There are two types of Degree Day based on the daily average temperatures for a given day against the Reference Value.
Cooling Degree Days (CDDs)
The number of degrees difference between the mean daily temperature and the Reference Value. Cooling Degree Days are used comparatively to measure the relative severity of a summer.
Calculation:
(Temperature Average – Degree Day Reference Point) or zero, whichever is greater
Example:
Temperature Average = 85°F
Degree Day Reference Point = 65°F
CDD = (85 – 65) = 20
Heating Degree Days (HDDs)
The number of degrees difference between the mean daily temperature and the Reference Value. Heating Degree Days are used comparatively to measure the relative severity of a winter.
Calculation:
(Degree Day Reference Point – Temperature Average) or zero, whichever is greater
Example:
Temperature Average = 30°F
Degree Day Reference Point = 65°F
HDD = (65 – 30) = 35