Degree Days are defined as the difference in the mean daily temperature (taken as the average of the maximum and the minimum) and a specified base temperature. When the mean daily temperature is below a given base value, it is a Heating Degree Day, and if it is above the base value then it is a Cooling Degree Day. If the mean temperature is the same as the base value, the difference is zero and there are no heating or cooling degree days.
On a monthly or annual basis degree days are added to give a cumulative total. For example, let's assume a HDD base of 60°F. If the mean daily temperature on January 1 is 0°F, the HDD would be 60 degree days. If we assume a mean temperature of 10°F on January 2nd, the HDD for this day would be 50, and the two day cumulative HDD is 110.