SF weather is pretty unpredictable. I'm not sure if its global warming or el nino fallout, but in general it is pretty ridiculous. In the past month and a half or so, it has been both 80 degrees and 50 degrees with a delightful mix of fog, rain and unbelievable sunshine. In the summer, it is often cold, whereas in September and October it is generally sunny and warm. December is clear and beautiful but doesn't go much above 50 degrees.
Beyond this, different areas of the city have completely different micro climates. While the Mission and Potrero Hill are generally fog free and warm, the Sunset and the Richmond (both close to the beach) are almost always fogged in and breezy. I remember a month or so ago (November 15th -- I remember the day because I bragged to all my friends living on the East Coast) it was 75 degrees. My girlfriends and I actually laid out at Ocean Beach IN NOVEMBER. There were a ton of surfers out, lots of dogs, and families all frolicking along the sand. I love that when it IS beautiful in SF that everyone takes advantage of it, too. So basically, the weather is yet another reason why SF is amazing...I swear I'm not biased!