posed. The effect of these motions constitutes a
condition which we term heat or light, according
as it affects certain nerves. A portion of the ra-
diant energy is used up in this generous perform-
ance, or in other words is said to be absorbed.
Relatively, more heat can be derived from the
long wave rays of red colour, and more light
OPTICAL PHENOMENA OF THE ATMOSPHERE. 143
from the short wave rays of violet colour, but
both are produced all through the spectrum.
Heat and light, therefore, are simply effects of
the same wave motion according as it specially
affects our senses of sight or feeling, and they both
inseparably belong to the same radiant energy
of wave motion of the aether, started by a body
already in a state of incandescence like our sun.
Rays near the violet end of the spectrum pro-
duce the chemical action noticed in photography
besides being converted into heat and light. Dawn
and twilight have ever formed expansive themes
to the poet. " Rosy-fingered dawn " is a familiar
metaphor of the immortal Homer. These half
lights are the result of reflection of the sun's rays
when below the horizon, chiefly by the small dust
and water particles at great heights in the atmos-
phere. The fingered appearance alluded to by
Homer is due to the light passing between clouds
or mountains below the horizon. The reddish
colours of the clouds at both times are chiefly
due to the selective scattering which is exerted
by the dust and vapour suspended in the air.
The smaller waves corresponding to the blue
rays, as we have already remarked in Chap. VI.,
are more easily turned aside than the larger ones
corresponding to the red rays, and this dispersion
reaches its greatest effect when the sun is shining
through a great thickness of air at its rising and
setting. Consequently red rays predominate at
these times and tint the clouds as they succes-
sively receive its parting or coming rays. Occa-
sionally when a sunset has disappeared below the
western horizon it is brilliant enough to cause a
second sunset on clouds near the western horizon
by reflection, just as though it were the sun itself.
144 THE STORY OF THE EARTH'S ATMOSPHERE.
When the dust ejected by the volcano of Kra-
katoa Island in 1883 had spread in a layer above
50,000 feet all over the world we had such bril-
liant primary and reflected sunsets, which often
lasted i hours after the sun had disappeared.
The reflection was assisted by a peculiar ac-
tion called diffraction, by which white light meet-
ing fine dust is split up into its coloured elements,
just as though it passed through a glass prism.
In this way a huge coloured ring called a corona
was produced round the sun. This ring is blue
inside, and exhibits thence all the spectral colours
in turn, ending with red at its border.
Inside this ring a white central glow was pro-
duced even when the sun was high up in the sky.
When it was setting this diffraction glow became
pink and finally red through the extinction of all
other colours except the reds owing to the great
length of air traversed by the rays. Such glows
are always present to some extent, due to dif-
fraction by suspended water particles, but when
the Krakatoa dust was still in the upper atmos-
phere they were intensified, and the ordinary re-
flections prolonged far beyond their .usual limits.
Similar small coronse are produced when small
clouds pass over the sun and moon. On a small
scale they can be frequently observed when we
look through our eyelashes at the flame of a
candle or gas lamp.
The smaller the particles of cloud the larger
the corona. Hence the large corona seen round
the sun after Krakatoa, called Bishop's ring from
its discoverer in Honolulu, showed that the ma-
terial was composed of very small particles.
A halo is a large ring seen when the sun and
moon shine through a thin sheet of cirrus or
OPTICAL PHENOMENA OF THE ATMOSPHERE. 145
cirro-stratus, and can only be produced by re-
fraction through ice-prisms. Consequently its
presence is one indication of the ascent of vapour
into very lofty regions, such as occurs in cyclones.
It is thus a signal of the approach of rainy and
A primary halo is always the same size 45
diameter. Sometimes, however, secondary halos
are formed by more complicated refractions and
reflections of light through the ice prisms.
For example, outside the ordinary halo, and
concentric with it, an extraordinary halo is occa-
sionally seen of 90 diameter. Intersecting these
halos, a huge circle passing though the sun and
parallel to the horizon makes its appearance. At
the points of intersection of these halos, the light
is so reinforced that the patches look like sepa-
rate suns, and form what are termed mock-suns or
parhelia. Similar appearances round the moon or
mock-moons are termed paraselenae. At the oppo-
site points of the sky similar mock-suns are occa-
sionally formed. Some years back the author
saw four mock-suns at the same time. Two in
front where the primary halo intersected the large
horizontal halo, 22^ on each side of the sun, and
two behind him, making angles of 157^ with the
sun on each side.
