EFFECTS OF TOPOGRAPHY ON CLIMATE – The easternmost mountain chains form a barrier that protects much of California from the extremely cold air of the Great Basin in winter.  There are occasions when cold air from an extensive high pressure area spreads westward and southward over California.  Even in these cases the warming by compression as the air flows down the slopes of the mountains into the valleys results in considerable moderation of temperatures.  The ranges of mountains to the west offer some protection to the interior from the strong flow of air off the Pacific Ocean.  As a result, precipitation is heavy on the coastal or western side of both the Coast Range and the Sierra Nevada and lighter on the eastern slopes.  Precipitation is also slightly reduced at the highest elevations of the Sierra Nevada because the range extends above the level of maximum transport of the moisture laden winds from the Pacific.

Temperature trends toward uniformity from day to day and from season to season on the ocean side of the Coast Range and in coastal valleys.  East of the Sierra Nevada temperature patterns are continental in character with wide excursions from high readings to low.  Between the two mountain chains and over much of the desert area the temperature regime is intermediate between the maritime and the continental models.  Hot summers are the rule while winters are moderate to cold.