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 18°C 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 Variable | Defined as | Description |
|---|---|---|
|
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. |