it is no surprise that the san diego convention and visitors bureau boasts that san diego enjoys beautiful weather year round but a review of yearly temperatures supports this claim the average temperature for the city is a mild 70 degrees fahrenheit or 21 4 degrees on the celsius scalet is no surprise that the san diego convention and visitors bureau boasts that san diego enjoys beautiful weather year round but a review of yearly temperatures supports this claim the average temperature for the city is a mild 70 degrees fahrenheit or 21 4 degrees on the celsius scale