The amount of rainfall during the year varies greatly in different places. It amounts to nothing or only a few inches over some regions, as in parts of Peru where rain falls only on an average of once in five years. But in the Khasi Hills region of India it has been known to be over 600 inches; and over 40 inches, or about the average yearly rainfall for the eastern United States, has been known to fall in 24 hours. This was in the season of the southwestern monsoons.
The rainfall in different parts of the earth has been carefully measured and maps showing its average amount prepared. As agriculture is largely dependent upon the amount of rain and the season of the yearRAINFALL MAP OF THE WORLD.
in which it falls, these maps tell much about the relative productivity of different regions of the earth. An annual total of eighteen or more inches is necessary for agriculture; and this must be properly distributed throughout the year.
On examining the map of the mean annual rainfall, we see that there are large areas where it is not sufficient for agriculture without irrigation. Such areas are within the belts of dry winds or in continental interiors far from large bodies of water. The rain-bearing winds coming from the water are forced to rise and cool so that their moisture is deposited before reaching these interior regions.
The rainfall of a place depends largely: (1)upon its elevation, since most of the rain-bearing clouds lie at low altitudes; (2) upon the direction and kind of winds that blow over it; and (3) upon the elevation of the land about it. The sides of mountains toward the direction from which the rain-bearing winds approach will be well watered, while the opposite side may be a dry desert. Explain the cause of the dryness of five of the great dry regions as found on the map above.
A cylindrical vessel having vertical sides, called a raingauge (Fig. 74), is used to determine the amount of rain. ItFig. 74.
is placed in an open space away from all trees and buildings and after each rain the amount collected is measured. Snow is melted before it is measured. As a rule eight or ten inches of snow make an inch of rain.
If the temperature is below the freezing point, 32° F., when condensation takes place, the moisture of the air will form into a wonderful variety of beautiful six-rayed snowflakes. These float downward through the air and often cover the ground with thick layers of snow. Although snow is itself cold, yet it keeps in the heat of the ground which it covers, so that in cold regions soil which is snowcovered does not freeze as deeply as that without snow. Therefore, to keep water pipes from freezing, it is not necessary to bury them as deeply in localities where snow is abundant as in places equally cold where snow seldom falls.
If raindrops become frozen into little balls in their passage through the air, they fall as hail. Hail usually occurs in summer and is probably caused by ascending currents of air carrying the raindrops to such a height that they are frozen and often mixed with snow before they fall. Sometimes hailstones are more than a half inch in diameter. They occasionally do great damage to crops and to the glass in buildings.
Sleet is a mixture of snow and rain.
78.Rainfall of the United States. -An examination of a rainfall map of the United States (following page) will show that the distribution of rainfall can readily be divided into fourbelts which, although gradually shading the one into the other, are yet quite distinct. These belts may be called the north Pacific slope, the south Pacific slope, the western interior region, and the eastern region.
In the north Pacific coast region the storms of the "westerlies" are common, particularly in winter, when the westerly winds are strong and stormy. The yearlyrainfall here amounts to about seventy
inches.
SALMON RIVER DAM, IDAHO.
A typical irrigation dam in the United States.
From central California south the rainfall of the Pacific slope decreases until, in southern California, there is almost no rain in summer and the entire rainfall for the year averages about 15 inches. By reference to the isobar map of the world (section 69) it will be seen that the high-pressure area of the dry tropical calm belt moves sufficiently far north in summer to take this region out of the influence of the wet westerlies and into that of the drier belt.
The western interior region, extending from the Cascade and Sierra Nevada mountains to about the 100th meridian, is dry over the larger part of its surface, since thewinds have deposited most of their moisture in passing over the mountains to the west. On the mountains and high plateaus, however,there is a considerable fall
of rain, as the winds are
EAST END OF THE ASSUAN DAM ACROSS THE NILE.
The greatest irrigation dam in the world.
cooled sufficiently in passing over these to deposit their remaining moisture. In most of this region, as in southern California, irrigationmust be resorted to if agriculture is to succeed. The fall of rain on the mountains and high plateaus supplies rivers of sufficient size to furnish water for extensive irrigation, and so a considerable part of the area which is now practically a desert will in the future be reclaimed for the use of man. The government is at present engaged in extensive irrigation work in this territory.
From about the 100th meridian to the Atlantic Ocean there is a varying rainfall, but it is as a rule sufficient for the needs of agriculture. It gradually increases toward the east, moisture being supplied plentifully from the Gulf of Mexico and the Atlantic Ocean by the southerly and easterly winds. The rainfall is well distributed throughout the year and averages from thirty to sixty inches.
79.Electricity.
Experiment 82. -Place some small pieces of paper or pith balls on a table and after rubbing a glass rod with silk bring it near the pieces. Do the same with a stick of sealing wax or a hard-rubber rod rubbed with flannel or a cat"s skin. Note the action of the pieces.