If you’re hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in San Francisco: The hottest months are usually May and April, with an average temperature of 28°C, while the coldest months are February and January, with an average of 27°C. The rainiest months in San Francisco are July, June, September and October, with each month seeing an average of 312 mm of rainfall.