greater importance than the direction of prevailing winds. The
currents of air are warm or cold, wet or dry, according as they
have had their origin in warm or cold latitudes, and have traversed
inland tracts, or the expanse of ocean, in their advancing course.
With us, and in the northern half of the globe in general, north
and east winds are cold and dry, while south and west winds are
warm, and often accompanied by moisture. Within the Southern
Hemisphere these conditions are reversed, and the hottest currents
of air come from a northwardly direction. The prevailing winds of
western Europe are from the west and southwest; and it is to this
fact that we must mainly ascribe the high winter temperature, as
well as the comparative freedom from extremes of heat and cold
which distinguishes the countries of western Europe. The same cause
explains the abundant moisture which belongs to those regions in
general, and which distinguishes the western shores of our own
islands in a remarkable degree. Such winds have traversed the immense
expanse of the Atlantic, and come to the western seaboard of Europe
laden with the moist vapors gathered on their course. These vapors,
condensed upon the high grounds which line the western side of the
British Islands, or, further to the northward, upon the long chain of
the Scandinavian Mountains, fall to the earth in copious torrents of
rain. In the process of condensation, a vast quantity of latent heat
is disengaged, and the temperature is correspondingly raised. Warmth
and moisture are, indeed, speaking generally, concomitant conditions
of European climate, and are especially so in the case of western
Europe.
Even in the case of lands which nearly approach the tropic, the
influence of prevailing winds in raising or lowering the temperature
is strikingly seen. At New Orleans, bordering on the Mexican Gulf,
and throughout the adjacent portions of the United States, the
winters are often of excessive severity. Cold winds, generated in
the higher latitudes of the New World, and blowing for weeks in
succession from the northern quarter of the sky, are the cause of
this. The generally level interior of the North American Continent--a
vast lowland plain, bounded only to the east and west by the
Alleghanies and the Rocky Mountains--presents no obstacle to the
advance of these cold northerly blasts. The middle and eastwardly
parts of North America are subject to like influences, in this
regard, to the plains of eastern Europe. To the westward of the
Rocky Mountains, on the other hand, the conditions affecting climate
present greater analogy to those that belong to western Europe.
In the case of many countries, some local wind, of occasional
prevalence, forms a marked characteristic of climate. The most
remarkable of these local winds are the simoon, the sirocco, the
föhn, the harmattan, and the mistral.
The often-described _simoon_ of the desert is an intensely heated
and dry wind, which raises the temperature like the blast of a
furnace, and fills the air with particles of sand, of suffocating
quality. The same wind is known in the deserts of Turkestan as the
_tebbad_ (fever-wind), the terrible conditions of which are thus
described by the pen of a traveler. “The kervanbashi (_leader of the
caravan_) and his people drew our attention to a cloud of dust that
was approaching, and told us to lose no time in dismounting from the
camels. These poor brutes knew well enough that it was the _tebbad_
that was hurrying on; uttering a loud cry they fell on their knees,
stretched their long necks along the ground, and strove to bury their
heads in the sand. We intrenched ourselves behind them, lying there
as behind a wall; and scarcely had we, in our turn, knelt under their
cover, than the wind rushed over us with a dull, clattering sound,
leaving us, in its passage, covered with a crust of sand two fingers
thick. The first particles that touched me seemed to burn like a
rain of flakes of fire. Had we encountered it when we were deeper
in the desert we should all have perished. I had not time to make
observations upon the disposition to fever and vomiting caused by
the wind itself, but the air became heavier and more oppressive than
before.”
The _sirocco_ of the Mediterranean coasts is the hot wind of the
African desert, tempered, before reaching the coasts of southern
Europe, by its passage across the great expanse of inland waters.
The enervating influences of this wind are well known to the resident
on the shores of Sicily, the Italian mainland, or the islands of the
Archipelago. The same wind, when it reaches the high mountain regions
of the Apennines and the Alps, is known as the föhn.
