The Sierra Nevada of California is covered by a deep mantle of snow in winter and spring, but in spite of this the spring days are warm and even the nights are mild. This departure from the classic belief that snow surfaces must give rise to a severe climate is examined in great detail, and is traced to two main and a variety of subsidiary causes. First, the anticyclonic curvature of the air circulation at the 700 mb. (about 10,500 ft., 3200 m.) level on many days results in very dry subsiding air. This permits a great deal of solar radiation to penetrate to the ground, and also brings down heat from above by day though not by night. This accounts for the warm days. Secondly the climate is considerably modified by the open forests which cover 40 per cent of the area. The albedo of trees is low, and they absorb a great deal of the solar radiation; in calm weather leaves may be 10–12° C. wanner than the air. This heat is transferred to the air and ground, and some of it is conserved until night. Some other factors work in the same direction. The albedo of fresh snow is high, but in the long sequences of “weathering days” between storms it decreases rapidly, especially in spring. Melt water formed during the day sinks into the snow and some of it freezes again at night, releasing its latent heat. The large amount of run-off is itself evidence of the mild spring climate.
This University thesis is a mine of information about the Sierra Nevada (there are 84 tables of data), but also contains a great deal of well-documented material about the physics of snow in other mountain ranges. The bibliography of some 400 items is evidence of the author’s wide reading.
