San Francisco Bay Area - Climate

Climate

The same location during the summer months. Because rain is rare in the San Francisco Bay Area during this time, the surrounding hills quickly become dry and golden-hued in grassy areas

Because the hills, mountains, and large bodies of water produce such vast geographic diversity within this region, the San Francisco Bay Area offers a significant variety of microclimates. The areas near the Pacific Ocean are generally characterized by relatively small temperature variations during the year, with cool foggy summers and mild rainy winters. Inland areas, especially those separated from the ocean by hills or mountains, have hotter summers and colder overnight temperatures during the winter. San Jose at the south end of the Bay averages fewer than 15 inches (380 mm) of rain annually, while Napa at the north end of the Bay averages over 30 and parts of the Santa Cruz Mountains just a few miles west of San Jose get over 55. In the summer, inland regions can be over 40 degrees Fahrenheit (22 degrees Celsius) warmer than the coast. This large temperature contrast induces a strong pressure gradient, which results in brisk coastal winds which help keep the coastal climate cool and typically, foggy during the summer. Additionally, strong winds are produced through gaps in the coastal ranges such as the Golden Gate, the Carquinez Strait, and the Altamont Pass, the latter the site of extensive wind farms. During the fall and winter seasons, when not stormy, a high pressure area is usually present inland, leading to an offshore flow. While negatively impacting air quality, this also clears fog away from the Pacific shore, and so the best weather in San Francisco can usually be found from mid September through mid October. Winter storms are typically wet and mild in temperature during this time of year, being caused by cold fronts sweeping the eastern Pacific and often originating in the Gulf of Alaska. During November into mid March, winter storms are usually several days in length, wet and cool, with severely damaging storms rare. There is also recorded snowfall on San Francisco Bay Area peaks, such as Mount St. Helena, Tamalpais, Diablo and Hamilton. Snow levels range every given year from 1000 feet in Sonoma County to 2,000 ft in Contra Costa, San Mateo, and Santa Clara counties the during the winter. Greater recorded snowfall amounts are generally recorded once every 5 to 10 years. In February 2001, 30 inches (76 cm) of snow fell on Mount Hamilton (4360 ft), 17 inches on Mount Tamalpais (2,574 ft) and 10 inches on Mount Diablo (3,864 ft). Occasionally during the late Summer or early Autumn, spells of warm humid weather will drift over the Bay Area from the Southwest Monsoon or from the residue of Western Pacific hurricanes near Mexico, usually bringing high variable clouds as well, and more rarely, high-based thunderstorms.