The _föhn_, or warm south wind, is an important agent in modifying
the climate of the higher Alpine region, where its prevalence for a
few days in succession causes the snow-line to recede, and is often
accompanied by inundations occasioned by the suddenly melted snows.
Its absence during a longer period than usual is attended, on the
other hand, by a prolongation of the glaciers into a lower region
of the mountain valleys. The Swiss peasants have a saying, when
they talk of the melting of the snow, that the sun could do nothing
without the föhn.
The _harmattan_ of Senegambia and Guinea is a cold and intensely dry
wind, which blows from the northeast during the months of December
and January.
The _mistral_ of southern France possesses similar qualities to the
last-named wind, and blows, for days together, down the valley of the
Rhone.
Winds transport particles of dust, and, with them, the minuter forms
of vegetable and animal life, to vast distances. The phenomena known
to sailors as _red fogs_ and _sea-dust_ are evidence of this. In
the Mediterranean, and also in the neighborhood of the Cape Verde
Islands, showers of dust, of brick-red or cinnamon color, are
sometimes experienced in such quantity as to cover the sails and
rigging hundreds of miles away from land. Among this sea-dust,
examination with the microscope has detected infusoria and other
organisms native to the tropical regions of South America.
The prevailing currents of the atmosphere, or _winds_, constitute an
important feature in the climate of any country, and it belongs to
Physical Geography to explain the prevalent winds which distinguish
great regions of the globe. Such explanation is more easily made
in regard to the warmer latitudes of the earth, where alone the
direction of the wind is constant, than might be at first supposed
by those whose personal experience is limited to such countries as
Britain, and other temperate lands, where the variable condition
of the atmosphere is the well-known subject of common observation
and remark. But within those parts of the globe which experience
a vertical sun, and for a few degrees beyond the exact line which
marks the limit of the sun’s vertical influence on either side of
the equator, the conditions either of perennial calm, or of currents
of air that constantly blow in one given direction, are the uniform
characteristics of climate.
Throughout a zone of a few degrees in breadth, which extends round
the globe in the neighborhood of the equator, and the limits of which
undergo a certain amount of variation, dependent on the sun’s passage
of the equinox, the variation of temperature throughout the year
is confined within very narrow limits, and the result is a general
prevalence of calms--that is, of undisturbed atmosphere. Wind is air
set in motion, mainly by the existence of different conditions of
temperature between adjacent bodies of air--of colder and denser air
pressing against warmer and lighter air, and taking the place which
is left vacant by the latter, as it rises into the higher regions
of the entire aerial sea. Between the heated air of the tropics in
general, and the comparatively cooler air of the regions lying some
distance north and south of the tropics, for example, there is a very
manifest difference as to temperature, as well as in regard to other
conditions; but for a few degrees in the immediate neighborhood of
the equator there is no such obvious difference, and, consequently,
nothing to occasion disturbance (temperature alone being considered)
in the general equilibrium of the atmosphere. Hence the prevalence
of calms in that region. Within the parallels of 8° or 10° on either
side of the line, the angle at which the solar rays reach the earth
is at no time more than a few degrees from the perpendicular, for
the equator divides the total amount of angular difference which is
involved in the entire yearly path of the sun.
The average breadth of the calm latitudes--or the _Zone of Calms_,
as it is the custom, in books and maps, to term it--may be stated
at about six or seven degrees. The mid-line of this zone does not
coincide with the equator, for the reason that the equator does
not represent the line of the earth’s highest temperature, owing
to the preponderance of land in the Northern Hemisphere. Hence
the Zone of Calms is, for the most part, to the northward of the
equator--extending, with varying seasonal limits, from about the
first to the seventh or eighth parallel of north latitude. But the
limits oscillate with the sun’s passage of the equinox and consequent
place in the heavens vertically over either side of the equator.
The calm latitudes are the dread of the mariner, whose ship is often
delayed for weeks together within their limits. The wearisome and
tantalizing nature of this delay can, perhaps, only be adequately
appreciated by those who have experienced the monotony attendant on
a calm in mid-ocean, when, with a still and glassy sea around, a
glittering atmosphere, and a burning sun overhead, the sails hang
idly by the yards, and the vessel makes no appreciable progress.
