Frost drainage is a curious part of the climate of hills and moun tains. On cold nights, when there is no wind, the coldest air, being heaviest (Sec. 66), runs down into the valleys, which thus get colder than the hillsides above. If the valley does not have time to fill up with freezing air before sunrise, the hilltops and higher slopes do not freeze. A place having frost drainage is sometimes called a thermal belt. It does not reach very high up the mountainside. As the hillsides and hilltops are safer from frost than the flat valleys below them (Fig. 179), many orchards in Southern California and elsewhere in the world (Sec. 283) are planted on the slopes and tops of hills (Fig. 244).
Orchards on the slopes in dry countries must be supplied with water by irrigation. With much labor, ditches have been dug which carry water out on the South ern California slopes, where orange groves bedeck the foothills on both sides of the San Bernardino Valley as in Fig. 179. Below on the floor of the valley, instead of fruit trees there are fields of alfalfa, wheat, barley, and other crops that frost does not injure. Where the foot hills cannot be irrigated, grapes are sometimes grown, because the vines are able to send roots far into the earth, and thus they get the small amount of necessary water.
185. Very intensive agriculture.—Agricul ture is more intensive in this district than in any other region of the United States. Many families support themselves by selling the oranges, peaches, grapes, tomatoes, celery, cabbage, or vegetables which they grow on five or ten acres of irrigated land. In some localities the sugar beet is grown. This crop requires much work, but it gives a large har vest. The farms are so small that the farm houses are close together. As one rides along the road, one seems to be passing through a village which extends for miles, but really the buildings are just a suc cession of farmhouses.
186. The fog and the beans.—Near the seacoast there is a heavy fog every summer morning for weeks at a time. Foggy weather seems to suit lima beans, and nearly all the beans that are dried and sold in the United States are grown in this district. The rain fall is only ten or fifteen inches a year, but the beans grow without irrigation because the fog dampens the ground each night.
187. Hillside pastures.— Flocks of cattle, sheep and goats pasture on the slopes above the irrigation line, and above the pastures the moun tainsides are clad with for ests of oaks and evergreens.
188. The need of forests.
—The people of these valleys insist that the Government shall keep all mountain summits well cared for and forest-clad (Sec. 168), so that there may be water for irrigation.
189. The Los Angeles aqueduct.—Years ago the people of Los Angeles found that the city must have a new and larger water supply. This supply was secured at great labor by building an aqueduct through moun tains and over hills, valleys, and plains, in order to bring water from Owens River near the foot of Mt. Whitney, in the Sierra Nevada Mountains, two hundred and forty miles away (Fig. 182). This new water sup ply is one of the things which has helped the city to grow so rapidly. Hydroelectric power plants have been built at points where the aqueduct comes down the mountainsides. The power is sold in cities to help repay the twenty-five millions of dollars spent in build ing the aqueduct. (Fig. 194.) 190. Fuel and cities.—There is so little coal on the Pacific Coast, that ships coming to California for grain formerly brought coal from our Atlantic Coast, from Australia, and even from England. Then petroleum fields were found in Southern California and in other parts of the state. (Fig. 44.) This oil is a much cheaper fuel than the coal that was imported. It is used instead of coal on Pacific railroads and many steamships and United States Government war vessels. The manufacturers in Los Angeles can now run engines as cheaply as they can be run near the coal fields of the eastern states. This is one of the reasons why Los Angeles grew more rapidly between 1910 and 1920 than any other large American city except De troit. Los Angeles is the trade center of this region. It has a good harbor, steamship lines on the Pacific, and is growing as a manu facturing center.
191. Future.—Much more land may be irrigated by preventing all waste of water. What was said in the study of the South western Plateaus (Sec. 151) about crops that grow on unirrigated land? Will the delightful climate of this section continue to attract people from other regions? What does the success of the film industry indicate as to the future of other manufac turing industries?