Degree Days are a practical method for determining cumulative temperatures over the course of a season. Originally designed to evaluate energy demand and consumption, degree days are based on how far the average temperature departs from a human comfort level of 65°F. Simply put, each degree of temperature above 65°F is counted as one cooling degree day, and each degree of temperature below 65°F is counted as one heating degree day. For example, a day with an average temperature of 80°F will have 15 cooling degree days. The number of degree days accumulated in a day are proportional to the amount of heating/cooling you would have to do to a building to reach the human comfort level of 65°F. The degree days are accumulated each day over the course of a heating/cooling season, and can be compared to a long term (multi-year) average, or normal, to see if that season was warmer or cooler than usual. (Source: NESDIS, NOAA).

Daily Temperature VariableDefined asDescription
Cooling
Degree Day

(CDD)
(T - 65)
Daily CDD
T is daily Average Temperature (°F).
If T is less than 65°F, CDD=0.
Heating
Degree Day

(HDD)
(65 - T)
Daily HDD
T is daily Average Temperature (°F).
If T is greater than 65°F, HDD=0.
Average (Mean) Temperature of the day Tmax + Tmin
2
Tmax (High) & Tmin (Low) are whole integer values.