after a clear day is due to the presence of water
drops in the air, and indicates conditions favorable for rain,
since, unless the air were saturated to a considerable altitude, the
comparatively warm sunshine of the afternoon would favor evaporation
rather than condensation of moisture. A gray sunrise sky has, as
a general rule, just the opposite meaning. It often indicates the
presence in the air of water drops formed on dust particles during
the night, after the manner of dew, because the upper air has been
_dry_ enough to permit rapid radiation from the dust. These drops will
be speedily evaporated by the rising sun, and the general dryness of
the atmosphere will not favor further condensation. Several familiar
weather proverbs are thus justified, e. g.:
Evening red and morning gray
Help the traveler on his way;
Evening gray and morning red
Bring down rain upon his head.
There are many other interesting optical phenomena connected with
sunrise and sunset, including, first of all, the morning and evening
twilight. When the sun, or any other heavenly body, is only a little
below a clear horizon, it is still visible, on account of the
bending of its rays by the atmosphere. This lifting effect, known
as _astronomical refraction_, amounts to about half a degree (at
the horizon), which is about equivalent to the apparent diameter of
the sun or moon. As the sun sinks farther below the horizon, in the
evening, the only daylight that comes to us is that reflected from
the upper levels of the atmosphere, which are still illuminated. This
is called twilight, and it lasts until the sun is about 18 degrees
below the horizon, when total darkness sets in. The period as a whole
is sometimes called _astronomical twilight_, in distinction from the
briefer period known as _civil twilight_, during which there is light
enough for outdoor occupations; the latter lasts from sunset until the
sun is about 6 degrees below the horizon. Morning twilight is more
commonly called “dawn.”
An interesting succession of light and color effects is observed before
and after sunset and, in inverse position and order, about sunrise.
Considering sunset only: After the sun has sunk out of sight, a broad
band of golden light, called the _bright segment_, is seen along the
western horizon. Above this, in the western sky, appears a more or less
circular expanse of rosy glow, known as the _purple light_. In the
eastern heavens, after sunset, there rises steadily from the horizon
the so-called _dark segment_, which is the blue or ashy shadow of
the earth on the sky. This is bordered above by the pink or purplish
_antitwilight arch_. As time goes on, the purple light in the west,
after increasing in brightness for a while, finally sinks behind the
bright segment; while in the east the rising dark segment encroaches
upon and finally obliterates the antitwilight arch. Sometimes, in clear
weather, there is a fainter repetition of these lights and colors
(_second purple light_, etc.).
Among the Alps and other snow-capped mountains, these sunset and
sunrise phenomena assume a particularly beautiful form, known as the
_Alpenglow_. In fine weather, just before sunset, the peaks to the
eastward begin to show a reddish or golden hue. This fades gradually,
but in a few minutes, when the sun is a little below the horizon of
the observer, but the peaks themselves are still bathed in direct
sunlight, an intense red glow, beginning down the slopes, moves upward
to the summits. This is identical with the antitwilight arch described
above. Presently this glow is succeeded by an ashy tint, as the peaks
are immersed in the rising shadow of the earth (the dark segment).
Their rocks and snows assume a livid appearance, aptly described by
the inhabitants of Chamonix, whence the phenomena in question are
well seen on the summit of Mont Blanc, as the _teinte cadavéreuse_.
In ordinary weather darkness succeeds without any further notable
phenomena, but occasionally there occurs a remarkable renewal of rosy
light upon the peaks, known as the _recoloration_ or _afterglow_. At
Chamonix this is termed the “resurrection of Mont Blanc.” The afterglow
has been variously explained, but it is probably due, mainly at least,
to the reflection of the purple light in the western sky. Sometimes
it lasts until an hour after sunset, and it passes away from below
upward. On very rare occasions there is a second afterglow, presumably
the reflection of the second purple light mentioned above. Similar
phenomena are often seen in reverse order at sunrise.
