Summer (June, July and August)
In short, Summer in San Francisco is cool, with lots of fog and overcast days. Average temperatures in summer are lower than other major cities in the United States, temperatures vary greatly from day to day and drop down due to the fog. Usually a T-shirt, short-sleeved shirt or long-sleeved shirt is good, in addition, it is important to pack several items that you can layer.

Fall (September, October and November)
Fall in San Francisco is truly the best time, the weather is sunny and beautiful. But November is a little chilly and there are more rainy days. Generally in this season a short-sleeved shirt, long-sleeved shirt, cardigan and sweater are comfortable, however, you will need light winter clothes as winter approaches.

Winter (December, January and February)
Winter is the rainy season, but snow is rare. You should prepare for the rain. During this season a jacket is good, sometimes light winter clothes are necessary.

Spring (March, April and May)
Spring is also a good season, especially April and May, the skies are clear and beautiful. March is still in the rainy season. In Spring, you will need a long-sleeved shirt, cardigan or sweater. But March can be also winter day, so you will need the same clothes as winter depending on the weather.