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San Francisco has around 5 months a year of intermittent rain. San Jose, just 50 miles south and nestled between mountains instead of between waters, has perhaps only 1 month of rain. San Francisco also has notably colder winters and summers than San Jose, to a fairly shocking degree.

Geographical differences are three, primarily. First of all, San Francisco is further north, and closer to the Pacific Northwest, but only by 50 miles, or 80 kilometers. Second, San Francisco is surrounded by water, with the San Francisco Bay to the east and the Pacific Ocean to the West, while San Jose is isolated from the Pacific by the Santa Cruz mountains. Third, San Francisco has a far higher population density, with more smog and greenhouse gases as a result, which may or may not be a significant contributor to rain (nucleation sites?).

Why do these two so proximate cities have such different climates?

Topographical map of San Fransisco and San JoseClimate comparison between San Francisco and San Jose

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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.

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  • $\begingroup$ As someone who lives in San Jose I would add that in addition to the rain shadow effect of the Santa Cruz Mountains the cloud systems associated with winter storms from the Pacific often do not reach as far south as San Jose, but their fringes often do reach as far south as San Francisco. $\endgroup$
    – njuffa
    Jan 9, 2021 at 14:36
  • $\begingroup$ If you have ever biked between San Jose and Santa Cruz, it's pretty obvious that it's the mountains. Going up from San Jose (in the summer) it may well be 90s and sunny. Cross the ridge and go downhill a bit, and you're in the marine layer with fog & temps in the 60s (or lower: definitely chilly!) It's pretty much the same all the way up to the coast, until you get to the Golden Gate where the marine air can get in, or over the lower hills. (WRT population, it was much the same in the '70s, when much of the South Bay was farms.) $\endgroup$
    – jamesqf
    Jan 11, 2021 at 4:54
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I would submit this as a comment rather than an answer, but I don't have enough reputation yet.

Could the geography around San Jose make it more susceptible to descending air from further inland? This could give it drier weather than slightly further north, in addition to the rain shadow effect already mentioned by Fred. https://en.wikipedia.org/wiki/Diablo_wind

The more traditional example of this is the Santa Ana winds in southern California: https://en.wikipedia.org/wiki/Santa_Ana_winds

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