A pretty phenomenon observed chiefly in the late afternoon and early
morning consists of beams of light radiating from the sun, known
technically as _crepuscular rays_. The beams are made visible by the
presence of abundant dust or water droplets in the atmosphere, and the
intervening dark spaces are the shadows of clouds. When the sun is
above the horizon and the beams are directed downward, the phenomenon
is popularly described as “the sun drawing water” and is regarded as a
sign of rain. Sailors call these beams the “backstays of the sun,” and
they have several other names based upon the legendry associated with
them in different parts of the world. After sunset or before sunrise
a fanlike sheaf of the beams often extends upward from the western or
eastern horizon, respectively. The Homeric expression “rosy-fingered
dawn” probably refers to this phenomenon. In all cases the apparent
divergence of these beams is an effect of perspective, as they are
really parallel. A rarer phenomenon is that of _anticrepuscular rays_,
which appear to converge to a point opposite the sun. In this case the
beams and shadows are projected entirely across the sky, but their
paths can very seldom be traced in the upper part of the heavens
because in this direction the observer’s line of sight passes through a
comparatively shallow extent of dusty atmosphere.
An analogous phenomenon is seen in the shadows which near-by isolated
mountain peaks frequently cast upon the sky opposite the sun at sunrise
and sunset. Travelers have described such shadows cast by Adam’s Peak
in Ceylon, Pike’s Peak in the Rocky Mountains, and Fujiyama in Japan.
The phenomenon is said to be especially striking in the polar regions,
where the air is often heavily charged with particles of ice.
One more optical phenomenon of sunrise and sunset that requires mention
here seems to be comparatively little known to the nonscientific
public, notwithstanding the fact that it has supplied the subject and
title of a diverting novel by Jules Verne. The conditions required
for its appearance are a clear and steady atmosphere and a sharply
defined horizon, such as that of the ocean. At the instant the sun is
appearing or disappearing, and when only a very small segment of its
disk is visible above the horizon, this portion appears to be colored
a bright emerald green, sometimes blending into blue. This transient
phenomenon is known as the _green flash_. It is best explained as due
to the different degrees of refraction undergone by rays of different
wave lengths coming to us from the sun. The effect of refraction in
elevating the solar image as a whole when near the horizon has already
been mentioned. This effect is a little greater for the green and blue
rays than for the orange and red. It is still more pronounced for the
violet and indigo rays, but these are mostly sifted out of the solar
beams in their long passage through the atmosphere when the sun is low.
Hence at the upper edge of the solar image there is a narrow green or
blue fringe, which is not, however, perceptible except when a screen
is interposed between the eye and the bright image of the sun. A sharp
horizon furnishes such a screen. Through a telescope it is possible,
in suitable weather, to see the green flash--and also a corresponding
“red flash” at the lower edge of the sun--by placing an opaque
diaphragm in the focal plane of the object glass. Another explanation
of the green flash--which could, however, account only for its
appearance at sunset and not at sunrise--is that it is a physiological
effect; the eye, fatigued by the reds and yellows that predominate in
the light of the setting sun, sees an “after image” of complementary
hue the instant after the real image has disappeared.
We now turn to a group of phenomena, also due to atmospheric
refraction, which includes some of the most bizarre of optical
illusions. The simplest of these phenomena consists of a slight
apparent elevation of all objects in the surrounding landscape through
_terrestrial refraction_, which is identical in principle with
astronomical refraction and depends upon the difference in density, and
hence of refractive power, of the air at different levels above the
earth.
Normally the air decreases in density at a nearly regular rate with
increasing altitude. Sometimes, however, this change in density is
greatly modified by local effects of temperature. Over a cold surface
of land or water the adjacent air may be abnormally dense, resulting
in an unusually rapid decrease of density with altitude. Over a hot
surface, as in the case of a desert under strong sunshine, the adjacent
air may become so much rarefied that, for a certain distance upward,
there is actually an increase of density with ascent, instead of the
reverse. The rays of light coming to us from distant objects are bent
in different directions and to various degrees by virtue of these
abnormalities in the density of the atmosphere. The apparent positions
of such objects depend upon the angle at which the light rays, coming
from them, strike the eye of the observer. Sometimes the objects appear
to be lifted far above their true positions (a phenomenon known as
_looming_) and sometimes depressed far below them; and occasionally
local irregularities in air density produce curiously distorted images
of these objects.