The mirage, or serab (illusion), as the Arabs
term it, is a phenomenon which has often formed
a subject for the poet as well as the artist.
The thirsty traveller in the dreary and parch-
ing wastes of the Sahara and Arabian deserts
frequently sees looming up in the distance a
beautiful lake dotted over apparently with islands
and trees. This lake is an illusion produced by
the bending or reflection of the light that occurs
146 THE STORY OF THE EARTH'S ATMOSPHERE.
at the boundary of two strata of air of different
temperatures. In this case a layer of cool air
overlies one of very hot air just above the heated
sand. Any object, such as a tree or mound above
this layer, has its image inverted by reflection,
while the light from the ground is thrown back
by what is termed internal reflection. Conse-
quently the effect is just the same as though a
layer of water were really present. A special kind
of mirage is termed " looming." In this case
objects which are ordinarily below the horizon
are seen raised above it, sometimes inverted and
sometimes erect. These effects are due to a great
increase in the ordinary refraction which takes
place near the horizon, due, probably, to a cold
and dense layer of air over the sea, overlain by a
warmer layer derived from the neighbouring
land. The famous Fata Morgana or castles of
the witch Morgana of Reggio are an instance of
this kind of mirage. During certain conditions
of the air the inhabitants of Reggio see castles
and men and trees, etc., suspended above the sea
in the direction of Messina, whose reflected image
they really are. A southern imagination con-
verts them into enchantments.
A curious effect of looming occurred once at
Malta, where the top of Etna appeared by refrac-
tion like an island in the sea. Several ships sailed
out to take possession of this supposed new island,
but soon the image vanished and the quest was
seen to be vain.
This story was paralleled more recently when
the gorgeous Krakatoa sunsets first made their
appearance in America. A local fire brigade in
a raw Western township, seeing the sky so red,
with more zeal than wisdom harnessed up and
OPTICAL PHENOMENA OF THE ATMOSPHERE. 147
set forth with all speed to put it out. When
they ultimately found out their mistake they
were not a little put out themselves.
In the polar regions, where the sea is usually
colder than the air, the images of objects below
the horizon are frequently reflected to the ob-
server from the top warm layer and appear in-
verted. If the upper warm layer is of no great
thickness, there is thus often both a direct and
inverted image. Scoresby once recognised his
father's ship, the fame, by observing its inverted
image through a telescope. The real ship was
afterwards found to have been thirty-five miles
The rainbow has always been a majestic sym-
bol of the union between earth and heaven.
Iris, the goddess of the rainbow, was one of
the most graceful of the Grecian deities. She
was represented as the messenger between
Olympus and his earthly subjects.
According to the Teutonic mythology the
rainbow was the bridge over which the heroes
passed to the festive abode of Walhalla.
Robbed of its fanciful mysticism, the rainbow
loses nothing of its beauty when we know that it
is the result of the refraction of the white light
from the sun as it enters the raindrop subse-
quently reflected from the back of the drop to
our eyes. The whole operation is so wonderful.
The different coloured rays which make up the
white ray when they meet the new surface, part
company according to their wave frequency, and
travelling along separate paths are reflected by
the mirror back as though they were painted in
the sky. The tiny violet waves being more bent
inwards, appear inside the bow, while the longer
148 THE STORY OF THE EARTH'S ATMOSPHERE.
red waves form the external boundary. Ordina-
rily the earth cuts off the lower half of the bow,
and when the sun is more than 40 above the
horizon, the entire phenomenon disappears.
Inside the bow the violet is occasionally seen
repeated in what are termed supernumerary bows,
while the external bow is often visible in which
the colours are reversed. The explanation of
these belongs rather to a book on optics. The
" Spectre of the Brocken " is simply a shadow of
the spectator projected on to a screen of vapour
rising up from the surrounding valleys, and may
be seen on any mountain where the conditions
The " ignis fatuus," or wandering flame occa-
sionally seen in marshy land, or over church-
yards, where it is called the "corpse candle," is
believed to be merely a distillation from the soil
of phosphoretted hydrogen gas which has the
property of self-ignition on emerging into the
The "aurora polaris " or "northern lights"
are a manifestation of quiet electrical discharge
round either pole, attaining its greatest brilliancy
and frequency near the magnetic poles, which are
at some distance from the true geographic poles.
