Despite being only about 49 square miles (i.e., approximately 7 miles by 7 miles), San Francisco's rainfall is nearly as diverse as its many cultures. Strongly influenced by its topography and proximity to the Pacific Ocean, its wettest location receives 34% more rainfall than its driest.
Normal (i.e., 30-year averages from 1981 to 2010) annual rainfall data was extracted from PRISM (Parameter-elevation Relationships on Independent Slopes Model) for 315 gridpoints spaced at 0.01 degree (~0.55 miles) intervals across approximately 60 square miles covering San Francisco, adjacent waters, and extreme northern San Mateo County. To visualize this, the data was plotted and isohyets (i.e., contours of equal rainfall) were drawn.
San Francisco's rainfall patterns are the byproduct of moist air from the Pacific being forced upward by the terrain (i.e., orographically), resulting in the windward slopes seeing enhanced precipitation amounts as the air cools and condenses. On the leeward side downward forcing causes the air to become drier with less rainfall resulting.
Over San Francisco, the moist southwesterly flow coming onshore with many storms from the Pacific Ocean, is nearly perpendicular to the north-south ridge running pretty much through the center of The City. As it ascends toward this ridgeline the rainfall amounts increase from about 22” along the Great Highway to a maximum of 28.26” annually near Laguna Honda Hospital; just southwest of Twin Peaks. [Note that there is an even higher maximum of 30.17” over the higher terrain of Mount San Bruno in San Mateo County].
The air then dries out as it descends downward into the eastern side of San Francisco with a minima of about 21” along the waterfront between the Bay Bridge and Candlestick Point.
It is important to that this analysis is based upon the 1981-2010 30-year normals, and the “new" 1991-2020 normals which will be published next year, will be on the order of about 1 inch (~4%) lower.