Most of these strange effects are known collectively as _mirage_. There
are many varieties. There is the “desert mirage,” first made famous
through the experience of Napoleon’s soldiers in Egypt. There are
mirages that suspend the images of remote objects in the sky; sometimes
inverted, sometimes right side up. There is the lateral mirage,
occasionally seen when one looks along the face of a heated wall or
cliff. Lastly, there are the complex displacements and distortions
of objects known as the _Fata Morgana_--a name originally applied to
a phenomenon of this kind visible, on rare occasions, at the Straits
of Messina, but now used generically for similar appearances in other
parts of the world. Some of the finest examples of Fata Morgana are
witnessed in the polar regions.
In the desert mirage an image of the lower part of the sky is brought
down to earth and simulates the appearance of water, while the images
of terrestrial objects, also depressed and inverted by the mirage,
look like the reflections of the same objects upon the liquid surface.
Humphreys says: “This type of mirage is very common on the west coast
of Great Salt Lake. Indeed, on approaching this lake from the west one
can often see the railway over which he has just passed apparently
disappearing beneath a shimmering surface. It is also common over
smooth-paved streets, provided one’s eyes are just above the street
level.” The confusing and obscuring effects of the desert mirage
were illustrated during the fighting between the British and Turks in
Mesopotamia, in April, 1917, when, according to the report of General
Maude, a battle had to be suspended on account of one of these optical
disturbances.
[Illustration: FATA MORGANA ON THE COAST OF GREENLAND
(_From drawing by Scoresby_)]
The strong vertical contrasts in air temperature that occur in the
polar regions produce many remarkable examples of mirage. The pictures
and descriptions of those observed a century ago along the coast of
Greenland by Captain William Scoresby, Jr., have become classical. A
recent episode connected with mirage was the expedition sent north in
1913 to explore “Crocker Land,” which Peary believed he had sighted
from an elevated point in Grant Land in 1906, and which for a time
figured on all maps of the Arctic. The later explorers found no land at
the place indicated, but they observed the same mirage that Peary had
mistaken for distant hills and mountains.
Currents of air of different densities produce, through their varying
effects on atmospheric refraction, the twinkling or scintillation of
the stars, as well as of distant terrestrial lights. Twinkling is much
more violent near the horizon than near the zenith, and more pronounced
on some nights than others. The shimmering of the air over heated
surfaces faces and the “boiling” of celestial objects as seen in the
telescope are analogous phenomena.
[Illustration: INFERIOR MIRAGE
(_From American Museum Journal. Drawing by Chester A. Reeds._)
Out-of-doors when a layer of warm rarefied air arises from contact
with heated ground or warm water, occupying a position below the
colder, more dense normal air, two images of a distant object may be
seen--one inverted beneath the other. This is “_inferior mirage_” and
is explanatory of the appearance of trees and their reflections, which
haunts the desert traveler with the hope of water.]
[Illustration: SUPERIOR MIRAGE
(_From American Museum Journal. Drawing by Chester A. Reeds._)
When a zone of warm rarefied air is sandwiched between normal air above
and colder air below, a “_superior mirage_” of distant objects may be
seen. Three images are produced, one above the other, the middle one
inverted.]
In the refraction phenomena that we have thus far considered the air
is the medium in which the light rays are bent and distorted. In the
production of the _rainbow_, light undergoes refraction, dispersion
(separation of the spectral colors) and reflection by passing through
drops of water in the atmosphere; especially falling raindrops.