In the northern hemisphere the belt of greatest
frequency (80 auroras per annum) occurs from
latitude 50 to 62 in America, and from latitude
66 to 75 over Siberia. From thence they dimin-
ish both north and south.
The Aurora exhibits various forms. Stream-
ers, curtains, bands, and rays, and it frequently
coruscates, whence the name " Merry Dancers."
It is believed that the Aurora is a sheet of rays
which converge downwards toward the magnetic
WHIRLWINDS, ETC., OF THE ATMOSPHERE. 149
axis of the earth, a kind of luminous collar, the
top of whose arch is as much as 130 miles above
the earth, though parts of it are believed to be
quite near the earth. It is therefore an electrical
discharge taking place in highly rarefied air or
vacuum. Lemstrom of Finland recently suc-
ceeded in causing an artificial aurora by suitably
imitating what is believed to occur in Nature.
The Aurora is certainly closely connected with
the magnetic condition of the earth and also of
the sun. When any great sun-spot appears on
the latter orb, the magnetic balance of the earth
is affected, as shewn by the irregular movements
of the magnetic needles and the simultaneous
appearance of aurorse at both poles.
WHIRLWINDS, WATERSPOUTS, TORNADOES, AND
THUNDERSTORMS OF THE ATMOSPHERE.
BESIDES the large cyclones, there is a peculiar
group of local disturbances or storms of the at-
mosphere which, according to their violence, occur
in one or other of the above forms. The harm-
less dust-whirl we see arise on a still day in early
summer, and sweep across the young corn, is but
the embryo of the terrible tornado of the Middle
The dreaded simoom of the Arabian Desert is
simply a larger whirlwind laden with the dust of
the desert. Where the whirl is broader and high-
er, and the air is moist, we have the common
thunderstorm of Europe with or without hail, the
150 THE STORY OF THE EARTH'S ATMOSPHERE.
"nor'-wester" of India, the "pampero" of the
Argentine, and the so-called " arched squall " and
"bull's eye squall" of the tropical seas.
When the action is very intense and concen-
trated, we have the " tornado " which is common
in the Mississippi Valley. The freaks of some of
these tornadoes, while generally of the tragic or-
der, occasionally border on the ridiculous. Thus
even in India where they occasionally occur in a
mild form it is stated that in the district of the
Brahmaputra, on March 26, 1875, after a tornado
had passed the village of Uladah a dead cow was
found stuck in the branches of a tree some 30
feet from the ground.
In America, in the tornado of June 4, 1877, at
Mount Carmel, Illinois, the spire, vane, and gilded
ball of the Methodist Church were carried fif-
teen miles to the north-eastward. In other cases
ploughshares and even houses (generally of wood)
have been carried up into the air, and, so to speak,
transplanted. In the recent terrible visitation at
St. Louis, in June 1896, it was stated that a car-
riage was lifted from the road up into the air and
gently let down again 100 yards off without dam-
age, while at the end of this remarkable perform-
ance the coachman's hat was declared to have re-
mained securely attached to his head. This last
circumstance sounds extreme, but there is no ob-
vious exaggeration in that given by one spec-
tator who informed the writer that he looked up
a street in St. Louis and saw everything horses,
carriages, people, and furniture being whisked
along in tumultuous chaos towards him as the
centre of the tornado passed over it.
When the centre of a tornado passes it seems
to sweep everything movable along with it, often
WHIRLWINDS, ETC., OF THE ATMOSPHERE. 151
destroys the most substantial buildings and cuts
a clear lane through a forest. In all these cases,
the prime cause appears to be a local instability
of the air due to an aggregation of heat near the
surface, combined with an incursion of cold air
in the stratum above. These together cause a
rapid fall of temperature in a vertical direction.
In such a case even dry air may temporarily
ascend in a narrow column and burst through the
When once this has taken place the surround-
ing air rushes in to supply its place, and there
ensues a whirling round just as in the case of
water running down through a sink.
