You can visit San Francisco comfortably any time of year. Possibly the best time to visit San Francisco is September and October, when the city's most summerlike weather packs the calendar with outdoor concerts and festivals. The climate here always feels Mediterranean and moderate–with a foggy, sometimes chilly bite. The temperature rarely drops below 40°F, and anything warmer than 80°F is considered a heat wave. Be prepared for rain in winter, especially December and January. Winds off the ocean can add to the chill factor, so pack warm clothing. That old joke about summer in SF feeling like winter is true at heart, but once you move inland, summers are warmer.