If you spend more than a few days inside San Francisco Bay, you’ll experience the phenomenon of microclimates. If you travel within San Francisco, you can experience several microclimates just traveling from the bayside waters to the breakers on the coastal shore. These are not minor events. There have been days in San Francisco when dewfall has been heavy enough to need windshield wipers in the Sunset, yet sunscreen is required in the bleachers at AT&T Park. These are seven miles apart; the tallest geographic feature between them is less than one thousand feet high.
This effect becomes less pronounced but doesn’t disappear entirely as you move away from San Francisco. The combined effects of geography and water depth smooth the ranges of temperature. You have the option of a cool coastal drive from Pacifica to Santa Cruz, or a warm cruise from Millbrae to San Jose. It can be downright toasty along the spine of the mountains from Highway 92 to Highway 9.
All this is a prelude to the three pictures below: