WIND--WHY IT BLOWS.
We have said that globules of moisture, released by the action of the
sun's rays in the process of evaporation, tend to rise because they are
lighter than the air. Right here let it be said that all material
substances have weight; even hydrogen, the lightest known gas, has
weight, and is attracted by gravitation. If there were no air or other
gaseous substances on the face of the earth except hydrogen, it would be
attracted to and envelop the earth the same as the air now does. Carbon
dioxide is a gas that is heavier than the air. If we take a vessel
filled with this gas and pour it into another vessel it will sink to the
bottom and displace the air contained in it until the air is all driven
out. If we fill a jar with water up to a certain height and then pour a
pint of shot into it the water will be caused to rise in the vessel
because it has been displaced at the bottom by the heavier material. Now
if we remove the shot the water will recede to the level maintained
before the shot was put in. On the contrary, if we should pour an equal
bulk of cork or pith balls into the jar the water would not be
displaced, because the balls are lighter than the water and would lie on
top of it; if, however, the water is removed from the jar, the cork will
immediately go to the bottom of the jar, because the cork is heavier
than the air which has taken the place of the water. We wish to impress
upon the mind of the reader the fact, that all substances of a fluidic
nature, whether in the fluid or gaseous state, have weight, and obey the
laws of gravitation, and the heavier portions will always seek the lower
levels, and in doing this will displace the lighter portions, causing
them to rise. There is no tendency in any substance to rise of itself,
but the lighter substance rises because it is forced to do so by the
heavier, which displaces it. This law lies at the bottom of all of the
phenomena of air currents.
If we are at certain points on the seashore in the summer time we may
notice that about 9 o'clock in the morning a breeze will spring up from
the ocean and blow toward the land; this will increase in intensity
until about 2 o'clock in the afternoon, when it has reached its maximum
velocity, and from this time it gradually diminishes, until in the
evening there will be a season of calm, the same as there was in the
early morning. The explanation of this peculiar action of the air is
found in the fact that during the day the land is heated much more
rapidly on its surface than the water is.
The radiant energy from the sun is suddenly arrested at the surface of
the earth, which is heated to only a very shallow depth, while in the
water it is different; being transparent it is penetrated by the radiant
energy to a much greater depth and does not suddenly arrest it, as is
the case on land. As the sun rises and the rays strike in a more and
more vertical direction the earth becomes rapidly and intensely heated
at its surface, and this in turn heats the stratum of air next above it,
which is pressing on it with a force of fifteen pounds to the square
inch at sea-level. When air is heated it expands, and as it expands it
grows lighter. The stratum lying upon the earth as soon as it becomes
heated moves upward and its place is occupied by the heavier, cooler air
that flows in from the sides. We can now see that if there is a strong
ascending current of air on the land near the ocean the cooler air from
the surface of the ocean will flow in to take the place of the warmer
and lighter air that is driven upward, really by the force of gravity
which causes the heavier fluid to keep the lowest level. As the earth
grows hotter this movement is more and more rapid, which causes the
flow of colder air to be quickened, and hence the increasing force of
the wind as the sun mounts higher in the heavens. But when it has passed
the point of maximum heating intensity and the earth begins to cool by
radiation, the movements of air currents begin to slow up, until along
in the evening a point is reached where the surface of the earth and
that of the ocean are of equal temperature, and there is no longer any
cause for change of position in the air.
The earth heats up quickly, and it also cools quickly, especially if
there is green grass and vegetation. While they are poor conductors of
heat, they are excellent radiators, so that when the sun's rays are no
longer active the earth cools down rapidly and soon passes the point
where there is an equilibrium between the land and water. The water
possesses the opposite quality. It is slow to become heated, because of
a much larger mass that is affected, and is equally slow to give up the
heat. And the consequence is that after the sun has set, the land cools
so much faster than the water that we soon have the opposite condition,
and the sea is warmer than the land, which makes the air at that point
lighter, and which in turn causes the denser or colder air from the land
to flow toward the ocean, and displace the lighter air and force it
upward; hence we have a land instead of a sea breeze. So that the
normal condition in summer is that of a breeze from the ocean toward the
land during part of the day and a corresponding breeze from the land to
the ocean during part of the night, with a period of no wind during the
morning and evening of each day.
