LOCAL WINDS.
There are so many causes that will produce air motion that it is often
difficult to determine just what one is the chief factor in causing the
direction of the wind at any particular time. There are very many
instances, however, where the cause can be traced without difficulty;
many of these have already been mentioned and there are many more that
might be. Of course, as has been often stated, there is only one remote
cause for all winds, and that is the sun, coupled with the movements of
the earth. But there are certain local conditions that are continually
modifying the phenomena of air movement. The velocity of winds as they
occur from day to day varies very greatly with the height above the
surface of the earth; ordinarily the velocity at 1000 feet above the
earth will be more than three times greater than it is at 50 or 60 feet
above, and even at 60 feet the velocity is much greater than at the
surface of the earth. This is due partly to the retarding effect of
friction caused by contact of the air with the earth's surface, but more
particularly by trees, inequality of surface, and other obstructions on
the earth.
There is a variety of wind called mountain winds that arise from
different causes. As has been stated in a former chapter, under ordinary
conditions the air is more dense at sea-level than at any point above,
and the density is constantly changing from denser to rarer the higher
we ascend. Suppose at a certain point, say halfway up a mountain side,
the air has a certain density, and if it is at rest the lines of equal
density or pressure will seek a level, just as water would under the
same conditions. Suppose we start at a given point on the side of a
mountain and run out on a level till we are 100 feet in a perpendicular
line above the side of the mountain, the air contained within those
lines will be in the shape of a triangle. If now the sun shines upon the
side of the mountain the air is warmed and expands according to a
well-known law, and the amount of expansion will depend upon the depth
of the volume of air; hence the point of greatest expansion in our
figure will be where the air is 100 feet deep, and will gradually
decrease as we go toward the mountain till we come to the point where
our horizontal line makes contact with the mountain side. At that point,
of course, there is no expansion, because there is no depth of air; and
the effect will be that the expanded air will overflow toward the
mountain, and be deflected up its sloping side. If we apply this same
principle to the whole mountain side we can see that there will be,
during the day, a constant current of air flowing up the mountain. As
night comes on this upward movement will cease and there will be a
season of quiet until the earth has become colder than the air, and we
have a phenomenon of exactly the opposite kind, when the air contracts
instead of expands, which produces a downward current from the mountain
top.
These currents are as regular at certain seasons of the year as the land
and sea breeze. Of course, they may be obliterated for the time being,
by the presence of a stronger wind due to some other cause, such as
during the prevalence of a storm. In some of the regions of California
hottest during the day time, the nights are made endurable, and even
delightful, by the cool breezes that sweep down from the tops of the
mountains. It often happens that on the shady side of a high and steep
mountain where the sun's rays strike it so obliquely, if at all, that
the earth will be but little heated, there will be a vast mass of cold
air stored up. After the valley has become intensely heated by the sun
there is an ascending current of air which in turn causes a down rush
of the cold body of air from the mountain side. These local winds are
frequently very severe, only lasting, however, for a short time, until
an equilibrium of temperature and density has been established. A
wonderful exhibition of this sort of wind is said to occur at certain
times of the year on the coast at Tierra del Fuego, where a blast which
they call the "Williwaus," comes down from the mountain side, without
warning, with such tremendous force that no ship could stand the strain
if it should continue for any length of time. Fortunately the shock does
not last more than eight or ten seconds, when it is followed by a
perfect calm. It is as though a great volume of air had been fired from
some enormous cannon from the top of the mountain to the sea. The water
is pulverized into a spray that is driven in every direction.
Sometimes these violent blasts occur in the Alps, but from a very
different cause. Avalanches of great extent often take place on the
sides of the mountains, when a vast amount of material, equal to three
or four hundred million cubic feet of earth, will fall several thousand
feet. Often an avalanche of this kind will produce a wind, which is
confined, of course, to a restricted area, that is said to be so violent
as to tear one's clothes into shreds. This is not caused by any
difference of temperature, but by a violent compression.
There is a peculiar wind that occurs in Switzerland, often, between the
months of November and March. These winds last from two to three days
and are of great violence--especially near the mountains. They are warm
and dry and are caused by an area of low barometer and an ascending
current of air occurring at some point north of the Alps, which causes
the air from Italy to flow over the Alpine range, causing a tremendous
precipitation of snow and rain, which not only takes the moisture from
the air, but sets free in the form of heat the energy that was stored in
the process of evaporation, and this, together with the compression of
the air as it flows down the slope of the mountains, makes it hot and
dry. This wind is called the "Fohn."
There is a similar condition of things existing on the eastern slope of
the Rocky Mountains which has a modifying effect upon the climate of
parts of Colorado, Wyoming, Montana, also extending up into British
America. This wind, which is here called "chinook," arises from causes
similar to those that are active in Switzerland that give rise to the
"fohn" wind.
