CONSTANT WINDS.
“Up from the sea I sprang, O voyager,
Ere Aphrodite rose from out its foam.
I am a banned, unresting wanderer,
Doomed o’er the surface of the deep to roam.
Without being aged, o’erwhelmed with days,
The end of being is my only dream.
I trod the earth ere man’s ephemeral race,
And onward flee long as yon sun shall beam,
Ever, forever,
Here, and wherever,
Turneth the earth, must I course forever!”
The phenomena of climate and seasons are too familiar to need especial
comment or description. They are dependent, in the first place, upon the
annual journey of the earth about the sun, the inclination of the
earth’s axis to its orbit, and the distance of any particular region in
question, from the equator.
But the changes thus constantly made are greatly modified by other
factors. Chief among these agencies are the form and extent of the
continents, their position relative to each other and the water areas,
and the currents of the air and sea.
Men usually identify climate with atmospheric conditions. A warm
atmosphere is for them the whole of a warm climate: it is really but one
of its factors, at most: it is often to be considered as a result,
rather than a cause. On lofty plateaus, or in mountainous regions, the
heat is not oppressive, even in the tropics; but here the moderate
temperature is due to the elevation. France is as far north as
Labrador; but there is no similarity whatever in climatic conditions, as
there should be, were climate dependent only on the heating of the local
atmosphere by the rays of the sun. Who would think of instituting a
comparison of sunny Rome or Madrid with the city of New York? Yet the
three are nearly on the same parallel: Rome furthest north. So there is
little resemblance between the warmth of sunny Florida and the scorching
heat of the Sahara: or between the climates in those portions of our own
Pacific and Atlantic coasts that lie between the same parallels. So we
find that though there is a general relation between the climate of a
region and its distance from the equator, there are many other
conditions to be considered. First, let us note atmospheric currents and
disturbances.
“The wind bloweth where it listeth, and no man knoweth whence it cometh,
or whither it goeth.”
“The world do move.” The illustration so full of meaning two thousand
years since has lost much of its force. The truth of yesterday is the
error of to-day. The fact of to-day may be the phantasy of to-morrow. So
it has come to pass that in our day the origin and laws of air currents
are believed to be as well understood as those of any other forces in
nature. Yet scientific theorists are, after all, divided on not a few
points.
Two general classes of winds are recognized: the constant, and variable.
Constant winds are those that blow all the year in the same direction.
The beautiful concept of Kingsley, in the preceding chapter, contains
the leading points of our knowledge concerning them.
All the various phenomena of air currents are dependent upon one
unchanging law: that gaseous bodies--and all but two others--always
greatly expand under the influence of heat. There are two noted partial
exceptions: one of these prevents our globe from becoming a complete
iceberg, and is as important as the law itself. Iron expands, till its
melting point; but in its liquid state it occupies less space than when
solid. Water contracts under the influence of cold, until the
temperature of 39° is reached; after that it expands: and when frozen
occupies about one-eighth more space than before. This wise provision of
the Creator is second to none in importance, as regards its influence
upon the climate of the earth at large. Had it been otherwise--did ice
sink instead of float, our rivers and seas would in time become solid
masses of ice; for water is so poor a conductor of heat, that its
under-currents warm very slowly. Any one who plunges into a lake in
mid-summer may often find the water warm at the surface, and of almost
icy coldness a short distance beneath. The great Polar current comes
down from Baffin’s Bay, and off the coast of Newfoundland it plunges
beneath the warm, lighter current of the Gulf Stream; but it is not
warmed by it. Registering thermometers detect its icy coldness almost
unchanged in the realms of the tropics, far beneath the surface.
Note some simple illustrations of the expansive force of freezing water.