The rainbow, perhaps because it is such a common sight, is seldom
observed with careful attention. Hence few people realize that there
are many varieties of this beautiful meteor, and various erroneous
ideas about it are prevalent. The rainbow is always seen in the part
of the sky opposite the sun--or the moon, in the case of the lunar
rainbow--and is high in the heavens when the luminary is low, and low
when the luminary is high. Generally less than a semicircle of the
bow is visible, and never more, except from an eminence. (Aeronauts
occasionally see a complete circle.) The outer border of the bow is
red and the inner blue or violet. Contrary to popular belief and to
statements sometimes found in reference books, it is almost never
possible to distinguish all seven of the spectral colors in a rainbow;
four or five is the usual limit.
The ordinary or _primary rainbow_ has a radius of about 42 degrees at
its outer edge. Very commonly a _secondary_ rainbow is seen, concentric
with the primary bow, and having a radius of about 50 degrees. The
secondary is fainter than the primary, and its colors are in opposite
order--red inside and violet outside. Additional bands of color,
chiefly red and green, may often be detected adjacent to the inner
edge of the primary bow and, less frequently, along the outer edge of
the secondary bow. These are known as _supernumerary bows_. The space
between the primary and secondary bows is somewhat darker than the rest
of the sky.
The common rainbows differ much among themselves in the number and
purity of their colors, the width of the bows, etc., these differences
depending especially on the size of the raindrops. The minute drops of
a fog sometimes give rise to a bow that is almost devoid of color--the
“white rainbow,” or “fog bow.” The rainbows produced by the moon
commonly show little color, on account of the relative faintness of
the light, but the brighter lunar rainbows are often very distinctly
colored.
[Illustration: INTERSECTING RAINBOWS
(_After a sketch by T. Hodge. Courtesy of Scientific American._)]
_Reflected rainbows_ are sometimes seen upon a sheet of water; and
again the image of the sun, as reflected by such a surface, may give
rise to both primary and secondary rainbows in the sky, which appear
to intersect those produced by the sun directly. A horizontal layer of
water drops below the level of the observer’s eye occasionally produces
the so-called _horizontal rainbow_. This may be formed over a bedewed
field or other surface (the “dew bow”); or the drops may be those of
a low-lying sheet of fog, or of water deposited on a floating film of
oil, or, finally, actual raindrops, seen from an elevation, such as the
summit of a mountain. Horizontal rainbows formed by rain have been seen
from the Eiffel Tower.
The common saying,
A rainbow in the morning
Is the shepherd’s warning;
A rainbow at night
Is the shepherd’s delight,
is, on the whole, well justified for the following reasons: We see the
rainbow where rain is falling, while the sun is shining in the opposite
part of the sky. Our rainstorms usually come from the west and pass
away to the east. A morning rainbow can only be seen in the west, and
indicates that rain is approaching us. An evening rainbow (ignoring
lunar bows) is seen only in the east, and shows that the rain area is
receding from us, giving place to clear skies.
[Illustration: THE CIRCUMZENITHAL ARC
(_From a drawing by L. Besson in La Nature._)
Parts of the halos of 22° and 46°, upper tangent arc of the 22° halo,
and two parhelia are also shown. The circumzenithal arc is always
brightly colored.]
Ice crystals in the atmosphere, such as those composing the higher
clouds, produce a great variety of optical phenomena, known as _halos_.
Some phenomena of this class are common, others exceedingly rare.
Moreover, there are several theoretically possible forms of halo
of which observations have never yet been reported, so that halo
observing can be recommended to the amateur meteorologist as offering
opportunities for making interesting discoveries.
Halos take the form of narrow rings of definite angular size around
the sun or moon (not to be confused with the coronas, of variable
dimensions, described below), rings passing through the luminary, arcs
in various other positions, and roundish spots of colored or white
light. They may be seen separately or in combination. In rare cases,
a dozen or more different forms of halo are visible at the same time,
producing a most spectacular display. One of the most remarkable
displays of this kind in the history of science was seen, in different
degrees of development, over the eastern United States on November 1
and 2, 1913; an event which greatly stimulated interest in the study
of halos in this country. Complex halos are quite common in the polar
regions; where they are seen not only in the sky, but also in the air,
charged with ice particles, close to the earth.
