San Francisco, a city known for its fog, is actually a composite of microclimates. Last week, I was visiting the Bay Area and was reminded of its unique climate situation.

The area, in general, has a Mediterranean climate with mild, wet winters and dry summers. It is rare for the city to get warmer than 70°F in summer or cooler than 45°F in winter. Situated on a peninsula along the California coastline, San Francisco is kept mild by the Pacific Ocean’s chilly currents and local coastal upwelling.

Famous for its hills, the city’s complex topography is another major influence on its climate. The Golden Gate, a break in the mountainous Coast Range, funnels Pacific air into the Bay Area. The hills and basins of the peninsula then capture and divert the circulating marine air in intricate ways, forming a variety of  microclimates. As a result, weather conditions can vary widely across short distances, such as 10°F between neighborhoods.  In San Francisco, the forty hills that form the center of the city create a general weather divide. The western side of the city usually bears the brunt of the incoming Pacific air, with cool temperatures, strong winds, and fog.  The more sheltered eastern side generally sees more sun and warmer temperatures.

On a larger regional scale, temperature differences have an even wider scope.  For example, the average high temperature in July in the city is 68°F, while temperatures can reach 100°F in the Sacramento Valley, just 50 miles inland.