If you expect it to drop to 20F (60 - 40), that's below freezing! I didn't realize it got that cold in SF. Even here it only goes down to 10C (about 50F) at night.
Sorry, I wasn't clear. Even in winter it rarely gets below 40 here. In summer it's often foggy and in the 50s. My example was to show how much colder it can be here in the city than nearby--the 40 degree drop was from about 95-55 in 10 minutes of driving.