In California, much of the climatic variation results from the patterns of global weather systems. However, prominent mountain ranges in California and western Nevada also have an important influence on moisture distribution in the region.
Temperatures are cooler in the higher altitudes of the mountains.
As eastward-moving, moist, unstable air masses rise up the western slopes of the mountains, the air is cooled and water vapor condenses and falls as rain, snow, or ice. When these air masses descend the eastern slopes, they become warmer and more stable and thus retain most of the remaining moisture. The extreme northern part of California has slightly wetter summers than the rest of the segment. Fog occurs frequently on the coast and provides some additional moisture that is used primarily by vegetation. Mountain ranges that parallel the coast also affect temperature distribution.