A degree day, by definition, is the difference between the mean daily temperature and 65 degrees.
The mean daily temperature is the value which is the average of the high of the day and the low of the day. This information is important in determining how much it will cost to both heat and cool a home in a certain area. It can be done both with a heating and a cooling aspect separately.
--Susan 13:01, 5 October 2007 (PDT)