There is a wind called the "blizzard" that is felt most keenly in
Montana and the Dakotas during the winter, which is exceedingly cold and
lasts sometimes for a period of 100 hours. The temperature falls at
times 30 or 40 degrees below zero and the wind maintains a velocity of
from forty to fifty miles an hour. These winds spread eastward as far as
Illinois, but not with the same severity, and they move southward to the
Gulf of Mexico, spreading over the States of Texas and Louisiana, and
are there called "northers." It is exceedingly dangerous to be caught in
a blizzard in the Dakotas, where the wind reaches its greatest velocity
and the cold its lowest temperature--especially when the wind is
accompanied, as it frequently is, by severe snowing. By the time it
reaches the Gulf States it is very much modified as to temperature, but
it is a very disagreeable wind in that portion of the country, because
of the exceeding dampness of the air. One would be much more comfortable
in dry, still air, even if it were many degrees below zero, than in an
air freighted with moisture, although the temperature has not fallen to
the freezing point.
There are hot winds called by different names according to the
localities in which they occur. In southern California at certain
seasons of the year the inhabitants are afflicted with what they call a
desert wind that blows from the heated regions of Arizona toward the
Pacific Ocean. The temperature sometimes reaches 120 degrees Fahrenheit,
and persons have been known to perish from the effects of these hot
winds in open boats out on the water before they could reach land.
Hot winds prevail on the plains of Kansas during the months of July and
August that are phenomenal in their intensity, so much so that if they
were widespread and of long continuance, like the northern blizzard,
they would be attended with great loss of life and destruction to
vegetation. Fortunately, they come in narrow streaks and in most cases
do not blow more than from ten to thirty minutes at a time. These hot
belts are sometimes not over 100 feet wide, and again they are as much
as 500. They are so hot and dry that green leaves and grass are rendered
as dry as powder in a few minutes. These winds are probably caused by
the fact that at this season of the year, when the prevailing wind is
southwesterly, the air becomes heated to a great height, and are the
resulting effect of certain combinations of air currents in the higher
regions of the atmosphere that force the already heated air toward the
earth. As the air descends it is more and more compressed, which causes
it to become more and more heated. We have already described the heating
effect of compression upon air as shown by the experiment with the fire
syringe. It was shown that air at normal temperature could be suddenly
compressed into so small a space that the condensed heat, which was
before diffused through the whole bulk of air at normal pressure, was
sufficient to cause ignition. A cubic yard of air on the surface of the
earth would occupy a much larger space if carried a mile above it. From
this it is easy to see that if a volume of air at that height had a
temperature of 70 or 80 degrees it would be very hot when condensed into
a very much smaller volume, as it would be if it were forced down to the
surface of the earth. These winds are the result of some superior force
that is active in the upper regions of the atmosphere, because it is
natural for heated air to rise, and this is what happens when the power
that forced it down to the earth is no longer active to hold it there.
Reference has been made in a former chapter to tornado winds; they are
rather exceptional phenomena and not thoroughly understood. The winds
seem to blow in from all directions toward an area of very low pressure
at a single point. The spiral motion that is common to all cyclones, in
a tornado seems to be gathered up into a condensed form, like a funnel.
The direction of movement is the same as that of the cyclone--that is,
in the reverse direction to that of the hands of a watch. The upward
motion of the air inside of the funnel is at a rate of over 170 miles an
hour. The onward movement of the whole system is about thirty miles per
hour.
Tornadoes occur with greater frequency in the United States than in any
other section of the globe. Tornadoes seldom occur in winter, except
perhaps in the Southern States. They are more frequent in the month of
May than at any other time during the year, although they occur
sometimes in April, June, and July.
Between 1870 and 1890 about sixty-five destructive tornadoes occurred in
the United States, involving great loss of life and property. When a
tornado moves off the land on to the ocean it may become what is termed
a waterspout. These probably never originate on the water, but after
they have once formed may be carried over the water to a considerable
distance. A tornado was never known to originate on the shores of Lake
Michigan, but there are a few instances (the most notable one being the
Racine tornado) when they have reached the lake after having traveled
from some distant point inland.
The Racine tornado--so called because it destroyed a large portion of
that city--happened fifteen or more years ago. The tornado originated
about 100 miles southwest of Racine, Wis., in northern Illinois. The
funnel-shaped cloud passed over the lake, but the tornado character of
the storm was broken up before it reached the other shore.
When a tornado passes from land to water it becomes a waterspout only
when the cloud-funnel hangs low enough and the gyratory energy is
sufficiently great. There is a great pressure on the water outside of
the funnel and almost a perfect vacuum inside. This latter fact
contributes largely to the destructive power of the tornado. When a
funnel is central over a building a sudden vacuum is created outside of
it and it bursts outwardly from the internal air pressure.