Every housewife knows that a bottle left full of water will burst when
the water freezes. The same power is shown in the gradual disintegration
of rocks by alternate freezing and thawing. Water freezing in the
crevices bursts off small particles, or even large fragments; so that
rocks long exposed to the weather, crumble more or less. Every one is
familiar with the appearance presented by steep clay-banks, in late
winter and early spring, of ragged masses and fragments ready to fall at
any time. Still another instance of this destructive power is shown in
the killing of vegetation by freezing. Plants are built of myriads of
tiny cells. The moisture within freezes and bursts the cell-walls,
destroying the plant life. Certain plants have cells more elastic than
others, which in consequence are not destroyed by freezing. But as an
expanded cell does not readily shrink to its former size, subsequent
freezings, when the cell contains more water than before, may finally
destroy it. So wheat is “winter-killed,” by too frequent freezing. So
globes of steel may be burst by this force.
To show the poor qualities of water as a conductor of heat, take a long
glass tube and fill with water. Then put a piece of ice in one end. The
water at the other end may now be brought to the boiling point by means
of the flame of a lamp, ere the ice at the other end is melted.
Every one is familiar with the fact that heated air rises; but not all
inquire why it does so. Take a foot-ball or bladder and partially
inflate it; then hold it near a hot fire, and it may be swollen almost
to bursting. Now, there is no more air in it than before; and if it be
laid in a cold place, it will shrink to its first inflation. This shows
how great is the expansive power of heat on the atmosphere. The same
weight occupying a much larger bulk, we perceive that heated air is much
lighter, and must rise. This, then, is the cause of what are known as
constant winds.
As the earth revolves on its axis, the air is unequally heated, that
nearest the equator becoming the warmest, in consequence of its
receiving the most direct rays. Here, then, the air rises most rapidly;
while the cooler air to the north or south must flow southward or
northward to fill the vacuum. Now, the earth turning on its axis from
west to east, whirls the northward and southward currents to the
westward, so that they appear to blow from the northeast and southeast.
The result of this loss of direction is gradual; so that when first
perceptible, they are almost from a due northerly or southerly
direction. As they near the equator, they are more rapid, and turn more
decidedly to the west, never becoming violent, however; rarely exceeding
fifteen to eighteen miles per hour.
It would appear that at the point where these meet each other, or come
in contact with the ascending warm current, there must be a region of
calms or light, variable winds, and occasional tempests. Such, in fact,
is the case. This belt is from two hundred and eighty to four hundred
miles in width, and lies along the thermal equator, or line of greatest
average heat. This is not the same at the earth’s equator, properly so
called; for, as the land has greater capacity for absorbing and
retaining heat than the sea, and as most of the land lies in the
northern hemisphere, it is evident the highest mean temperature must be
north of the equator. So this belt of calms must lie in the same region;
and, in fact, in the Atlantic ocean it lies between 3 and 9° north
latitude, and in the Pacific, between 4 and 8°. As the sun travels
northward during the first half of the year, this region of calms shifts
slightly, also, so as to always nearly coincide with belt of the
greatest mean heat.
At first sight, it appears curious that the motion of the earth should
deflect these winds to the west. It would appear that the earth,
atmosphere and all, must revolve as a unit about its axis; else, if the
atmosphere lose time, its speed to the westward should be constantly
accelerated, and long ago should have reached a velocity that would
shake the mountains themselves; while, in fact, there is no variation
perceptible.
It should be remembered that at the equator the earth is about
twenty-four thousand miles in circumference; and as one complete
revolution is made every twenty-four hours, a point on the equator is
carried eastward at the rate of one thousand miles an hour. But if a
circle be drawn around the earth parallel to the equator, at some
distance from it, it is at once seen that any object in this circle,
having a shorter distance to traverse, is carried eastward at a slower
rate; so that a point only a few yards from either pole must necessarily
advance but a few feet per hour. So then, a body of air moving from
either pole toward the equator, must needs advance very slowly if the
friction of the upper reverse currents and of the surface of the globe
are to have opportunity to overcome its relative inertia and give it the
same velocity as that of any point over which it may pass.
