Weather in San Francisco is quite interesting in that it is usually between 50 and 70 degrees every day of the year, and by that I mean it covers that range every day.
Summer is often colder and foggier than winter in that the high valley heat pulls in the marine layer and the coast can't burn it off. Heat waves can hit into the low 100s in the City proper, but never last more than 1-2 days before "nature's air conditioning" (as the marine layer is called) comes back and plummets the temperature over 30 degrees in a few hours.
I've been joking with my new housemate that since she just moved from Chicago, the City is showing off for her. Her first day here, January 15, it was 5 degrees when she left Chicago in the AM, and that evening in SF she was walking around the Castro in a t-shirt with no jacket on due to our low 60s temp that night. We were all joyously stunned.
BTW, the quote "the coldest winter I ever spent was a summer in San Francisco" was NOT really said by Mark Twain, despite many attributions of this line to him. I don't know who said it, but sadly, I have to say it's true.