December in San Francisco is the rainiest and the coldest month of the year. In December, the weather in San Francisco is typically warm. Average daily high temperature in December is about 13 degrees above zero. On average, San Francisco has 13 sunny, 11 partly cloudy, and 7 rainy days in December. The rains are usually moderate. December as well as January, February, and March is considered the most uncomfortable time to visit San Francisco.