Use of the barometer. The barometer may be used to determine elevations. At sea level the barometer will read 30 inches. If the barometer be carried to elevations above sea level, the mercury gradually falls. It falls at the rate of one inch for a rise in elevation of about 910 feet.

Changes in air pressure are also very closely connected with changes in the weather. In general it may be said that a rapid rise of the mercury in the barometer indicates fair weather, and a rapid fall indicates stormy weather. The barometer is thus of great assistance in predicting the general trend of the weather.

Questions

1. How can you prove that air has weight?

2. What is the weight of a column of air over one square inch at sea level?

3. Why is not the atmospheric pressure on the top of a mountain the same as it is at the bottom?

FIG. 50. A standard mercurial barometer.

4. Why are we not always conscious of the great pressure of the air around us? 5. What are some of the effects of high altitudes on the human body?

6. How high will the atmosphere hold a column of water? Of mercury? How do you account for the difference in these heights?

7. Does the height of the column of mercury held by the atmosphere vary with the diameter of the tube? 8. How can you prove that air presses in all directions?

9. Does the pressure of the atmosphere vary? If so, under what conditions?

10. What is the structure of a barometer?

11. Why do we use mercury in a barometer instead of some other liquids?

12. What are the uses of a barometer?

13. What kind of weather does a high barometer indicate? A low one?

CHAPTER XX

WINDS AND STORMS

Winds. We are all familiar with the sometimes regular, but more often fitful and irregular movements of the air, which we call winds.

Winds are important in many ways. They transfer great masses of air from one part of the earth to another; they carry away the impurities of city air; they furnish power for windmills and sailing vessels; they cool hot regions and warm cold ones. Most changes of weather are due to changes in the direction of the wind. Some winds are agreeable and favorable to life. Some bring suffering, destruction, and death. Even light breezes effect a continual change of air which is beneficial for both plants and animals. To man, winds are as a rule stimulating and invigorating, while calm air is often enervating.

Cause of winds. A piece of wire gauze is placed over the flame of a Bunsen burner.

is

FIG. 51. When air heated it expands, becomes lighter, and is pushed up by the colder air which is drawn down beneath it.

If very small pieces of cotton are dropped upon the

gauze they soon rise, being carried upward by the cur rents of heated air.

We already know that when air is heated it expands and becomes lighter. The surrounding colder air, because it is heavier, is then forced into the place occupied by the heated lighter air. The heated air is then pushed up and produces an air current. This, on a small scale, is a wind. If for any reason the air above one place becomes heavier than that above another, there will be a transfer of air from the place where the pressure is greater to that where it is less. This movement of the air we call wind. Winds are caused by unequal pressure. The inequality of pressure is usually the result of unequal heating.

It is a well known fact that all parts of the earth are not equally heated. It is also known that the amount of heat received by the earth at any given place varies from hour to hour, day to night, and month to month. These variations in temperature are largely the result of the shape of the earth, its movements, and its position with reference to the sun.

Just as the earth is unequally heated, so likewise is the air immediately above it unequally heated. As a result the pressure of the air varies in different places, being lightest where the air is hottest, and heaviest where its temperature is lowest. In those portions where the earth is greatly heated, the warm air will be pushed up by the cold air from the surrounding regions which will come in to fill its place.

General effect of unequal heating. The effect of heating the earth is to cause the air to rise from the surface. Since all parts of the earth are not evenly heated, the air does not rise evenly. It rises most over the equator and in the tropics where the earth receives the greatest amount of heat. This warm upper air moves from equatorial regions toward the poles. As it does so, it gradually becomes cooler and consequently heavier, until in the regions of 30 degrees north and south of the equator, it settles down upon the earth. This movement of the air establishes a region of high pressure in each

30

N

30

one toward the equator. The elements of

the general circula

FIG. 52.

Diagram of the general circulation of the air. The trade winds blow toward the equator, the prevailing westerlies blow toward the poles.

tion, then, are: 1. the rising of warm air in the tropics, 2. the moving poleward of this air, 3. its settling, 4. the moving poleward of part of this air and the moving equatorward of the remainder.

If the earth did not rotate, the poleward moving