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 18 C *. Simply put, each degree of temperature above 18 C is counted as one cooling degree day, and each degree of temperature below 18C is counted as one heating degree day. For example, a day with an average temperature of 27 C will have 9 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 18 C. 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 - 18)
Daily CDD
T is daily Average Temperature (°C).
If T is less than 18°C, CDD=0.
Heating
Degree Day

(HDD)
(18 - T)
Daily HDD
T is daily Average Temperature (°C).
If T is greater than 18°C, HDD=0.
Average (Mean) Temperature of the day Tmax + Tmin
2
Tmax (High) & Tmin (Low) are whole integer values.