WIND--CONTINUED.
In our last chapter we discussed the winds that prevail in the regions
of the tropics called trade winds, because they follow a direct course
through the year, with the exceptions noted in regard to their shifting
to the north or south with the changing seasons; we also described the
phenomena of land and sea breezes, which during certain seasons of the
year reverse their direction twice daily. We will now describe another
kind of wind, called monsoons, that prevail in India.
India lies directly north of the great Indian Ocean, and the lower part
of it comes within the tropical belt lying south of the Tropic of
Cancer. During the summer season here the earth stores more heat during
the day than it radiates or loses during the night. This causes the wind
to blow in a northerly direction from the sea both day and night for six
months each year, from April to October. During these months the land is
continually heated day and night to a higher temperature than the water
in the ocean south of it. The winds are probably not so severe during
the night as through the day, as the difference between the temperature
of the land and the water will not be so great during the night; and
difference of temperature between two points usually means a
proportional difference in the velocity of the wind. There is a time in
the fall and spring, while there is a struggle between the temperature
of the land and water for supremacy, when the winds are variable,
attended with local storms somewhat as we have them in the temperate
zone. But after the sun has moved south to a sufficient extent the land
of India loses more heat at night than is stored up in the day; hence
the conditions during the winter months are reversed, the water is
constantly warmer than the land, and there is a constant wind blowing
from the land to the ocean, which continues until April, when after a
season of local storms the conditions are established in the opposite
direction. These winds are called "monsoons."
The word monsoon is probably derived from an Arabic word meaning
"seasons." It is a peculiarity of this monsoon that in summer it blows
in a northeasterly direction from the sea and in the winter in a
southwesterly direction from the land. This divergence from a direct
north and south is caused by the rotation of the earth and the
explanation is the same as that we have given for the trade winds.
In the southern latitudes there is a comparatively constant condition
of wind and weather, because the surface of the globe in these regions
is mostly water; but in the north, where most of the land surface is
located, we have a very different and a very complicated set of
conditions, as compared with the southern zones.
The freaks of wind and weather that we find prevailing upon the North
American continent are not so easily accounted for as the phenomena
heretofore discussed. In the northern part the land reaches far up
toward the north pole, while on the west lies the Pacific Ocean, which
merges into the Arctic Ocean at Bering Strait. The climate of the
western coast is affected by a warm ocean current that sets up as far
north as Alaska, while high ranges of mountains prevent the effects of
this warm current from being felt inland to any great extent; all of
which helps to complicate any theory that may be advanced regarding
changes of weather. Aside from the changes of temperature that are due
to the seasons, which are caused by the oscillating motion of the earth
between the limits of the Tropic of Cancer on the north and the Tropic
of Capricorn on the south, there are other changes constantly taking
place in all seasons of the year. While it is not difficult to account
for the change of seasons and the gradual change of temperature that
would naturally follow--owing to the difference of angle at which the
sun's rays strike the earth--it is more difficult to account for the
violent changes that occur several times during the progress of a
season, as well as the less violent ones that come every few days. In
fact, it rarely happens that the temperature is exactly the same on any
two successive days during the year. The diurnal changes are easily
accounted for by the rotation of the earth on its axis each day. But
there is another class of phenomena with which the "weather man" has to
struggle when he is making up a forecast of the weather from day to day.
In order that we may proceed intelligently, let us say a word about the
barometer. We speak of high and low barometer, and we make the
instrument with graduations marked for all kinds of weather, which
really mean but very little. The reading of a single barometer alone
will give us but a faint idea of what is really going to happen from day
to day. But if we have a series of barometers located at different
stations scattered all over the continent and connected at headquarters
by telegraph, so that we can have the readings from a whole series of
barometers at once, then it becomes a very useful instrument. A
barometer may read low at one station by the scale, but may be high with
reference to some other barometer that reads very low.
What is a barometer? If we should take a glass tube closed at one end,
the area of the cross section of which is one inch square, and fill it
with mercury, and while thus filled plunge the open end into a vessel of
mercury, it will be found that the amount of mercury remaining in the
tube above the level of the mercury in the vessel will weigh about
fifteen pounds, if the experiment has been performed at sea-level. This
will vary, however, according to the temperature of the air. Of course
barometers are tested when the air is at a certain temperature. If the
weight of mercury in the tube is fifteen pounds, since it is sustained
by the air pressing down on the mercury in the open vessel, it shows
that the air-pressure on that open vessel is equal to fifteen pounds to
the square inch. In practice, of course, the tubes are made very much
smaller. If the air changes so that it is lighter than normal the
mercury will fall in the tube, because the pressure on the mercury in
the open vessel is less than fifteen pounds to the square inch. And,
again, conditions may arise that will condense the air and make it for
the time being weigh more than fifteen pounds to the square inch, in
which case the mercury will rise in the tube. Thus it will be seen that
the barometer will register the slightest change in air pressure.
Let us dwell for a moment on the causes of what are commonly called
"changes of weather," when we will again revert to the use of the
barometer.
The use of the telegraph in connection with the establishment of a
weather bureau having stations for observation at convenient points
throughout the country has contributed much to the science of
meteorology. It is found that there are areas of high and low pressure
existing at the same time in different parts of the country. These
usually have their origin in the far northwest, and follow each other,
sweeping down the eastern side of the Rocky Mountains and gradually
bending easterly and from that to northeasterly by the time they reach
the Atlantic coast. The areas of low pressure are called cyclones, while
the areas of high pressure are called anti-cyclones. (By cyclone we do
not mean those cloud funnels commonly called by that name that form at
certain times of the year in certain sections of the country and produce
such destruction of life and property. These storms are usually confined
to a narrow strip and are short-lived. They arise undoubtedly from local
conditions. A description of these tornadoes--for such is their true
name--will be given in some future chapter.)