In Tornadoes the whirling round of the air is
not due as in that of the large cyclones to the
deflection caused by the rotation of the earth, since
this would be practically insensible for move-
ments within such limited areas. It is due to the
rapid development of gyration as the air is
forced inwards towards the centre when once
such gyration has started. The slightest devia-
tion to one side of the direct path to the centre
is enough to start a gyration, and any slight
irregularity in the flow suffices to cause a devia-
After the whirling has once started, the gyra-
tions near the centre become so rapid that ulti-
mately a funnel shaped column of highly rarefied
air is produced, which is marked by the appear-
ance of a sheath of cloud or water within which,
in extreme cases, is a nearly complete vacuum.
Round and up the sides of this the air ascends,
flows out above, and again quietly descends over
a wider area.
When the air is dry, the action, as we know,
152 THE STORY OF THE EARTH'S ATMOSPHERE.
cannot continue very long, since the uprising air
soon reduces the vertical temperature differences
below i in 180 feet. Dust whirls and sand
storms are consequently short-lived and never of
When, however, the lower air is very damp as
well as hot, the action can go on for a much
longer time and with far greater energy.
Lieut. Finley of the U. S. Navy, who has made
a special study of American tornadoes, estimates
that the velocity of the wind rotating near the
centre of a tornado may reach as much as 500
miles an hour, and exert a pressure of 250 Ibs. to
the square foot. Even the upward velocity near
the vortex probably amounts, in many cases, to
over loo miles an hour, otherwise it could not
sustain the objects it visibly does.
The awful effects frequently produced by the
arrival of such a piece of what may be termed
meteorological dynamite can therefore be understood.
The central column of rarefied air by reason of
its expansion is cooled below dew point. Hence,
whatever vapour exists there, becomes condensed
into a visible sheath. This is the cause of what
are termed waterspouts, which are only a mild
form of tornado. In the real tornadoes, the black
funnel shaped cloud, which forms one of their
most marked features, is due to the same causes.
The popular notion of a waterspout accounts for
the water by imagining it to be drawn up from
the sea. But this is erroneous. When water-
spouts pass over the sea, they cause a disturbance
and slight upward rise round their bases, but the
long visible column, often half a mile in length,
which dips down from the clouds, is entirely com-
posed of vapour, condensed out of the inflowing
WHIRLWINDS, ETC., OF THE ATMOSPHERE. 153
air. As Ferrel puts it " the cloud (or rather the
conditions which favour the production of cloud)
is here drawn down towards the earth by the re-
duction of pressure produced by the rapid whirl-
ing of the air."
At the same time, the downward dip is only an
apparent and not a real descent of water. As
long ago as 1753, indeed, the great Franklin cor-
rectly explained this where he says
" The spout appears to drop or descend from
the cloud though the materials of which it is com-
posed are all the while ascending, for the moisture
is condensed faster in a right line downwards, than
the vapours themselves can climb in a spiral line up-
The freshness of the water in a marine spout
is clearly testified to in a story quoted by Prof.
Davis in his Meteorology :
A waterspout had fallen upon a vessel and
poured its contents so freely over the captain,
that he was nearly washed overboard. He was
asked afterwards, rather jocularly, if he had
tasted the water ? " Taste it," said he, " I could
not help tasting it. It ran into my mouth, nose,
eyes and ears." Was it then salt or fresh asked
his querist ? " As fresh," said the captain, " as
ever I tasted spring water in my life."
Waterspouts occur mostly in the tropics, and
during the day hours. They are children of the
The prevailing funnel shape, tapering down-
wards, of the waterspout or tornado cloud, is a
consequence of the increased pressure of the air
near the surface. Above the surface the absence
of friction and the lower pressure allows the cen-
tral area of rarefaction, produced by the rapidly
154 THE STORY OF THE EARTH'S ATMOSPHERE.
whirling air, to extend for some space laterally.
Lower down the centrifugal tendency of the ro-
tating air is met by increased inward pressure
and is thus confined to a narrower space. Out-
side the central core the air moves gently towards
the centre. When water in a basin is descend-
ing through a hole, a similar gentle flow may
be observed, the rapid whirling only extending
for a short distance immediately around the hole.
