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<br />2. <br />GEOGRAPH I C AND CLH1ATI C SETTH!G <br /> <br />THE REGION <br /> <br />The Pyramid Pilot Cloud Seeding Project field area, lying <br />to the east of the crest of the Sierra Nevada Mountains <br />(Fig. 2-1), possesses distinct geographic and climatic boundaries. <br />Topography not only distinguishes the field area from ot~er parts <br />of the region, but also accounts for many of the observed varia- <br />tions in climatic elements within it. <br />The series of features which trend roughly northwest- <br />southeast and characterize the regions surrounding the project <br />area are clearly defined on the topographic map (Fig. 2-1). <br />California rises out of the Pacific Ocean in a series of ridges <br />and valleys known as the Coast Ranges. -Although some of the <br />ridges exceed 1,500 m (5,000 ft) in elevation, most are 900 m <br />(3,000 it) or less above sea level. The Great Valley of <br />California, lying immediately east of the Coast Ranges, is a <br />broad, alluvial topographic low which extends for almost 650 km <br />(400 mil through the central portion of the state. <br />The Sierra Nevada is a part of the 80 to 160 km (50-100 mil <br />wide Sierra-Cascade mountain system which extends for over <br />1,600 km (1,000 mi) through four states. The Sierra, which <br />reached its present height only within about the last two <br />million years, is a large mass of granitic rock which is bounded <br />on its eastern margin by a fault zone which is still active <br />(witness the Owens Valley earthquake early in this century). <br />Vertical movement along this fault zone has resulted in the <br />uplift of the Sierra as a tilted block with a long, gentle <br />western slope and a short, steep eastern face. As a consequence <br />of this structure, virtual1y all of the major Sierran rivers <br />drain to the west (Fig. 2-2). Relief ~ithin the mountains has <br /> <br />2-1 <br />