to that which produces the monsoon of eastern seas--that is, the
influence of the sun heating in various measures the lands and
seas, and with them the superincumbent air. Successive movements
are generated in the atmosphere according as different portions of
the whole acquire, with difference of temperature, various degrees
of density. During the hours of midday heat, the air over the land
becomes relatively hotter, by many degrees, than the air which is
above the adjacent water, for it is the well-known attribute of land
to experience much greater extremes of temperature than water does.
As afternoon, with its sultry temperature, advances, this continued
heat occasions the land-air to form an ascending current, while the
cooler (and relatively denser) air from the neighboring waters flows
in to take its place. This cooling breeze is an effort of nature to
restore equilibrium in the atmosphere, the heavier portions of the
whole body of air assuming the place of lower strata, and the higher
portions spreading over the superior regions. This effort continues
until the desired balance is attained, and, with the approach of
midnight, the air is again calm and settled. But during the night,
while the water retains a nearly uniform temperature, the land
rapidly parts with the heat, so that the air over the land becomes
at length colder than that over the water. This latter, therefore,
relatively the warmer of the two, tends to rise, while the cooler air
of the land fills its place. A wind blowing from off the land is thus
generated. In some localities this blows during great part of the
night. But the period of its commencement varies in different places,
and the intervals of calm between both land and sea-breezes are often
of uncertain duration.
The land and sea-breezes repeat, on a scale of diurnal variation, the
phenomena shown by the monsoons on a scale of yearly change. They
show how readily the atmosphere yields to the slightest pressure,
and how powerful an influence on the laws of climate, and, with
them, on the condition of mankind, is exercised by every change,
of temperature, or otherwise, to which it is subject. Similar
winds--alternating from opposite quarters of the heavens--are
experienced in inland districts, as on the banks of the Tapajos
River, in South America.
The rotary storms which occur, at uncertain intervals, in particular
latitudes, are to be included among the exceptional phenomena of
atmospheric change. They prevail, however, over larger areas than
was formerly supposed, and perhaps belong to a general system of
atmospheric movements in which electric and magnetic influences fill
an important place. The hurricanes of the West Indies, the tornadoes
and cyclones of the Indian Ocean, and the typhoons of the China Sea,
are winds of this description. Within the Southern Hemisphere, the
direction of the rotating circle is always found to correspond to the
movement of the hands of a watch (_i. e._, from west to north, east,
and south): to the north of the equator, the circle of wind follows
an opposite direction (or west to south, east, and north). By a
knowledge of this law, combined with careful observation of the track
usually taken by such storms, mariners are enabled to avoid some of
the dangers incident to their occurrence. The destruction which they
occasion, however, within maritime tracts exposed to their influence,
as well as upon the high seas, is at times fearfully great.
Waterspouts are another form in which the rotary movements of the
air are manifested. In the case of these phenomena, a taper column
of cloud, descending from above, is joined by a spiral column of
water which winds upward from the agitated surface of the sea, the
two together forming, by their union, a continuous column which moves
over the sea. Waterspouts seldom last longer than half an hour. They
are more frequent near the coast than on the high seas, and more
commonly seen in warm climates.
SQUALLS, WHIRLWINDS, AND TORNADOES
--SIR RALPH ABERCROMBY
If we watch the stages of gradually increasing wind, we find that as
the strength rises the tendency is more and more to blow in gusts.
