Why is there a big change of climate after you pass the San Bruno area in San francisco California?

It is the Bay Effect.

San Francisco is known for its different weather patterns.

Mark Twain once said, "The coldest winter I ever spent was a summer in San Francisco."

Once you get to San Bruno you are leaving that turbulent area.

Peggy: Site like Snopes and others claim that Mark Twain never said that but they have no proof that he did not. They just can't find it in any of his writings. It was never written down according to those who were most familiar with him but it was said on the stage while introducing a relative of his. He had several similar statements in writing so it indicates it was his thinking and would not be too hard to believe the quotation was attributable to him. It certainly is more believable that AGW.

South of San Bruno the San Francisco Peninsula widens out and a low range of mountains partially blocks the prevailing westerly winds of the cold Pacific. This makes the daytime highs warmer to the south, particularly in the summer, when the sea breeze is more prevalent.

Sagebrush: There is no evidence that Mark Twain ever said that, although it is widely attributed to him.

Microclimates caused by the proximity to, or distance from, the ocean - with the bay also factoring, as well as the Santa Cruz mountains. Mainly, blame Mr. Ocean - he is a fickle boss.