Now, in the case of these constant winds, the inertia is very nearly
overcome, as they start from a circle in which the velocity to the
eastward is about 750 miles per hour. If the inertia were fully
overcome, there would be no perceptible wind; as the velocity is
actually but fifteen to eighteen miles per hour, it appears that the
friction encountered actually destroys from thirteen-fourteenths to
fifteen-sixteenths of the inertia. Hence, we find these constant air
currents toward the west are, in reality, the result of the earth
carrying any object on its surface a little more rapidly than the
atmosphere moves; so that these winds are precisely the same in
principle as the well-known fact that when you run rapidly in still air
(so-called), it seems that the wind is blowing directly in your face.
In like manner, it appears that a wind from west to east is merely an
air-current moving a little more rapidly than the earth revolves at that
point. The relative difference between the velocity of air-currents must
vary greatly; for a violent easterly or westerly wind very near the
poles may equal or even exceed the speed of the rotation of that point;
while the most violent tropical storms average between one-twentieth and
one-eighth of the local rotation. The latter is not often exceeded. But
whatever the relation of the respective velocities, it is clear that the
velocity of the wind in general must depend largely on the amount of air
abnormally heated, and upon the rapidity with which it is heated. So men
have come to recognize that a period of unusually oppressive heat
forebodes a storm of some sort. But few regard the unusual warmth as a
reason of the storm. They are linked, in the popular mind, as antecedent
and consequent, rather than as cause and effect.
These constant winds near the equator have been named trade-winds,
because of their importance to commerce. Unknown before the first voyage
of Columbus, they filled the minds of his crew with fear that they could
never return home, if the wind blew always in one direction. The same
gentle wind bore Magellan in his voyage around the world, and caused him
to give the name of “Pacific,” or “peaceful,” to the great ocean on our
west; and the same steady breezes made the fortune of many a noble
galleon in the days when Peru was an Ophir, Mexico an El Dorado, and the
Philippine Isles a Tarshish where they took shipping for the distant
land of gold.
Owing to the fact that the continents intercept the regular trades by
reason of their elevation and irregular conformation, and also because
of their much greater specific heat, whereby they set in motion many
other local currents, the trades are found to begin only a considerable
distance to the west of the continents. Yet the influence of the trades
is sufficient to make easterly winds the prevailing ones on the great
inland plains: as in the Sahara, Arabia, Southern Siberia, and portions
of North and South America.
It is clear that other nearly constant currents must exist to supply the
vacuum that would be otherwise caused by the trades. These are found to
the south and north of the trade belts, and, as might be expected, blow
nearly in the opposite direction, being descending currents; while the
trades, as before stated, are ascending. The column of hot air from the
equator starts toward the poles above the trades, while a polar current
sets in toward the equator; but as the amount of air displaced at the
equator is by far the greatest, much of it can, of course, never reach
the poles. On meeting the polar current, the two partially mingle and
descend, forming what is called the return trade. This blows, most of
the year, to the southeast, the equatorial current prevailing and coming
from a region whose easterly rotation is more rapid. At certain seasons
of the year, however, the polar current prevails to some extent, though
not sufficiently to overcome the eastward trend; so the wind in this
belt blows alternately to the southeast and the northeast.
Between the region of trades and alternating winds is a belt, on either
side of the equator, of calms and variable winds, which shift northward
or southward, parallel to the belt of calms between the trades. These
two zones, however, are much less clearly defined than the great central
one, and are not liable to such extraordinary disturbances.
Such is the great constant wind, with its dependents. So long as the sun
has warmed the earth, it has hurried on its course, subject to unceasing
law, and destined to cease only when the heavens and the earth shall
pass away, and chaos or annihilation shall end the things that be. A
Wandering Jew of the atmosphere, it flies ever onward, bearing the
merchant to his port, and the rain-cloud to the land; ever and anon
desolating the isles with its bursts of fury; then resuming its restless
course, like the remorseful Salathiel.