The wind on the surface of the earth is not usually in the samedirection as that high up. The strength of the wind depends upon differences in air pressures. As the air pressure is measured by the barometer, the wind is commonly spoken of as due to a difference in barometric pressure or to the barometric gradient. Winds are named from the direction from which they come. A west wind is a wind that blows from the west.
If there were no other forces that affected the movement of the air,except the high and low pressures, the transfer would be in a straight line from one place to the other, and it could always be told in what direction the high and low pressures were, by direction of the wind. But obstacles like mountains and hills deflect the air currents. There are also other causes which influence the direction of the movement; chief among these is the rotation of the earth on its axis.
71.Velocity and Effect of Wind Action. -The velocity of airmovement varies from a gentle breeze which has not force enough to stir the leaves, to the terrific and al-most irresistible blast of the tornado, which sometimes attains a velocity of a hundred miles an hour and sweeps everything before it. The velocity of ordinary wind is measured by an instrument called an anemometer, which usually consists of four alu- minum cups attached by horizontal arms to a vertical spindle, the numberof revolutions of which is recorded on
Fig. 71.
a dial by a train of cogwheels geared to the spindle (Fig. 71).
When the wind has great velocity, it can be estimated only by thepressure which it exerts. A measure for the velocity of wind which needs no apparatus is given by Professor Hazen and is as follows:
1.Calm.
2.Light; just moving the leaves of trees.
3.Moderate; moving branches.
4.Brisk; swaying branches; blowing up dust.
5.High; blowing up twigs from the ground, swaying whole trees.
6.Gale; breaking small branches, loosening bricks on chimneys.
7.Hurricane, or tornado; destroying everything in its path.
Although the wind in its great paroxysms of rage is sometimes very destructive, it is ordinarily a most beneficent force. It is the circulatory medium for the earth; as the blood is for the animal and the sap forA DUTCH WINDMILL.
Windmills are widely used to pump water.
the plant. Without it the activities of the earth would stagnate. It spreads over the land the water evaporated from the sea. It cools the hot regions with the invigorating breath from the mountains and the uniformly tempered sea. It warms the cold places by bearing to them the heat taken from the warm ocean waters and the parched places of the earth. It bears man"s commerce across the seas and by the power of the water which it has borne over the land, furnishes him the means for his manufacturing. It scatters the seeds over the fields and sweeps the smoke and foul air away from his cities.
EFFECT OF WIND ON THE GROWTH OF TREES.
The trees have grown in the direction in which the prevailing wind blows.
72.The Effect of the Earth"s Rotation on Winds.
Experiment 80. -Revolve a globe from left to right and while it is revolving draw a piece of chalk from the pole toward the equator. Does the line as marked on the globe follow a meridian? What is its general direction in lower latitudes?
The rotation of the earth affects the direction of movement of all bodies free to move over its surface. Thus if a current of air starts from the north pole to flow south,it will, as it goes along, tend to move toward the right, and so when it reaches middle latitude it is no longer moving south but southwest. Why this is so can be fairly well understood if the conditions of this moving body of air are considered.
As the earth is about 25,000
miles in circumference and
A SAILING VESSEL.
Showing how the wind is used in commerce.
turns on its axis once in 24 hours, a body situated at the equator is carried from west to east at the rate of about 1000 miles per hour, whereas a body at the poles simply turns around during a revolution. Thus as we go on the surface from the poles toward the equator, each point has an increasing west to east velocity.
A body of air, not being attached to the surface, will have this west to cast velocity imparted to it very slowly by friction. Thus as it goes from higher to lower latitudes, it will lag behind particles on the surface which have this west to east velocity, and so will appear to have an east to west motion, just as a person sitting in a train that is just starting appears to be sitting still and the objects outside seem to move in the opposite direction. The combination of the north to south movement with the apparent east to west movement gives a northeast southwest direction to the air current.
It can be proved mathematically that all freely moving bodies on the earth"s surface are deflected toward the right in the northern hemisphere and toward the left in the southern hemisphere. This statement is called Ferrel"s law.
73.Planetary Wind Belts. -As the air at the equator receives a large amount of heat, it becomes warm and light, while that near the poles is cold and heavy. The air would thus have a constant tendency to move along the surface of the earth toward the equator and in an upper current from the equator toward the poles, just as in the dishes where water and oil were connected. But this direct movement is affected by the rotation of the earth and by certain atmospheric conditions so that between 25° and 35° both north and south of the equator there is an area of high pressure. These high-pressure areas can be seen on the isobar maps of January and July.
From these areas of high pressure the surface currents move both toward the equator and toward the poles. On account of the earth"s rotation the directions of these movements are not north and south but in the northern hemisphere northeast and southwest. Winds of this kind must occur on every revolving planet having an atmosphere; hence these winds are called planetary winds.