Gradually these gusts get still more violent, and in their highest
development come with a boom like the discharge of a piece of heavy
ordnance. This is what sailors call “blowing in great guns,” and
these are the gusts which blow sails into ribbons, and dismast ships
more than any amount of steady wind. These gusts only last a few
minutes, but they seem to be very closely allied to the simplest form
of squalls. In a true, simple squall the wind generally need not be
of the exceptional violence which causes “guns”; but after it has
rather fallen a little, the blast comes on suddenly with a burst,
and rain or hail, according to intensity, or other circumstances,
while the whole rarely lasts more than five or ten minutes. At sea
one often sees two or three squalls flying about at a time. Then we
readily observe that over the squall there is firm, hard, cumulus
cloud; that the disturbance only reaches a short distance above the
earth’s surface; that the squall moves nearly in the same direction
as the wind; and that there is little or no shift of the wind before
or during the squall. We also see that the shape of the squall is
merely that of an irregular patch, with a tendency rather to be
longer in the direction of the wind than in any other quarter; and
that the motion of the squall as a whole is much slower than that
of the wind which accompanies the first blasts. If, at the same
time, we watch our barometer closely, we find that if the squall
is sufficiently strong, the mercury invariably rises--sometimes as
much as one-tenth of an inch--and returns to its former level after
the squall is over. No difference is observed in this sudden rise,
whether the squall is accompanied with rain, hail, or thunder and
lightning; and though we are unable exactly to explain why the wind
sometimes takes this irregular method of blowing, we have still to do
with a comparatively simple phenomenon.
The simplest kind of thunderstorm may more properly be described as
a squall accompanied by thunder and lightning, instead of only with
wind and rain. On a wild, stormy day, with common squalls, one or two
of these, which are exceptionally violent, will be accompanied by one
or two claps of thunder with lightning. The principal interest which
attaches to this type of thunderstorm consists in the proof which
is afforded that there is no essential difference between a common
squall and another which may be associated with electrical discharge,
except intensity. The look and motion of the clouds, and the sudden
rise of the barometer, are identical in both cases. In western Europe
this class of thunderstorm is much more common in winter than in
summer, which is the reverse of what takes place with all other kinds
of thunderstorm. So much is this the case that in Iceland there are
no summer thunderstorms, but only winter ones, of this simple squall
type. In Norway both types occur; and the winter ones are there
found to be the most destructive, because they are lower down, and
therefore the lightning is the more likely to strike buildings. In
that country, however, the summer thunderstorms are not nearly so
violent as in more southern latitudes.
We must now just mention a class of thunderstorms which are more
complicated than a simple squall, and yet differ in many ways from
line-thunderstorms. They are associated with secondary cyclones,
and are much commoner in England than line-thunderstorms, but none
have been tracked over a sufficiently long area to allow us to say
anything about their shape or motion. All we know is, that as surely
as we see a secondary on the charts in summer, so certainly will
thunderstorms occur during the day, though we can not say in what
portion of the small depression.
The special features of this class of thunderstorm are the calm
sultry weather with which they are associated, so different from
the squall of a line-thunderstorm, and the limited rotation of
the surface-wind during the progress of the storm. Another very
remarkable feature is that this surface circling of the wind extends
only a very short distance upward, and whenever a glimpse can be
caught of the drift of the upper clouds, they are found to move in
the same direction throughout the whole period of the disturbance.
This is the familiar class of thunderstorm which we associate with
sultry weather, and with the thunder coming against the wind.
One of the first things which must strike everybody is, that even
in the temperate zone some countries are far more ravaged by
thunderstorms than others. For instance, France suffers more than any
other part of Europe, and England the least. We may probably find at
least two causes which modify the development of thunderstorms. In
the first place, the geographical position of the country relative to
the great seasonal areas of high and low pressure. From this point
of view we can readily see that France is far more exposed to the
influence of small secondaries, which come in from the Atlantic,
and which die out before they reach central Europe, than any other
portion of that continent.
In Great Britain, though the bulk of winter rain is cyclonic, a
great deal of summer rainfall is non-isobaric; in Continental Europe
a still larger proportion is of the latter character; so are most
tropical rains, except the downpour of hurricanes; while the whole of
the heavy rain on the equator, and all that falls in the doldrums, is
also absolutely non-isobaric.
A moderate whirlwind may be two hundred feet high, and not above ten
feet in diameter. The dimensions, however, are very variable, for a
whirlwind may vary in intensity from a harmless eddy in a dusty road
to the destructive tornado of the United States.