Whenever a thin veil of cirrus or cirro-stratus clouds overspreads the
sky there is a likelihood that halos will be visible. Those formed
near the sun, however, frequently pass unnoticed, on account of the
dazzling brightness of that luminary. Smoked or tinted glasses greatly
facilitate their observation.
[Illustration: DIAGRAMS OF THE PRINCIPAL FORMS OF HALO
(_After Besson, Monthly Weather Review, July, 1914._)
1. (Upper). Perspective view of the sky, showing the sun (_S_);
ordinary halo of 22° (_a_); great halo of 46° (_b_); upper tangent arc
of the halo of 22° (_c_); lower tangent arc of the halo of 22° (_d_);
ordinary parhelia of 22° (_e_, _e’_); Lowitz arcs (_f, f′_); parhelia
of 46° (_g, g′_); circumzenithal arc (_h_); infralateral tangent arcs
of the halo of 46° (_i_); the parhelic circle (_m_); a paranthelion of
90° (_q_); light pillar, (_u, u′_); the observer (_O_). 2. (Lower).
Perspective view of the sky, showing the observer (_O_); the parhelic
circle (_m_): ordinary paranthelia of 120° (_p_); the paranthelion
of 90° (_q’_); the oblique arcs of the anthelion (_r, r′_); and the
anthelion (_n_).]
The commonest halo is a circle of 22 degrees radius (the _22-degree
halo_) about the sun or moon. When formed by the sun it generally
shows a distinct reddish inner border and traces of other spectral
colors. The lunar 22-degree halo usually appears colorless. This halo
is visible, in whole or in part, to the attentive observer about once
in three days, on an average. Less common, but by no means rare, are
the _parhelia_ or “sun dogs” of 22 degrees (called _paraselenæ_ or
“moon dogs” when formed by the moon), the beautiful _circumzenithal
arc_, and a few other members of the halo family. Most forms of halo
are so uncommon that their appearance is an event of some scientific
importance.
The accompanying diagrams, by Dr. Louis Besson of the Observatoire de
Montsouris, show the positions, with respect to the sun (or moon), of
the majority of known halo phenomena. The upper diagram shows the halos
that occur on the same side of the sky as the sun (or moon), and the
lower those that appear on the opposite side. Most of these halos, when
bright, show the spectral colors. The circumzenithal arc, _h_ (commonly
described, by the uninitiated, as a “rainbow”), and the parhelia of 22
degrees, _e, e′_, are especially brilliant in their coloration. The
_parhelic circle, m_, which sometimes extends entirely around the sky,
is white, and so are a few of the rarer forms of halo.
The _upper_ and _lower tangent arcs of the halo of 22 degrees_, _c_ and
_d_, undergo striking alterations, with changes in the altitude of the
sun. When the luminary is more than about 40 degrees above the horizon,
these two arcs become joined at their tips to form the _circumscribed
halo_, and at still greater solar altitudes this halo contracts from an
elliptical to a circular form, thus blending into the 22-degree halo
as shown on the next page, where the solar altitudes corresponding to
the different forms of the halo are indicated. The positions of the
parhelia of 22 degrees, _e, e′_, also depend upon solar altitude. When
the sun is on the horizon these “sun dogs” are 22 degrees from the
luminary, and therefore lie in the 22-degree halo; at greater solar
altitudes they lie outside this halo.
The reader who wishes to acquaint himself further with the
different forms of halo and the methods of observing them will find
a comprehensive article on the subject (devoid of mathematical
discussions) in the “Monthly Weather Review” (Washington, D. C.) for
July, 1914.
[Illustration: SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF
THE 22° HALO
When the sun is high they unite to form the “circumscribed halo.”
(Altitude of sun shown in the center of each figure.)]
The ice crystals that produce halos consist of hexagonal plates or
columns, occasionally including complications of structure, such as
pyramidal bases, combinations of plates and columns, etc. These have
the well-known effect of prisms in refracting and dispersing light
that passes through them. It is evident that there are many possible
paths for the light rays through the sides and bases of such crystals,
resulting in different deflections and corresponding differences in the
forms and positions of the halos produced. The attitudes assumed by the
crystals as they slowly sink through the air, and the oscillations
they undergo, are further points to be considered in working out the
theory of each form of halo by the application of the laws of optics.
