PERIODIC WINDS.
“Earth has each year her resurrection hours
When the spring stirs within her, and the powers
Of life revive; the sleeping zephyrs rouse,
The blushing orchards clothe their naked boughs,
The swallow skims above the lakelet’s verge.
Swift summer speeds with fire in every vein,
And autumn’s glories crimson hill and plain.
Then warmth and life from Nature take their flight,
And winter robes her in a shroud of white,
While mournful Boreas chants her funeral dirge.”
So the seasons tread their ceaseless round in the temperate zones, and
to a certain degree in the colder regions of the earth. But when we
examine the change of seasons in the tropical world, we find a state of
things so different that we are at once led to inquire the reason: and
it will be found primarily in certain periodical winds.
When the sun is north of the equator: that is, while our northern summer
is in progress, India enjoys a steady sea wind from the southwest, which
brings a rainy season to the corresponding coasts of Hindostan and
Farther India. When the sun returns to the south, the winds set in from
the opposite direction, coming down across the great upland plateau of
Central Asia, sometimes called, from its immense height and extent, the
“Roof of the World.” These periodic winds are called monsoons: a
corruption of the Arabic word _Moussin_, season. They are in reality a
modification of the trade-winds.
A glance at a map will show that the northern half of the great Indian
Ocean is enclosed by land masses as no other large body of water is.
Consequently, while in the southern section the southeast trade is
present, the northeast trade of the northern part is so modified by the
surrounding land areas as to almost entirely lose its distinctive
character. Hence, most tropical regions have, properly speaking, but two
seasons: the rainy, and the dry. As the clouds swept in meet with an
intensely heated region, the trade never chills them sufficiently to
produce snow, except in extremely elevated regions.
This is the direct cause of the monsoons: During the northern summer,
southern Asia, being under the rays of the vertical sun, becomes
intensely heated; and the cooler and denser air of the adjacent ocean,
and of southern Africa, flows towards it, producing the southwest
monsoon, which lasts from April or May to September or October. The time
of its beginning and its close varies in different latitudes, according
to the time at which the sun is vertical in each.
During the southern summer, southern Africa being under the vertical sun
and intensely heated, the cooler air of the surrounding seas, and of
southern Asia, flows towards it. This produces the northeast monsoon,
which lasts from October or November to April. This monsoon is, in fact,
only the regular northeast trade-wind somewhat intensified.
A similar exchange takes place between Asia and Australia, but it is
less marked, owing, perhaps, to the great islands lying between these
continents.
The period of transition of the monsoons, in spring and autumn, is
marked by sudden and violent gales, and terrific thunder storms.
Destructive hurricanes, also, are of frequent occurrence. This
corresponds with the period of equinoctial storms in higher latitudes.
There are narrow monsoon belts in the Atlantic, along the coast of
Africa and of Brazil, also on the Pacific coasts of North and South
America; but the phenomena they exhibit are of a much less striking
character. On the African coast, in general, the winds blow from sea to
land in summer, from land to sea in winter; on the Brazilian, the wind
is from the northeast in summer, while in winter the southeast trade
resumes its sway. The monsoons of the Pacific coast of America blow from
the northwest and north during the southern summer; from the southwest
and south during the northern. The regular trade-wind makes itself so
strongly felt in northern Brazil, which is unusually level, that a boat
can sail almost as rapidly up the swift current of the Amazon as it can
row down: and Humboldt records that he found it of great strength at the
foot of the eastern slope of the Andes.
Another modification of the northeast trade is found in the Etesian
winds of Greece and the adjacent archipelago. This is a true
intermittent trade, blowing only in the daytime, however, and lasting
from July to September. The cool air of the peninsula rushes toward the
extremely heated regions of the Mediterranean and north Africa.
Somewhat similar are the northers, or blizzards, of our Western States.
By the laws already given, it is seen that northerly winds can prevail
in any region only when some region further south is unusually heated.
Now, the northern portion of America may be roughly compared to a
trough. The cold polar current sets to the southward across the
continent, and is turned to the east by the Rocky Mountain range, giving
it a general southeast course. Hence, when the southern summer is in
progress, our prevailing winds are from the northwest; and when the
heated portion of the world is north of the equator, we have the return
trade, giving us as our prevailing wind that from the southwest. When
our return trade is unusually prolonged, we have a late fall; and if
the southern summer is unusually warm, we have the polar current longer
than usual, and a late spring in consequence. The polar current seldom
makes its presence felt beyond the Texan plains; though occasionally it
reaches the Mexican plateau, or sweeps across the Gulf to the Antilles.