The forces that work to produce all the varying phenomena of air
currents on different portions of the earth are difficult to explain, as
there are so many local conditions of heat and cold, and these are
modified by the advancing and receding seasons. The unequal distribution
of land and water upon the earth's surface; the readiness with which
some portions absorb and radiate heat as compared with others; the tall
ranges of mountains, many of them snow-capped; the lowlands adjacent to
them that become intensely heated under the sun's rays; the diversity of
coastline and the fact that there is a zone of continually heated earth
and water in the tropical regions--all these conditions, coupled with
the fact that the earth rotates on its axis once in twenty-four hours,
are certainly sufficient to account for all the complicated phenomena of
aerial changes on the various portions of the earth's surface.
The trade winds are so called because they blow in a certain definite
direction during certain seasons of the year, and can be reckoned upon
for the use of commerce. If you trace the line of the equator you will
notice that for more than three-quarters of the distance it passes
through the water. The water, as we have explained in the last chapter,
becomes gradually heated to a considerable depth, and when once
saturated with heat is slow to give it up. It can easily be seen that
there will be a zone extending each way from the equator for a certain
distance that will become more intensely heated than any other parts of
the earth, with the exception of certain circumscribed portions of the
land. The result is that this heated equatorial zone is constantly
sending up warm air caused by the inrush of colder air, which is heavier
than the air at the equator, expanded by the heat. The warm air at the
equator is forced up into the higher regions of the atmosphere, and here
it overflows each way, north and south, causing a current of air in the
upper regions counter to that of the lower. As it travels north and
south it gradually drops as it becomes cooler, and finally at some point
north and south its course is changed and it flows in again toward the
equator. As a matter of fact, the trade winds do not flow apparently
from the north and south directly toward the equator, but in an oblique
direction. On the north side of the equator we have a northeasterly
wind, and a southeasterly wind on the south side. This is caused by the
rotation of the earth from west to east. The direction of the trade
wind, however, is more apparent than real.
The earth in its diurnal revolutions travels at the rate of a little
more than 1000 miles an hour at the equator. But if we should travel
northward to within four miles, say, of the north pole, the surface
point would be moving at the rate of only about a mile an hour. At some
point equidistant between the north pole and the equator the surface of
the earth will be moving at a rate, say, of 500 miles an hour. If we
could fire a projectile from this point that would have a carrying power
to take it to the equator some time after the projectile was fired,
although it would fly in a perfectly direct line, it would appear to
anyone at the equator who observed its approach to be moving from a
northeasterly direction. The reason is that the earth is traveling twice
as fast at the equator as it is at the point whence the projectile is
fired. Therefore it will overshoot, so to speak, at the equator, and not
be dragged around by the increased motion we find there.
To make this still plainer, suppose the earth to be standing still and a
projectile be fired directly across from the north pole in the direction
of the lines of longitude and required one hour to reach the equator,
the projectile would appear to anyone standing at the equator to come
directly from the north. If, however, the earth is revolving at the rate
of 1000 miles an hour at the equator to the eastward, and the
projectile was fired from the pole, where there is practically no
motion, in the same direction along the longitudinal lines as before,
the observer would have to be in a position on the equator 1000 miles
west of this longitudinal line in order to see the projectile when it
arrived; therefore the apparent movement of the projectile would not be
along the line at the instant that it was fired, but along a line that
would cross the equator at a point 1000 miles west. When a southward
impulse is given to the air it follows, to some extent, the same law, so
that to one standing on the equator the northern trade wind will blow
from the northeast and the southern trade wind from the southeast.
Owing to the fact that the air rises in the heated zone there is always
a region of calms at this point where there is no wind and no rain.
There are two other regions of calms in the ocean, one at the north at
the tropic of Cancer and another at the south near the tropic of
Capricorn. As has been stated, there are currents flowing back in the
upper regions at the equator north and south, and these are called the
upper trades--the lower currents being called the lower trades. These
upper trades gradually fall till they reach the tropic of Cancer on the
north, where the lower part of the current stops and bends back toward
the equator, now becoming a part of the lower trade wind. This causes a
calm at that point where it turns. The upper parts of this current
continue on, in a northerly and southerly direction, on the surface
until they meet with the cold air of the north and south polar regions,
where there is a conflict of the elements--as there always is when cold
and warm currents meet.
The only point where the trade wind has free play is in the South Indian
Ocean, and this is called the "heart of the trades."
If the whole globe were covered with water there would be a more
constant condition of temperature; but owing to the great difference
between the land and water, both as to altitude and the ability to
absorb and radiate heat, we have all of these varied and complicated
conditions of wind and weather. The trade winds shift from north to
south and vice versa with the advancing and receding seasons, due to the
fact that the earth has a compound motion. It not only revolves on its
axis once in twenty-four hours, but it rocks back and forth once a year,
which is gradually changing the direction of its axis; and in addition
to these motions it is traveling around the sun as well.