But by far the most striking non-isobaric rain in the world is the
burst of the southwest monsoon in the Indian Ocean. The quality of
the rain, if nothing else, distinguishes the monsoon from cyclonic
precipitation. The rain in front of a Bengal cyclone seems to grow
out of the air, while that of the monsoon falls in thunderstorms and
from heavy cumulo-form clouds. The only rational suggestion which has
been made to account for this burst of rain would look to a sudden
inrush of damp air from the region of the doldrums as the source
of the change in weather, but not of the direction of the wind, or
of the shape of the isobars; for the burst is apparently almost
coincident with the disappearance of the belt of high pressure to
the south of the Bay of Bengal.
The word “pampero” is, unfortunately, used in a very vague manner
in the Argentine Republic and neighboring states. Every southwest
wind which blows from off the pampas is sometimes called a pampero;
and there is a still further confusion caused by calling certain dry
dust-storms _pamperos sucios_, or dry pamperos. The true pampero
may be described as a southwest wind, ushered in by a sudden short
squall, usually accompanied by rain and thunder, with a very peculiar
form of cloud-wreath.
The barometer always falls pretty steadily for from two to four days
before the pampero, and always rises for some days after the squall.
Temperature is always very high before the squall, and then the
sudden change of wind sends the thermometer rapidly down, sometimes
as much as 33° in six hours. Thunder accompanies about three out of
four pamperos; but more or less rain always falls, except in the
rarest cases. The wind before this class of pampero almost invariably
blows moderately or gently for some days from easterly points, and
then with a sudden burst the southwest wind comes down with its full
strength, and, after blowing thus from ten to thirty minutes, either
ceases entirely or continues with diminished force for a certain
number of hours. In all cases but one the upper wind-currents have
been seen to come from the northwest before, during and after the
pampero.
The general appearance of a pampero will be best understood by a
description of an actual squall. “In the early morning of a day
in November, the wind blew rather strongly from the northeast. The
sky was cloudy, but not overcast, save in the southwest horizon.
The clouds were moving very slowly from the west, or a little
south of it, throwing out long streamers eastward. About 8 A. M.
the threatening masses in the southwest had advanced near enough
to show that at their head marched two dense and perfectly regular
battalions of cloud, one behind the other, in close contact, yet
not intermingling, and completely distinguished by their striking
difference of color, the first being of a uniform leaden gray, while
the second was as black as the smoke of a steamer. On arriving
overhead, it was seen that the front, although slightly sinuous, was
perfectly straight in its general direction, and that the bands were
of uniform breadth. As they rushed at a great speed under the other
clouds without uniting with them, preserving their own formation
unbroken, their force seemed irresistible, as if they were formed of
some solid material rather than vapor. The length of these wonderful
clouds could not be conjectured, as they disappeared beneath the
horizon at both ends, but probably at least fifty miles of them
must have been visible, as the ‘Cerro’ commands a view of twenty
miles of country. Their breadth was not great, as they only took
a few minutes to pass overhead, and appeared to diminish from the
effects of perspective to mere lines on the horizon. At the instant
when the first band arrived, the wind--which was still blowing, and
something more than gently, from the northeast--went round by north
to southwest; at the same time a strong, cold blast fell from the
leaden cloud, and continued to blow till both bands had passed.”
A whirlwind may be described as a mass of air whose height is
enormously greater than its width, rotating rapidly round a more or
less vertical axis.
A tornado is simply a whirlwind of exceptional violence; if it were
to encounter a lake or the sea, it would be called a waterspout.
Its most characteristic feature is a funnel, or spout, which is the
visible manifestation of a cylinder of air that is revolving rapidly
round a nearly vertical axis. This spout is propagated throughout the
northern temperate zone in a northeasterly direction at a rate of
about thirty miles an hour, and tears everything to pieces along its
narrow path.
The diameter of the actual spout often does not exceed a few yards,
and the total area of destructive wind is rarely more than three
or four hundred yards across. The height of the spout is that of
the lowest layer of clouds, which are then never high; and, as in
thunderstorms, the upper currents are unaffected by the violent
commotion below.
The spout as a whole has four distinct motions:
1. A motion of translation generally toward the northeast at a
variable rate, but which may be taken to average thirty miles an hour.