Between the oscillating limit of the Zone of Calms and the parallel
of 28° in the Northern Hemisphere, on one side of the globe, and
between the correspondent limit and the parallel of 25° south
latitude, on the opposite hemisphere, there prevail through above
two-thirds of the earth’s circumference, steady winds, blowing
with almost undeviating uniformity from the eastward. These are
the trade-winds. More precisely, _the trade-wind of the Northern
Hemisphere is a wind blowing from the northeastward_--that is, a
_northeast wind_. _The trade-wind of the Southern Hemisphere blows
from the southeastward_, and is _a southeast wind_.
The trade-wind belts stretch round more than two-thirds of the
earth’s surface. They comprehend (within the latitudinal limits
already defined) the Atlantic and Pacific Oceans, with the countries
that lie adjacent to those vast areas of water. In the Pacific,
however, their limits are less distinctly marked, and their influence
less powerful, to the southward of the equator than to the north of
that line. Over the Indian Ocean and its shores, the atmospheric
currents follow, during portions of the year, an opposite course.
The trade-winds of the Atlantic and Pacific--blowing constantly,
and with almost undeviating steadiness, from the eastward--regulate
the course of the mariner across those oceans. They, of course,
facilitate the passage of either ocean in a westerly direction--that
is, from the shores of the Old World to the eastern seaboard of
America, or from the western coast of the New World to the eastern
shores of the Asiatic and Australian Continents. It was the
trade-wind of the Northern Atlantic that carried Columbus to the
westward, on the adventurous voyage which resulted in the discovery
of the New World, inspiring terror in the breasts of his companions,
while in the mind of the great navigator himself it strengthened the
assurance of reaching land by pursuing the direction in which his
vessels’ prows were turned. On a like great occasion, the trade-wind
of the Pacific carried Magellan’s ship steadily forward through the
ocean which he was the first to cross, and facilitated the earliest
circumnavigation of the globe. On the other hand, the same winds
compel the return voyage across either ocean to be made in higher
latitudes, where westerly winds prevail.
The explanation of the trade-winds is found in the different measure
in which the sun’s heat is experienced by regions within or nearly
adjacent to the tropics, and by those of higher latitudes. They
are currents of air set in motion by the differences of density
consequent upon such various conditions of temperature--conditions
which are of uniform prevalence, and the result of which is also
constant.
To sum up, we may say that the trade-winds, like the currents of
the ocean, are due, _first_, to the sun,--that is to the different
measure in which the solar heat is distributed on the globe’s
surface; and, _secondly_, to the earth’s axial rotation, which
affects the direction of currents in the aerial ocean in manner
precisely analogous to that in which it affects the like currents
in the aqueous ocean. In truth, the ocean of water, and the ocean
of air--in contact with one another, and possessing many properties
in common--act and react upon one another, mutually imparting their
respective temperatures, movements, and other conditions. This is
only one among the instances of mutual harmony--one of the many mute
sympathies--which abound in the natural world.
The monsoons are winds which blow over the Indian Ocean, and the
countries adjacent to its waters. In general terms, it may be said
that they prevail within the same latitudes as those over which the
trade-winds of the Atlantic and Pacific blow. But the monsoons differ
from the trade-winds of the two greater oceans in the fact that they
are _periodical winds_, not perennial. The monsoon blows for half the
year from one quarter of the heavens, and for the other half from an
opposite quarter.
[Illustration: Charts Showing the General Directions of Wind and Tide
Currents]
Over the northerly portion of the Indian Ocean--from the
neighborhood of the equator to the shores of the Asiatic Continent,
including the Malay Archipelago and the adjacent China Sea--a
northeast monsoon blows during the winter months of the Northern
Hemisphere; that is, from October to March, inclusive. During the
summer months--April to September--and within the same limits,
the southwest monsoon blows. Southward from the equator to the
neighborhood of the tropic of Capricorn, the southeast monsoon
blows during the winter of those latitudes (April to September):
this is exchanged, during the other half of the year, for a
northwest monsoon in the neighborhood of the Australian coasts,
and for a northeast monsoon along the line of the African shores.
