Over the northern three-fourths of the State there are two primary mountain chains paralleling the coast, while in the southern one-fourth there is only one.  Isotherms run mostly north-south, parallel to the countours of the mountains, instead of east-west as is common in most parts of the temperate zone.  Along the western side of the Coast Range the climate is dominated by the Pacific Ocean.  Warm winters, cool summers, small daily and seasonal temperature ranges, and high relative humidities are characteristic of this area. With increasing distance from the ocean the maritime influence decreases.  Areas that are well protected from the ocean experience a more continental type of climate with warmer summers, colder winters, greater daily and seasonal temperature ranges, and generally lower relative humidities.  Many parts of the State lie within a transitional zone, where conditions range between these two climatic extremes.  The mixture depends upon local topography and its influence on circulation patterns.

Summer is a dry period over most of the State.  With the northward migration of the semi-permanent Pacific high during summer, most storm tracks are deflected far to the north. California seldom receives precipitation from Pacific storms during this time of year.  Occasionally, however, moist air drifts northward during the warm months from the Gulf of Mexico or the Gulf of California.  At such times, scattered, locally heavy showers occur, mostly over the desert and mountain portions of the State.

The Pacific high decreases in intensity in winter and moves further south, permitting storms to move into and across the State, producing widespread rain at low elevations and snow at high elevations.  Occasionally the broadscale circulation pattern permits a series of storm centers to move into California from the southwest.  This type of storm pattern is responsible for occasional heavy rains that may cause serious flooding.