Question: What can cuase it to rain in San Francisco on a warm cloudless night ?

Answer: How about a Buster Posey homerun !

Catcher Buster Posey of the hometown Giants had just launched a three-run home run into the deepest part of the park.

Whenever the Giants score a homerun at AT&T Park - two official demonstrations happen. A large foghorn located above center-field begins to sound. And four massive water cannon located
within the right-field archway columns discharge simultaneously. Here is what the resultant shower
looks like outside the park along the waterfront promenade.