The term _monsoon_--derived from a Malay word which signifies
“season”--expresses the periodical nature of these winds, and
indicates to how large an extent the climate of Indian seas and lands
is dependent upon their periodical recurrence.
The change from the one monsoon to that from an opposite quarter
is not accomplished at once. The breaking-up of the monsoon, as it
is termed, is attended by thunderstorms and other meteorological
phenomena, which prevail during some weeks, until the setting-in
of the coming monsoon is fairly accomplished. The nature of these
changes, and the general characteristics of the monsoon itself, are
admirably depicted in the following passage, by a master hand:
“Meanwhile the air becomes loaded to saturation with aqueous vapor
drawn up by the augmented force of evaporation acting vigorously
over land and sea; the sky, instead of its brilliant blue, assumes
the sullen tint of lead, and not a breath disturbs the motionless
rest of the clouds that hang on the lower range of hills. At length,
generally about the middle of the month, but frequently earlier, the
sultry suspense is broken by the arrival of the wished-for change.
The sun has by this time nearly attained his greatest northern
declination, and created a torrid heat throughout the lands of
southern Asia and the peninsula of India. The air, lightened by its
high temperature and such watery vapor as it may contain, rises into
loftier regions, and is replaced by indraughts from the neighboring
sea, and thus a tendency is gradually given to the formation of a
current bringing up from the south the warm humid air of the equator.
The wind, therefore, which reaches Ceylon comes laden with moisture,
taken up in its passage across the great Indian Ocean. As the monsoon
draws near, the days become more overcast and hot, banks of clouds
rise over the ocean to the west, and in the peculiar twilight the eye
is attracted by the unusual whiteness of the sea-birds that sweep
along the strand to seize the objects flung on shore by the rising
surf.
“At last sudden lightnings flash among the hills and shoot through
the clouds that overhang the sea, and with a crash of thunder the
monsoon bursts over the thirsty land, not in showers or partial
torrents, but in a wide deluge, that in the course of a few hours
overtops the river banks and spreads in inundations over every level
plain.
“All the phenomena of this explosion are stupendous: thunder, as we
are accustomed to be awed by it, affords but the faintest idea of
its overpowering grandeur in Ceylon, and its sublimity is infinitely
increased as it is faintly heard from the shore, resounding through
night and darkness over the gloomy sea. The lightning, when it
touches the earth where it is covered with the descending torrent,
flashes into it and disappears instantaneously; but when it strikes a
drier surface, in seeking better conductors, it often opens a hollow
like that formed by the explosion of a shell, and frequently leaves
behind it traces of vitrification. In Ceylon, however, occurrences
of this kind are rare, and accidents are seldom recorded from
lightning, probably owing to the profusion of trees, and especially
of cocoanut palms, which, when drenched with rain, intercept the
discharge, and conduct the electric matter to the earth. The rain at
these periods excites the astonishment of a European; it descends
in almost continuous streams, so close and so dense that the level
ground, unable to absorb it sufficiently fast, is covered with one
uniform sheet of water, and down the sides of acclivities it rushes
in a volume that wears channels in the surface. For hours together,
the noises of the torrent as it beats upon the trees and bursts upon
the roofs, flowing thence in rivulets along the ground, occasions an
uproar that drowns the ordinary voice and renders sleep impossible.”