2. A complex gyration. The horizontal portion of this rotation is
always in a direction opposite to that of the hands of a watch--that
is to say, in the same manner as an ordinary cyclone. But in addition
to this there is a violent upward current in the centre of the
cylinder of vapor or dust which constitutes the spout, and sometimes
small clouds seem to dart down the outer sides of the funnel whenever
these float in close proximity. There are, however, no authentic
instances of any object being thrown to the ground by the individual
effort of a downward current. The slight downward motion of a few
small clouds is probably only a slight eddying of a violent uprush.
3. A swaying motion to and fro like a dangling whip, or an elephant’s
trunk, though the general direction of the spout is always vertical.
4. A rising and falling motion, that is to say, that sometimes the
end of the funnel rises from the surface of the ground and then
descends again, and so on. Owing to this rise and fall, the general
appearance of the tornado changes a good deal. When the bottom of
the spout is some distance above the ground, the whole is somewhat
pointed, and does comparatively little harm as it passes over any
place. As the spout descends, a commotion commences on the surface of
the ground. This latter gradually rises so as to meet the descending
part of the spout, and then the whole takes the shape of an
hour-glass. This is the most dangerous and destructive form, because
the ground gets the whole force of the tornado.
The general appearance of the cloud over a tornado or whirlwind is
always described as peculiarly smoky, or like the fumes of a burning
haystack. The tornado is also never an isolated phenomenon; it is
always associated with rain and electrical disturbance.
The destructive effects of the tornado are very curious, from the
sharp and narrow belt to which the injury is confined. It appears
that in the passage of some tornadoes wind-pressures of various
amounts, from eighteen to a hundred and twelve pounds per square
foot, have been demonstrated by destruction of bridges, brick
buildings, etc. The upward pressures are sometimes as great as the
horizontal, and even greater. Downward pressures or movements of
wind have not been clearly proved. Upward velocities of 135 miles
per hour seem not to be unusual, and horizontal velocities of eighty
miles have been recorded with the anemometer. The destructive
wind-velocities are confined to very small areas. A destruction of
fences, trees, etc., is often visible over a path many miles long and
a few hundred yards wide, but the path of greatest violence is very
much narrower. The excessive cases above referred to are observed
only in small isolated spots, less than a hundred feet square,
unequally distributed along the middle of the track. Thus, in very
large buildings, only a small part is subject to destructive winds.
In different parts of this area of _maximum_ severity, the winds are
simultaneously blowing in different, perhaps opposite, directions,
the resultant tending not to overturn or carry off or crush in, but
rather to twist round a vertical axis. Buildings are generally lifted
and turned round before being torn to pieces. As the chances are very
small that a building will be exposed to the violent twisting action,
it is evidently the average velocity of rectilinear winds within the
path of moderate destruction that it is most necessary to provide
against in ordinary structures. These winds may attain a velocity
of eighty miles an hour over an area of a thousand feet broad, and
generally blow from the southwest; the next in frequency blow from
the northwest. The time during which an object is exposed to the
more destructive winds varies from six to sixty seconds. An exposed
building experiences but one stroke, like the blow of a hammer, and
the destruction is done. Hence, in a suspension-bridge, chimney,
or other structure liable to be set into destructive rhythmic
vibrations, the _maximum_ winds do not produce such vibrations. The
duration of the heavy southwest or northwest winds over the area
of moderate destruction is rarely over two minutes. The motion of
translation of the central spout of a tornado, in which there is a
strong vertical current, is, on ah average, at the rate of thirty
miles an hour.
Tornadoes mostly occur on sultry days and either in the southeast or
right front of cyclones, or in front of the trough of V-depressions.
