Q–Which spot in the nine-county Bay Area gets the most yearly average rainfall (and why) and which spot gets the least (and why)? And, of course, how much in each case?

Nearly all of the precipitation that we receive comes between November and April in the form of rain from moisture-bearing clouds off the Pacific. The San Francisco Bay area is well-known for its many micro-climates. Folks at the beach are freezing as the fog lingers in the Sunset District while others are sweltering in the bright sun in Livermore. The distribution and amount of rain is also all over this proverbial map.

At the Pt. Reyes Lighthouse, sticking way out into the ocean, you might expect rain to be plentiful. Wrong. Average rainfall there is only 15 inches. As storm clouds move inland and begin to rise up over the coastal mountains, the air cools. When air cools it cannot hold as much moisture and rain occurs. This is known as orographic precipitation and results in Kentfield getting 48 inches of rain per year from the same storms. And on the back side of these mountains rain shadows exist. So the city of San Jose may only average 16 to 20 inches of rain.

So the bottom line is that Mt. Tamalpais and the Santa Cruz mountains may get 45+ inches, Petaluma and San Francisco 23 inches and Livermore and Tracy 15 or less inches.

Michael Ellis – Ask the Naturalist – Bay Nature Magazine

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August 22, 2009