In the same way dew forms at night on blades of grass. The grass cools off quickly and makes the air near by so cool that it can not hold all the water vapor it has. We say the dew falls. It does not really fall, but it forms right where we see it, just as the moisture condenses on the cool glass. In fact, the moisture that we watch form on the glass is really dew.
157. Frost.—If you will watch the moisture con dense on a plate of ice cream in hot weather, you may see some of it freeze into tiny spikes, or crystals, of ice. They are frost— frozen water vapor. Why does frost, instead of dew, form on cold nights in fall and spring? 158. Rain in the western states.—Now we are ready to see why the Pacific Mountains have much rain and snow, while the Basins have so very little. When the west wind blows moist air from the Pacific Ocean toward Nevada, it has to cross two mountain ranges, and one of them is very high. (Fig. 157.) As the air climbs higher and higher it becomes cooler and cooler, and the moisture, or water vapor, condenses as it does in the case of the cold glass or the blades of grass. These little drops of water in the air are like those we see in the little cloud which forms above the mouth of a boiling teakettle. As the air goes higher and becomes still cooler, the little drops gather to gether making drops so large that they fall as rain. If the weather is cold enough, frost is formed instead, and falls as snow. Look at the rain fall map of the United States (Fig. 158). How many belts of rainfall can you pick out in each of the Pa cific States? What makes them? (Figs. 91, 157). How much rain falls in Nevada? How much on the Sierra? Can you get facts from Figs. 158 and 91 and make a figure like 157 from a line east and west through Seattle or Portland? Does the rainfall map show why nearly all the towns of Utah are in a string on the plain at the foot of the Wasatch Moun tains? 159. Uneven distribution of rain on the Pacific Moun tains.—Lower California is so far south that cyclonic storms occur there only in the winter. In summer it is a hot, dry region. There is so little rain that the mountain streams rarely reach the sea. The rainfall increases as we go north, but even at San Diego there is less than twenty inches of rain a year, and men must irrigate the land in order to grow most crops. But in
northern California, Oregon, and Washington, the west winds drop nearly a hundred inches of rain a year on the mountains. The winter snowfall is heavy. Sometimes it is piled up to the eaves of mountain houses. The railroads can run only by having snowsheds built over the tracks. At Donner, California, near the summit of the Sierra, the average snowfall is sixty-eight feet a year.
From the northern end of Van couver Island around to the Alaskan Peninsula, the Pacific Coast is in many places a rugged wall of moun tain, rising so directly from the sea that rarely is there room for a town or a farm (Fig. 165). The sea winds hurl moist air, warm from the ocean, against these mountain slopes. This causes very heavy rain and snow. So much snow falls on the high moun tains that glaciers bring millions of tons of ice down to the sea, where it melts in the warm water of the Japa nese current (Fig. 327). Because of this cur rent the temperature along the Alaskan coast is much warmer than that of eastern North America in the same latitude.
160. Forests and snowfields.—In Lower California the climate is so hot and the rain fall so slight that there is only a little forest on the top of the high mountains near the northern end of the peninsula. The moun tains are often only bare, dry rocks, with a few bushes clinging in the moister places.
The forest begins at 5000 feet on the moun tains of southern California. The valleys are treeless. In the Great Valley the western slopes of the southern Sierra Nevada are grass-covered for the first fifteen hundred feet, then scattering trees appear. At three thou sand feet the Sierras are covered with a solid forest of splendid trees, which extend upward to the 6000-foot level. The farther north one goes the greater is the rainfall and the lower this forest line on the mountains. The Coast Ranges, which are lower than the Sierra, do not have much forest south of San Francisco. What is the rainfall of the Coast Ranges? (Fig. 158.) In northern California, Oregon, and Washington the heavy rain makes the coast forest so thick that it seems almost like a great dark building beneath the trees. The great tree trunks stand like the columns of a temple. Between them stand masses of ferns as high as a man's head, and in winter they are dripping wet in this ocean climate.