When you arrive to SFO will you drive 30 minutes south to Sunnyvale or 15 minutes North to the City? SFO is in South San Francisco and is usually nice. Sunnyvale can be uncomfortably hot in the summer on some days. The city itself (SF) is usually nice but can be cooler when the fog rolls in.

There's a famous quote: "The coldest winter I ever saw was the summer I spent in San Francisco.--Mark Twain"

The quote doesn't reveal everything about the Bay Area which has an amazing variety (21?!) of micro-climates. Drive up into the hills onto dirt roads above Los Gatos at 11:00 am on a hot summer day and you can move from sweaty hot bright sun of Sunnyvale to cool misty forests with condensation dripping onto your windshield from the dark towering forest. Back down the hill at 2:00 and you're hot again. Amazing!

Enjoy your trip!