A similar cold wind from Central France toward the Riviera is locally
known as the Mistral. The cold winds from the south, which in crossing
the plains of Patagonia, are turned eastward by the Andes, are called in
Uruguay the _pamperos_, as their direction causes the popular belief
that they originate in the pampas, or grassy plains. In Malta the cold
wind becomes known as the _gregale_--in the Adriatic sea it is the
_tramontana_; in Trieste and Dalmatia it is the _bora_. In New Zealand
the corresponding cold blast comes from the south, and is known as the
buster. When loaded with drifting snow, as in the blizzard of the United
States, the cold wind of the Yenisei Valley, in Asia, is locally called
the _purga_; in the steppes of Central Asia it is the _bura_.
Eastern Asia receives its prevailing cold current from the northwest;
while western Asia and Europe receive their cold wave from the
northeast, there being no range of mountains, as in America, to deflect
the current, as the polar currents are disposed to follow the
continents, having their origin in arctic lands; while for a similar
reason the return trades reach their extremes on the ocean. Hence, lines
drawn through the places which possess the same mean annual temperature
reach a higher latitude at sea than on land.
These are the chief periodical winds of long periods. There is one other
class to be noted: the diurnal land and sea breezes. These occur along
all coasts, whether in the zone of trades or of variable winds; but the
phenomenon is more strongly marked in the tropical regions, and in the
summer of the temperate latitudes, because of the greater difference in
the temperature of land and sea by day and by night.
During the hottest part of the day the air over the land frequently
reaches a temperature of 100° Fahr., and even more, while that over the
sea rarely rises above 80°. During the night the land radiates its heat
with such rapidity that, towards morning, its atmosphere may be from 10°
to 15° colder than that of the sea.
Soon after sunrise, the land being warmer than the sea, a sea breeze
sets in, which increases in force until about three o’clock, when the
difference of temperature is greatest. It then gradually diminishes
until about sunset, when, the temperature of the land and sea having
become equal, the atmosphere is at rest, the calm continuing for an hour
or more.
Soon the land becomes cooler than the sea, and a gentle breeze from the
former sets in. It increases in force as the night advances, becoming
strongest a little before morning, when the temperature of the land is
lowest; after which it rapidly dies away, and is succeeded by a calm, to
be soon replaced by the sea breeze.
One other species of variable wind is to be noticed: the hot, dry,
dust-laden blast from desert regions. Such occur more or less
periodically, and are known by different names in different localities.
Tom Moore has told us that “love’s witchery” on the heart is
“Like the wind of the south o’er the summer lute blowing,
That hushed all its music, and withered its frame.”
The reference is to the simoom of Syria and Arabia. One who has not
experienced this wind can have little idea of its oppressiveness. Apt to
come at any hour during the hottest months of the year, with a
temperature so great that a piece of silver exposed to it becomes hot
enough to blister the flesh, and laden with the impalpable dust of the
desert, vegetation is scorched and withered by it, and animals flee from
it as from the pestilence. It may last but a short time: it may endure
several days.
At the first indication of its approach, people flee to their houses;
doors and windows are shut and every crevice that could allow any dust
to enter is tightly stuffed: while the wind lasts no one ventures out.
Such unfortunate animals as happen to be overtaken by it have literally
to struggle for their lives. The wind is not steady, but comes in fitful
gusts, sometimes differing as much as 20° in temperature. The streets
are deserted; and were they otherwise, a person could hardly be seen at
a few yards distance. Hours pass: that implacable enemy, the dust, sifts
in at unknown chinks. By degrees it covers everything. Valuable lace and
tapestry are nearly ruined. You put on a skull-cap; yet it penetrates
your hair. It finds its way beneath the garments to the skin, producing
distressing dryness and roughness. The lips parch and crack. The eyes
are red and inflamed. You drink as if famished, and gasp for breath. You
are excessively irritable; you reach the verge of complete nervous
prostration. At length the ordeal is over. You creep into the street, to
find your neighbors looking like corpses; some, it may be, actually dead
from nervous exhaustion. Dead birds and animals lie on the earth. It is
a case of the survival of the fittest. You pluck a leaf from a
neighboring tree; it crumbles to dust in your grasp.