The monsoons of the Indian Ocean are not divided by any such
distinctly defined belt of calms as separates the opposite
trade-winds of the northern and southern Pacific and Atlantic. The
southeast monsoon of the southern Indian Ocean passes gradually
into the southwest monsoon, which prevails at the same time in the
northern half of that ocean. Nor is the season of change from the
one monsoon to the other precisely the same over all parts of that
ocean. Indeed, the Indian Ocean, from the geographical conditions
already adverted to, is exposed in much higher measure than either
of the other oceans to the disturbing influences consequent upon
proximity to land, and its winds are hence affected in a vastly
greater degree by local conditions. Thus the Indian monsoon, the
Arabian and East African monsoon, and the monsoon of northwestern
Australia, assume in each case a direction which is dependent upon
the geographical position and contour of the lands whence they derive
their distinguishing names. In the Red Sea, the monsoons follow
the direction of its shores, and blow, for six months of the year,
alternately, up and down its long and trough-like valley, confined
and guided in their passage by the mountain-chains which bound it
upon either side.
We have hitherto spoken of the monsoons only in connection with the
Indian Ocean. But, in truth, a monsoon, or season-wind--which is what
the word monsoon means--is experienced upon a large portion of the
West African coasts, and thence far out into the mid-Atlantic, within
the proper region of the Atlantic trades. The evidence of this is one
among the many valuable results due to the Wind and Current Charts
of Maury, and the cause of it is precisely the same as that which
occasions the monsoon of the Indian coasts. Between the equator
and the parallel of 13° north, the intense heat of a vertical sun,
acting upon the western coasts and adjacent interior of the African
Continent, occasions a reversal of the ordinary wind of that region.
The intensely heated atmosphere of the land, owing to superior
rarity, ascends, and the cooler air of the neighboring sea sets in to
fill its place. The monsoon thus generated lasts as long as the sun
remains to the northward of the equator. Further to the south a like
phenomenon accompanies, in those localities, the passage of the sun
into south declination. The influence of these monsoons extends to
a distance of a thousand miles or more from land, the entire space
within which they prevail forming a cuneiform (or wedge-shaped)
region in the midst of the Atlantic, the base of which rests upon the
African Continent, while its apex is within ten or fifteen degrees of
the mouth of the Amazon.
A similar reversal of the trade-winds of the North Pacific occurs
off the western shores of Central America, capable of explanation
in precisely like manner--due, that is, to the excess of heat which
the summer sun brings to the adjacent lands, and the consequent
rarefaction and rising of the currents of air over those lands.
This, and the like instance of the West African monsoons, show in
the most striking manner how powerfully the land is affected by the
sun’s heat, and to how wide a distance the atmospheric movements
which are generated by such influences extend over the adjacent
seas. Even such limited tracts of land as the Society and Sandwich
Islands have a marked influence upon the winds experienced over the
surrounding waters. They interfere, says Maury, with the trade-winds
of the Pacific very often, and even turn them back, for westerly and
equatorial winds are common at both groups, in their winter time.
Upon the coasts of most countries that are within the warmer
latitudes of the globe, there occur daily, at or shortly before the
hour of early dawn, and toward the approach of sunset, breezes that
blow respectively _off the shore_ or from _off the adjacent waters_.
The former is known as the land-breeze; the latter as the sea-breeze.
These refreshing movements of the air are not confined to countries
within, or even very near to, the tropics, though they are more
powerful in the case of countries that are within the torrid zone
than in the case of other lands. But they are felt upon the coasts
of the Mediterranean, and in even much higher latitudes than those
of the Mediterranean, during the warmer portions of the year. The
hour at which they begin to be perceptible is not the same in all
localities; but, speaking generally, the land-breeze begins to be
felt about an hour before sunrise, and the sea-breeze toward the
early evening, as the time of sunset approaches. During the midday
hours the intense heat of the atmosphere, accompanied by general calm
and almost perfect repose of the animal world, is painfully felt by
all residents in warm countries, and the cooling sea-breeze which
sets in as the sun approaches the horizon is welcomed with intense
delight. To the sojourner in Indian lands, it is the signal for
outdoor exercise, and is accompanied by a general reawakening of
the outer world of nature. The dweller on the African or Australian
coasts equally rejoices in its refreshing power. The mariner within
Indian seas, frequently becalmed during the stillness of the
night-watch, finds like relief in the breeze which blows off the land
with the approach of early morning.