Weather will likely play an important role when you plan your trip to San Francisco, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually March and April with an average temp of 83°F, while the coldest months are December and November with an average of 79°F. The rainiest months in San Francisco are October, September, June, and May, with each month seeing an average of 17 inches of rainfall.