Even in destructive tornadoes the area of
dangerous damage and violent wind is confined
to comparatively narrow limits. The width of
the destructive path of the tornadoes in America
has been found by Finley to vary from 20 feet
to about 2 miles, the average being about 1369
The length of their paths is usually not more
than 20 miles, since the forces which give rise to
them, unlike those of cyclones, depend entirely
on specially marked vertical gradients of tem-
perature which seldom prevail simultaneously over
The mode in which the air travels up into and
round these phenomena, may be gathered from
the adjoining Fig. (36). Instead of rising up
vertically it travels along the lines which are
represented as winding spirally round the funnel
until it becomes cooled partly by ascent and
partly by expansion into the tornado-core and its
vapour becomes visible at a point C considerably
below the ordinary cloud level FH.
Tornadoes may be regarded as a kind of at-
mospheric eruption analogous to those by which
the volcanic energy of the earth's interior is ex-
pended in one spot.
They prevail where the local conditions favour
WHIRLWINDS, ETC., OF THE ATMOSPHERE. 155
the establishment of explosive heat conditions.
For example, where the geographical conditions
are favourable to the facile movement of cold air
from the north alongside or above warm air from
Such an area exists par excellence over the
flat river basins of the Mississippi, Missouri,
Fio. 36. Tornado funnel cloud.
and Ohio. The states lying in these basins are
those in which tornadoes are found to be most
The general north and south trend of the
mountains and hills in America favours the flow
of air of such contrasted conditions, while the
prevalent east and west ranges in the old world
make them act as preventive barriers.
The time of year most favourable to the pro-
duction of tornadoes is Spring or early Summer,
when the earth is heating up rapidly, and the air
156 THE STORY OF THE EARTH'S ATMOSPHERE.
above it is still cold from the effects of the pre-
ceding winter. Lieut. Finley found May to be
the month of greatest frequency of tornadoes,
while during autumn and winter they are almost
absent. The time of day at which they mostly
occur is in the afternoon when the accumulation
of heat in the lower layers has reached its great-
est amount. When the gun is loaded it only re-
quires the slightest pull on the trigger to release
an immense potential of energy. Half a degree
more temperature and the tornado is born and
starts off on its wayward journey.
The destruction caused by these tornadoes in
America is hardly realised in Europe which is so
happily exempt from them. At the same time
the deaths from this cause in the U. S. are esti-
mated to be less than those caused by fire and
Thunderstorms, like tornadoes, originate from
the uprise of a mass of warm moist air, but the
width of the column of uprising air is much
greater, and the whole action is much less con-
centrated and violent.
The vertical anatomy of a thunderstorm is
shewn in Fig. (37) where the spectator is sup-
posed to be standing to the right and viewing an
advancing storm. First of all he sees a layer of
cirro-stratus cloud (c) * commonly in the western
sky in the afternoon. Gradually this grows
thicker, and from its under surface festoons (/)
similar to those in the " Festooned Cumulus,"
Fig. (28), appear.
* This layer should extend as far again as the width of the
figure to the right. The exigencies of space have necessitated
its curtailment in the adjoining figure.
WHIRLWINDS, ETC., OF THE ATMOSPHERE. 157
The cirro-stratus may extend from 10 to 50
miles in advance of the storm. In this way as
soon as they are visible, thunderstorms may read-
ily be forecasted within a few hours by experts
such as the late Rev. Clement Ley. Then follow
the thunderheads (/) of cumulo-nimbus (as in
frontispiece) which represent the front portion of
the uprising current. Below these, a low level
FIG. 37. Thunderstorm in section.
base (b] of similar cloud is seen, underneath which
is a rain curtain (r). A ragged squall cloud (s)
rolls beneath the dark cloud mass, a little behind
its forward edge, and the whole structure moves
over the land at the rate of from 20 to 50 miles'
an hour. As the squall cloud comes overhead
the wind changes suddenly from an in-flow to an
out-flow, represented in the figure by two arrows
near the surface, with heads to the right. In con-
trast with the hot muggy air preceding the storm,
this squall is deliciously cool, especially in a Ben-
gal north-wester. Simultaneously with the arrival
of the squall, the barometer rises about ^V-h f
an inch, the rain or hail begins to fall, the light-
ning flashes and the thunder crashes right over-
head, until the centre passes, and everything
gradually resumes its former aspect, except the
temperature which has been permanently low-
158 THE STORY OF THE EARTH'S ATMOSPHERE.
ered. The outflowing squall is believed to be
very similar to the recoil of a gun when it is dis-
charged. The humid air in the centre of the