Bay are weather most likely fluctuates due to its location near the coast. Also, the fact that San Francisco rests on multiple hills/mountains contribute to the climate changes.
For instance, summer time may consist of foggy days and drizzling nights or simply have the usual scorching hot summer days.
Those who are located in the inlands find their summer days hotter and winter days much colder than those who are closer to the ocean.
Although a fantastic place to have summer vacations, beaches and other coastal locations are quite windy, especially during the night.
So, regardless of the weather, always make sure to wear a tank top under (in case it's scorching hot), a sweater/jacket (when it gets windy), and a convenient little umbrella (just in case it rains), hehe.
The mountain peaks of San Francisco Bay Area such as Mt. Tamalpais and Mt. Diablo have experienced snowfall.
Don't get me wrong, you can totally go there during June and July and still find a beautiful weather, but some days may be foggier and windier than you'd expect them to be.
The Bay Area has plenty of faults that could be a source for dangerous earthquakes. Remember the 1989 Loma Prieta earthquake? That's one of San Francisco's infamous greater earthquakes that did some major damage.
Storms most often occur during the months of November thru late February, and can vary in severity. I remember though, one December night I experienced some crazy thunderstorms (eek!).
The weather in San Franciso is most often quite windy, even during the Summer nights, so I suggest you always bring a sweater in your car or wrapped around your waist (um, I don't think that's in style anymore but whatever lol).
When you're going out to the city, try to check out the weather forecast first!
Man...you don't know how many times the weather in San Francisco has deceived me, lol. It could seem verrrrry hot and sunny early in day, but as the day progressed, it got cool and windy.
However, there are plenty of Summer nights where my friends and I just chill outside in the balcony and still enjoy a warm summer evening :)