Heating Degree Days

Chilly today.  And this is Memorial Day, the unofficial first day of summer.  Well not today.  56 degree F....brrrr.

I know something is wrong when I hear the furnace fire up.

https://commons.wikimedia.org/wiki/File%3AUnited_States_Heating_Degree_Day_map%2C_1961-1990.jpg

That brings me to the subject of Heating Degree Days, and for normally this time of the year, Cooling Degree Days.  For a formal definition try this link.