Such are the effects of an unusually protracted wind, even when most
favorably situated to encounter it. But if a caravan be overtaken by
such in the desert, happy are they who escape. The camels kneel and
thrust their
[Illustration: THE SIMOOM.]
noses into the sand, against each other, into a pack of goods--anywhere
to avoid breathing that poisonous blast. The men throw themselves upon
the ground behind the camels, and muffle their heads in their garments.
The storm is at hand; perchance attended by whirling columns of sand.
You raise your head: a thick, dun-colored cloud flies at you; a heat as
of red-hot iron, it seems, holds you in its choking grasp. You find your
way to your water bottle, and drink deeply. The lurid sun turns the
sweeping columns of sand to pillars of fire. Superstitious fear seizes
your Arab comrades. Gradually the storm passes on: the men pick
themselves up and endeavor to shake the irritating dust and sand from
out the folds of their clothing, and the party resumes its way, happy
that they are not numbered among the dead whose bones are bleaching by
the way. Tales are not wanting of great caravans completely overwhelmed
by the sandstorms of the desert.
These storms are met with in their greatest severity in Egypt and
Arabia. In Egypt, this wind is called the _Khamsin_, or fifty, referring
to the period of fifty days--the latter part of April, May, and early
June--when they may be expected. They never blow through the entire
season: rarely so long as fifteen days at a time. In Arabia the simoom
may travel from the center of the peninsula toward any point of the
compass; the Khamsin of Egypt blows from the southwest. Winds of the
same character cross the Mediterranean. In Spain the wind is known as
the _Solano_, or _Levanter_, or _Leveche_: in Sicily and Italy it is the
_Sirocco_. The distressing dryness is somewhat modified by the journey
across the Mediterranean. The same wind in Syria is called _Samiel_; and
a similar wind which blows from the Sahara southwest to the Guinea coast
is called the _Harmattan_. In California a similar dry hot wind blows
from the interior toward the coast, during the hot season, and is
called the desert wind. Such occasional hot blasts are experienced in
southeastern Dakota, coming from the “bad lands,” or sandy and rocky
wastes along the upper Missouri river.
All these periodical or varying winds may be very properly, from their
time and character, be called the season winds of the earth, as another
means of distinction from the constant trades: as they in part bring
changes of season, and in part are brought that way.
Into the question of climate and seasons one other element enters, of
especial importance in regard to those disturbances of the regular
winds, which we call storms. That factor is the quantity of moisture in
the atmosphere, and the consequent rainfall or snowfall of a region.
Without this element, the phenomenal disturbances known as tornadoes
would hardly occur: or if they did, there would be greater difficulty in
ascertaining their approach.
Water, in its vapor state, is but three-fifths the weight of the air,
and in consequence rapidly rises. This evaporation, as it is called,
goes on at all times: even when the water is frozen. A very thin sheet
of ice, hung in the open air, will finally disappear, even though the
temperature be always below freezing.
Now, all the phenomena of rain, snow, and hail, that are brought by
different seasons, in different climes, depend upon a single simple law:
that warm air can hold a much greater quantity of vapor than cold air.
The amount of moisture that may be held in suspension at different
temperatures is as follows:
Temperature Weight of vapor Temperature Weight of vapor
of Air. in a cubic foot of Air. in a cubic foot
of saturated air. of saturated air.
20 deg. Fahr. 1.30 grains Troy. 70 deg. Fahr. 8.00 grains Troy.
32 “ “ 2.13 “ “ 80 “ “ 10.95 “ “
50 “ “ 4.09 “ “ 90 “ “ 14.81 “ “
62 “ “ 6.15 “ “ 100 “ “ 19.79 “ “
This gives a second reason why storms of wind and rain closely follow
extremely hot weather.
Now, as the vapor is so much lighter than the air, their mixture must
also be lighter. So any unusual amount of moisture is at once detected
by the barometer, an instrument for measuring the pressure of the
atmosphere. If the air grow moister, and therefore lighter, the
barometer falls; a storm is approaching.
