San Mateo County is typical of California at its best. The days in summer are delightfully warm and balmy, being enhanced by clear skies, brilliant sunshine and clean, sweet air laden with the fragrance of blossom and cedar. The perfect ripening of fruit and the perennial bloom of flowers is convincing proof of this.
Those who first enter the county from the San Francisco boundary are impressed by the immediate change from the harsh weather of that place, with its high winds and fog banks, more or less prevalent throughout the entire year, to the agreeable warmth and brilliant sunshine which becomes more and more pronounced toward the south.
To the commuter and the motorist this is particularly noticeable. In the neighborhood of San Bruno, the fog banks disappear. Here the landscape will be found aglow with sunshine and not a cloud in sight except upon the hills to the right, where masses of fog-laden ocean clouds are piled high against the summits of the San Morena Mountains, which like giant hands hold them back in obedience to their perennial charge : “Thus far and no farther.”
This unusual climate-like a bit of sunny France or Italy transported bodily to San Mateo County, is the result of purely local topographical conditions or “the lay of the land.” It is caused by the protecting influence of the San Moreno range and the San Bruno hills, on the west and north respectively, and by the proximity of the warming waters of San Francisco Bay.
There are never any great extremes of heat or cold ; and even the hottest days of the year are tempered by cooling afternoon and evening bay breezes. The fogs and cold winds that are usually so prevalent in ocean counties are to a great extent lacking, so that outdoor sports are enjoyed the year around. On the ocean side of the peninsula the climate is more vigorous, winds from the Pacific cleansing the air frequently. But even these and the fogs that sometimes roll in, are not of a character that is disagreeable.
To understand the basic causes of San Mateo County’s climate, we must first analyze the general climatic conditions of central California.
This portion of the state is comparatively free from storms, as most of the recognizable disturbances pass far to the north. In the summer the climatic conditions here are in distinct contrast to those of other portions of the Pacific Coast; they create what might be termed a purely local climate. This reversal is caused by the great heat generated in the Sacramento and San Joaquin Valleys which causes the colder air from the Pacific Ocean to rush in from the coast. Because of the break in the coast mountains at the Golden Gate, this becomes the smaller end of a great funnel through which pass vast volumes of ocean air.
These air currents constitute the prevailing easterly winds and are intensified by the formation of the Gate. They are then deflected down the peninsula as north winds by the obstructions in the bay, consisting of the Contra Costa and Alameda shores, the Berkeley Hills, Angel Island, Yerba Buena and Alcatraz, the Sausalito Hills, Mount Tamalpais and the numerous hills of San Francisco.
In winter there is a reversal to normal of general climatic conditions when the prevailing winds are from the southeast and southwest.
The mean rainfall at San Francisco is about 23 inches, San Mateo about 21 inches and at San Jose about 15 inches, showing a steadily decreasing rainfall toward the south.