These centers of high and low pressure may be several hundred miles
apart. In the area of high pressure, if it is in the winter season, the
weather is unusually clear and cold, and generally clear and fairly cool
at any season, and while there may be some wind it is not so strong as
in the cyclone or low-pressure center. At this point it will be warmer
and winds will prevail, with rain or snow, the winds varying in
direction and intensity at a given point as the cyclone moves forward.
In the center of these cyclones and anti-cyclones there will be a region
of comparative calm, and the air is ascending at the center of the area
of low pressure while it is pouring in on all sides from the area of
high pressure where the air is compressed by a downward current from the
upper regions.
The high-pressure or anti-cyclone system usually covers a larger area
than the low-pressure system, where the air is ascending. While the air
moves laterally from high to low, it does not move in a direct line. The
air movement outside of the high-pressure center is usually not at a
very high speed, but in northern latitudes in the direction of the hands
of a clock. As it circles around it widens out spirally until it reaches
the edge of a low-pressure system, when it bends in its course and moves
in the other direction around this center, but constantly moving inward
toward it in a spiral form and in a direction that is reverse to that of
the hands of a clock. When the air current comes within the influence
of a low-pressure or cyclonic system the velocity of its movement is
very much accelerated until it has moved into the zone of quiet air in
the center, where it is ascending.
In the upper regions of the atmosphere there are counter currents
flowing in the opposite direction. The downward flow at the area of high
pressure compresses the air near the surface of the earth and rarefies
it in the higher regions of the atmosphere, while the opposite effect is
going on over the center of low pressure, the air being rarefied nearer
the surface of the earth, but condensed above normal in the higher
regions by the upward current, which causes an overflow back toward the
rarefied upper regions over the area of high pressure.
It will be observed that the ordinary storm has a compound motion. The
whole system moves in an easterly direction, while the winds are blowing
spirally about the storm center. If we should be in the track of a
moving storm so that its center passed over us the winds at the
beginning would blow in one direction and then there would come a
subsidence until it had moved forward through the quiet zone, when we
should feel the wind in the opposite direction until the area of low
pressure had moved forward into the region of high pressure. The
velocity of the wind will be determined by the difference of pressure
between the areas and by the distance that the areas of high and low
pressure are apart. The steeper the grade the more rapidly the fluid
will flow.
Let us now have recourse, for a moment, to Figs. 1, 2, and 3 in order
that the subject may be more fully understood. In looking at these
diagrams we should imagine ourselves looking South, with the left hand
to the East.
[Illustration: FIG. 1.]
Fig. 1 shows the general direction of the air movement between two
areas--one of high and the other of low pressure. The arrows show the
general direction of the wind. You will notice that in the upper regions
it blows in an opposite direction from the air movement on the surface
of the earth.
Fig. 2 shows in a general way how the wind moves spirally around both
centers. Over the area of high pressure the air descends spirally from
the upper regions, circling around a large area--it may be one hundred
miles or more in diameter--in the direction of the movement of the hands
of a clock.
[Illustration: FIG. 2.]
But then the wind at the high-pressure area is lighter than it is at the
low, and circles outwardly until it finally moves off in the direction
of a low-pressure area, gradually bending in the other direction until
finally it moves the reverse of the hands of a clock--although now it is
in a smaller circle, and with a more rapid motion. It moves spirally and
upwardly about the low-pressure area until it reaches a point in the
upper air, where it goes through the same gyrations in an opposite
direction. Now imagine the whole combination moving from west to east at
an average rate of thirty miles per hour, and imagine further that this
system is linked to other systems that are following along, and you have
some idea of the weather changes as they occur in the middle United
States.
By referring to Fig. 3 you will see why the wind changes its direction
when a storm center passes over any point. It has not only a spiral but
also a forward movement.
[Illustration: FIG. 3.]
Now let us go back to the barometer and see what part it plays in
predicting changes in the weather. At the area of low pressure the air
is ascending, as we have seen, and, owing to the peculiar way it
ascends--by circling spirally upward around a region of comparative
calm--it creates a partial vacuum, which is more pronounced in the
center of the area. At the area of high pressure the air will be
condensed by the descending current being arrested by the earth. The
descending current--coming, as it does, from the upper and colder
regions--accounts for the cool weather that most always prevails at a
high-pressure area. In order to know how great the change of weather is
likely to be, we must know what the readings of at least two barometers
are--one at the high- and another at the low-pressure area. If the
difference between the readings of the two barometers is very great, and
the areas are comparatively close together, we may expect the change to
be sudden and violent.
"High" and "low" as applied to a barometer are only relative terms.
There is no fixed point on the index of the instrument that can be said
to be arbitrarily high or low. For this reason a single barometer is not
of much use. If it begins to fall from any point, and falls rapidly, it
indicates that an area of a much lower pressure is approaching. The same
is true of a high-pressure area, if the barometer rises rapidly from any
point.
If we study the air motions in these systems sufficiently to get at
least an inkling of the law of their movements, it becomes a very
interesting subject.
Wind from whatever cause serves a wonderfully useful purpose in the
economy of nature. Without wind, heat and moisture could not be
distributed over the face of the earth and our globe would not be a fit
habitation for man. How wonderful is the machinery of Nature, that can
first forge a world into shape and afterward decorate it with green
grass and flowers that are watered by the "early and latter rain"!