San Francisco enjoys temperate marine weather, which stays mild year-round. Fog may roll in during the warm summer months (usually in the morning and evening), but it typically clears out during the middle of the day. Temperatures rarely fall below 40°F (5°C), or rise above 70°F (21°C) at any point throughout the year.
When visiting San Francisco it is best to be prepared with a light coat just in case. Layering is a good idea, as a cool foggy morning often turns into a warm, sunny afternoon in The City.