Since cold air can retain but little moisture, if a warm moist current
be chilled, it must lose a part of its vapor, which at once falls to the
earth as rain. If the cold be somewhat greater, the moisture is
crystallized into snow. Greely’s observations at Fort Conger show that,
varied as are the forms of snow crystals, those that fall during any
particular storm are invariably of the same types, even though they may
be collected from localities widely removed from each other. All
crystals of snow are hexagonal in plan, but there is much variety in
detail. The laws that produce one variety at one time, and a second at
another, are not yet known.
The subject of hail is a peculiarly perplexing one to the meteorologist.
Hailstones are more or less spherical in form, and are made of alternate
layers of soft opaque ice, and hard clear ice. It is evident that they
must acquire this structure by being whirled about between clouds of
different temperature and density. Some have supposed that they are
formed in a whirlwind, whose axis is horizontal, but for the present we
must be content with Lord Dundreary’s explanation, for “it ith one of
thothe thingth which no fellah can underthtand.”
Raindrops from a great height are larger than those from below, for they
increase as they pass through the vapor-masses. As the warmest currents
are also the highest, it will at once be understood why warm and
tropical rains fall in large drops, while drizzling rains, mists, and
fogs are characteristic of cold regions and cold seasons.
The masses of more or less condensed vapor in the upper air currents are
what are known as clouds. Their various forms and appearance are shown
in the cut on the ---- page.
The _cirrus_ and _cirro-cumulus_ clouds are the highest, are mostly in
the altitudes of perpetual frost, and are supposed often to consist of
minute ice crystals. In temperate latitudes they are usually formed in,
and move with, the upper air current, or return-trade from the tropical
regions.
The _cumulus_ clouds are characteristic of the tropics, and of the
summer days in middle latitudes, their height depending upon the
relative humidity of the air. They are formed by local ascending
currents, which carry a large amount of vapor into the cooler upper air.
There the vapors are condensed, and are gradually heaped up into those
heavy masses of sharply defined clouds, which look like vast snowy
mountains. Their base is horizontal, and marks the height at which the
dew point is reached and condensation begins.
The accumulation of vapors is often so great that these clouds form a
column several thousand feet high. In this case the difference in the
temperature and the electrical conditions of the upper and lower
portions is such that electrical discharges take place, accompanied by
condensation of a portion of the cloud, forming a thunderstorm.
_Stratus_ clouds are most frequently seen in the morning or evening, and
are always low. They are formed by the descent of the higher clouds and
vapors of midday into the lower air as the temperature decreases. They
are more frequent in winter and summer than in the intermediate seasons.
The _nimbus_ cloud is more dense and heavy than the others, which may
all be transformed into the nimbus by
[Illustration: FORMS OF CLOUDS.]
a diminution of temperature. It is of a dark-leaden hue, changing into
grey. This is the most common form of cloud in polar latitudes; and,
during the cold season, it is the most frequent of the temperate zones.
If a moist current cross a mountain range, it loses its moisture in the
cold region, and growing narrower as it descends the other slope,
presents the phenomena of a warm dry wind from the mountains. Thus the
wind that brings rain to Norway, gives warm fair weather to Sweden. Of
the same character are the hot winds of Switzerland, called Foëhn winds,
and the Chinook winds which blow from the eastward into Idaho,
Washington, and western Montana. Similar winds occur occasionally in
South Africa, Australia, New Zealand, and Peru. These hot winds must not
be confounded with the hot and poisonous winds from desert regions
described before.
Such, in fine, are the noted varying intermittent, or periodic winds.
However uncertain they may appear at first thought, they are obedient to
the same unchanging laws that bind the universe into one harmonious
whole. No doubt the ancients, if they had been acquainted with their
office, would have personified them as the nymphs of the seasons. But,
knowing naught of the wonderful immutable laws that bind them, they
could only say to each,
“We know not whence thou com’st, or whither goest,
When round our homes thy wizard blast thou blowest.”
In eternal law and harmony they found only endless confusion and wild
caprice. Man interpreted nature by man.
Yet they are the angels of the seasons, these air-spirits, sent by an
allwise Providence to bring the rain and the snow, and the sunshine and
storm in their season, to give seed to the sower and bread to the eater;
that while man shall dwell on earth, seedtime and harvest and summer
and winter may not cease. So they wander, clothing the tropics with
emerald cloaks of strangest beauty, and robing the poles with ermine and
crystal: painting with rainbow-tints the autumn leaves, and touching
with virgin blush the orchards in spring; in all things obeying the
decree of Him who hath set the seasons in order and made everything
beautiful in its time.