The Santa Clara Valley Rainshadow

 Often when it rains in the San Francisco Bay Area the lowest rain amounts are in San Jose and the surrounding environs of the Santa Clara Valley. To quantify this the normal (i.e., 30-year average from 1981 to 2010) rainfall over the region was analyzed. The data used was from PRISM (Parameter-elevation Relationships on Independent Slopes Model) at Oregon State University. Annual average rainfall was extracted for 600 gridpoints spaced at 0.05 degree (~0.55 miles) intervals over approximately 1300 square miles covering most of Santa Clara and the northern part of Santa Cruz Counties.

Rainshadows are the byproduct of moist air being lifted by the terrain (i.e., orographically) resulting in upward forcing on the windward slopes and thus enhanced precipitation amounts as the air cools and condenses. On the leeward side, the downward forcing causes the air to become drier with less rainfall resulting.


In the SF Bay Area, the moist southwesterly flow coming onshore from the Pacific Ocean is often perpendicular to the Santa Cruz Mountains, which are oriented from northwest to southeast. After ascending the mountains and producing sometimes copious amounts of rain, the air dries out as it descends downward into the Santa Clara Valley. This produces about fours time more rain near Ben Lomond (~60 inches) than near San Jose Airport (~15 inches) which is only 22 miles away. (Click here or on the map for full-size image)

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (408) 379-7500
Twitter: @ggweather


1 response
Living in Cambrian Park/Campbell on the 20" line the 60's, this was very evident by simple observations as a misty cloudless hole often opened up downstream of Loma Prieta while Saratoga/ Los Gatos was getting wet and Downtown San Jose dry.