Nearly all the known forms have been fully explained. A few species of
halo--notably the parhelic circle (called _paraselenic circle_ when
formed by the moon)--are due to simple reflection from the faces of the
ice crystals, and not to refraction.
The last group of optical phenomena that we shall consider consists of
those due to the process called _diffraction_, which occurs when light
is bent around objects in its path, instead of passing through them, as
in refraction. The process involves separation of the prismatic colors.
The diffraction phenomena of the atmosphere are produced by the water
drops of clouds and fog, or sometimes by fine dust.
Everybody is familiar with the nocturnal spectacle which Tennyson
describes as
... the tender amber round
Which the moon about her spreadeth,
Moving thro’ a fleecy night.
This diffuse reddish or rainbow-tinted circle is called a _corona_.
It occurs about the sun as well as the moon (though not easy to see
on account of the glaring brightness of the luminary), and also about
street lamps and other terrestrial lights when viewed through a misty
atmosphere. Unlike the halos, it has no definite angular size. It is
usually only a few degrees in radius. Small coronas are produced by
large water drops and large coronas by small drops, while the largest
of all coronas, known as _Bishop’s ring_, is due to exceedingly
fine dust in the atmosphere, and has been seen after great volcanic
eruptions.
In its commonest form the corona consists of a brownish-red ring,
which, together with the bluish-white inner field between the ring
and the luminary, forms the so-called _aureole_. If other colors are
distinguishable, they follow the brownish red of the aureole (in the
direction away from the luminary) in the order from violet to red; the
reverse of the order seen in halos. Sometimes the sequence of colors is
repeated three or four times.
Patches and fringes of iridescence are sometimes seen in the clouds at
a greater distance from the luminary than that of the ordinary corona.
Probably they are fragments of coronas of unusual size produced by
exceedingly fine cloud particles.
Similar in appearance to the corona is the _glory_; a series of
concentric colored rings seen around the shadow of the observer, or
of his head only, cast upon a cloud or fog bank. Such a shadow, with
or without the glory, constitutes the _specter of the Brocken_, often
seen from mountain tops and from aircraft. The colored circles are
sometimes called _Ulloa’s rings_, from the name of a Spanish _savant_
who observed the phenomenon among the mountains of South America in the
eighteenth century and has left us a vivid description of it.
The Brocken specter, though it owes its name to legends associated
with the famous German mountain where witches were once believed to
assemble on Walpurgis Night, is actually less frequently witnessed
there than in many other parts of the world. Whenever the sun is low
on one side of a mountain and a wall of mist arises from a near-by
valley on the other, the mountaineer is likely to see his shadow upon
the mist. If the latter consists of fine droplets of approximately
uniform size, the colored rings will probably appear, and occasionally
there is also a white fogbow outside of the glory. As all shadows cast
by the sun taper rapidly (on account of the angular breadth of the
solar disk), a well-defined Brocken specter can never be more than a
few yards away from the observer. Its distance is, however, commonly
overestimated--some observers have supposed it to be miles away!--and
hence the erroneous idea prevails that the specters are of enormous
size.
Rarely from a favorable point of vantage on a mountain, and very
frequently from aircraft, the specter, instead of being seen on a
vertical wall of mist when the sun is low, appears on a horizontal
sheet of cloud below the observer when the sun is high. The aeronaut
may thus observe the complete outline of his balloon or aeroplane,
encircled with the rainbow tints of the glory. During the World War the
appearance of the luminous rings was likened to the emblem painted on
the wings of the Allied aeroplanes and was regarded by superstitious
aviators as an omen favorable to their cause.
The glory is due to the light that is reflected back to the observer
after penetrating the cloud or fog a little way and is diffracted by
the superficial layer of drops in emerging.
The Brocken specter and the glory have occasionally been photographed.