Winter - Mornings and evenings tend to be chilly. A warm overcoat and sweater are appropriate attire. The Bay Area sees most of its rainfall in the winter months which makes packing an umbrella a good idea.
Spring - The spring months in San Francisco are true to season. Linen and cotton clothing is ideal, however, pack a sweater or medium-weight jacket for likely temperature changes. In the spring, the trees and flowers are in bloom and those prone to allergies should pack necessary medication.
Summer - Legend has it that Mark Twain once said the coldest winter he ever spent was a summer in San Francisco. Chilling morning and evening fog is common during summer months. Bring a sweater or a medium-weight jacket. You may encounter cool temperatures during a day on the bay, at Alcatraz or walking the bridge. Evenings are typically cool. Women should pack a blazer or dressy coat for a night out. Men will be comfortable in a sports jacket and long pants.
If you intend to travel to neighboring counties north (Marin or Napa) or south (San Jose to Santa Cruz), pack typical summer clothing such as shorts and T-shirts.
Fall - The City sees its best weather in the fall from September to October. Temperatures are known to rise into the 70s and 80s, a "heat wave" by San Francisco standards. Remember that the bay is breezy and a medium-weight jacket is always a good idea whatever time of year you choose to visit.