Why is the weather in San Francisco so different from that in San Jose?
San Francisco is more exposed the maritime westerly weather, whereas San Jose is in a rain shadow region with a mountain range between it and the Pacific Ocean.

San Jose, California, and adjacent cities are usually drier than the rest of the San Francisco Bay Area because of the rain shadow cast by the highest part of the Santa Cruz Mountains.

The exposure of San Francisco to westerly maritime weather and being surrounded by water on three sides are significant reasons why it has a milder climate. It's climate is largely influenced by the cold California Current.

Due to the Bay Area's diverse topography, the result of the clashing tectonic plates, the region is home to numerous microclimates that lead to pronounced differences in climate and temperature over short distances.

San Jose however is slightly inland and is surround by mountains of three sides, has less rain and an average of 301 sunshine days each year.