Range of totals for rain tied to mountains

Q I am looking to buy a new home, and although I have lived in the Bay Area almost all my life, I am curious about average rainfall and how it varies from city to city, particularly in the South Bay.

Bill Wrobleski
Sunnyvale

A The seasonal rainfall around the South Bay points to the great variability of the area's microclimates. Within a 20-mile radius of San Jose, there is as much as a fourfold increase in the average amount of rain received each year. San Jose and cities near the bay average 14 to 15 inches per year. Contrast this to a 60-inch average in remote parts of the Santa Cruz Mountains (see chart below).

The primary reason is the effect of the mountains surrounding the valley on approaching storms. A contour map of rainfall amounts closely resembles a contour map of the landscape.

Most of our rain comes from the southwest and is forced to rise over the Coast Range and then the Diablo Range. This creates lift to the clouds and helps to wring out the copious amounts of rain to which our Santa Cruz Mountains neighbors will attest. Because the Diablo Range is farther inland and its orientation to the wind flow is less ideal, rain amounts around Mount Hamilton are lower.

On the east side of these mountains, the airflow actually sinks, which minimizes the rain totals.

An excellent online map of average California rainfall is available at www.nws.mbay.net/CA_NORTH.GIF

Q Why is it so often foggy specifically in Daly City and Pacifica?

Dave Lee
San Jose

A Most of the low clouds and fog that invade the Bay Area form over the chilly coastal waters and then are brought inland by the prevailing westerly winds. These winds take the path of least resistance, through gaps in the coastal hills.

With the exception of the Golden Gate, the San Bruno Gap to the northwest of San Francisco International Airport is the lowest.

Q How does La Niña affect our winter temperatures -- are they colder, warmer or average?

Emily Holley
San Jose

A Much like rainfall in the San Jose area, temperatures for the winter during La Niña years end up near average. The coldest effects of La Niña are most pronounced in the Pacific Northwest and near the Great Lakes, while the warmest impact will be in the Southeast. Links to additional information about La Niña can be found at
www.wdc.ndin.net/lanina.html .

Jan Null, a certified consulting meteorologist and owner of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send him questions c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them to weathercorner@ggweather.com.