The average temperature in san francisco, california, for december, january, and february is 11°c (52°f). during the same three-month period the average temperature in richmond, virginia, is 4°c (39°f). yet, san francisco and richmond have nearly the same yearly total of

Heating Degree Days

In comparison to San Francisco, Richmond, Virginia has a higher high temperature and a lower low temperature. And San Francisco has a more steadily averaged temperature for both highs and lows throughout the year, less extreme variation.

And although the average winter temperatures discussed in the question is significantly lower for Richmond, the issue with having similar amounts of heating degree days, relates to the outside air temperature falling below 65 degree Fahrenheit.

A heating degree day corresponds to the time when it is necessary to heat a building due to the outside air temperature falling below this threshold - the level or number of 65 degrees. 

Though seasonal differences exist for both San Francisco and Richmond, as presented by the winter average differences, both of these cities fall below the 65 degree Fahrenheit level or Heating Degree Days at a similar rate, but San Francisco is not as much. Both cities transition to above this temperature during the warmer months, San Francisco by a smaller amount than Richmond, but like Richmond, still above the 65 degree level.