Well, November and December can be rainy, but are generally mild. In January there might fall a tiny bit of snow on the higher mountains every other year. A few years ago it snowed downtown, but the snow melted before it hit the ground. Temps are usually 50s and 60 during daytime, but 70s are not uncommon. At night it can get close to freezing. In the winter we have less fog and often very clear skies with great sunsets. For someone like me, who's used to freezing Boston winters, SF is extremely mild, and I'd almost say you won't notice any seasons, they blend. It's comparable to a mediterranean climate.