The general character of all tornadoes is so similar that the
description of one will do for all. We shall therefore give some of
the description furnished by an eye-witness to the United States
Signal Office, which is described in the reports as the “Delphos
tornado”:
“On Friday morning, May 30, 1879, the weather was very pleasant,
but warm, with the wind from the southeast, from which direction it
had blown for several days. The ground was very dry, and no rain
had fallen for a number of weeks. About 2 P. M. threatening clouds
appeared very suddenly in the west (against the wind), attended in a
few minutes by light rain, the wind still in the southeast It stopped
in about five minutes, and then commenced again, wind still the same,
accompanied by hail, which was thick and small at first, but rapidly
grew less in quantity and larger in size, some stones measuring
three and a half inches in diameter, and one was found weighing
one-fourth of a pound. This last precipitation continued for about
thirty minutes, after which a cloud in the shape of a waterspout
was seen forming in the southwest, and moving rapidly forward to
the northeast. The cloud from which the funnel depended, seen at a
distance of eight miles, appeared to be in terrible commotion; in
fact, while the hail was falling, a sort of tumbling in the clouds
was noticed as they came up from the northwest and southwest, and
about where they appeared to meet was the point from which the funnel
was seen to descend. There was but one funnel at first, which was
soon accompanied by several smaller ones, dangling down from the
overhanging clouds like whiplashes, and for some minutes they were
appearing and disappearing like fairies at a play. Finally one of
them seemed to expand and extend downward more steadily than the
others, resulting at length in what appeared to be their complete
absorption. This funnel-shaped cloud now moved onward, growing in
power and size, whirling rapidly from right to left, rising and
descending, and swaying from side to side. When within a distance
of three or four miles, its terrible roar could be heard, striking
terror into the hearts of the bravest.” The eye-witness judged that
the funnel itself would reach a height of about five hundred feet
from the ground. As the storm crossed a river, a cone-shaped mass
came up from the earth to meet it, carrying mud, débris, and a large
volume of water. The cloud then passed the observer’s house very
near to 4 P. M. The progressive velocity at the time was considered
to be about thirty miles per hour, although at Delphos, three and a
half miles distant, it had slackened down to near twenty miles. A few
minutes previous to and during the passage of the funnel, the air was
very oppressive; but ten minutes after the wind was so cold from the
northwest that it became necessary to wear an overcoat when outside.
The actual diameter of this storm appears to have been only
forty-three yards. On the right of the track, destructive winds
extended to a further distance of from one to two miles, sensibly
deflected winds for another mile and a half, beyond which only the
usual wind of the day was experienced. On the left or northern side
of the tornado path, the damage did not extend quite so far, for the
width of the belt of destructive winds was not more than twenty-eight
yards across and that of sensibly deflected winds one mile and a
quarter.
As a specimen of the damage done a large two-horse sulky plow,
weighing about seven hundred pounds, was carried a distance of twenty
yards, breaking off one of the iron wheels attached to an iron axle
one and three-quarter inches in diameter. A woman was carried to the
northwest two hundred yards, lodged against a barbed-wire fence, and
instantly killed. Her clothing was entirely stripped from her body,
which was found covered with black mud, and her hair matted with it.
A cat was found half a mile to the northwest of the house, in which
she had been seen just before the storm, with every bone broken.
Chickens were stripped of their feathers, and one was found three
miles to the northwest.
A few miles further on, another eye-witness says, “the dark, inky,
funnel-shaped cloud rapidly descended to the earth, which reaching,
it destroyed everything within its grasp. Everything was taken up and
carried round and round in the mighty whirl of the terrible monster.
The surrounding clouds seemed to roll and tumble toward the vortex.
“The funnel, now extending from the earth upward to a great height,
was black as ink, excepting the cloud near the top, which resembled
smoke of a light color. Immediately after passing the town, there
came a wave of hot air, like the wind blowing from a burning
building. It lasted but a short time. Following this peculiar
feature, there came a stiff gale from the northwest, cold and bleak,
so much so that during the night frost occurred, and water in some
low places was frozen.”
END OF VOLUME TWO
FOOTNOTES:
[1] The lakes of Sweden, which cover one-twelfth of the surface of
the country, exercise an important influence on climate according as
they are frozen or open.
[2] Another variety or species of seal inhabits Lake Baikal.
[3] Count von Helmersen, however, has stated his belief that for this
extreme northern prolongation of the Aralo-Caspian Sea there is no
evidence. The shells, on the presence of which over the Tundras the
opinion was chiefly based, are, according to him, all fresh-water
species, and there are no marine shells of living species to be met
with in the plains at the foot of the Ural Mountains.
[4] Archbishop of Spalato and Primate of Dalmatia.
TRANSCRIBER’S NOTE