Degree Days are the most common and popular weather variable utilized for energy trading, weather risk management and seasonal planning, used to determine 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. In very warm or cold locations, an alternative base such as 55 °F or 75 °F may be used.

Degree Day accumulations are proportional to the amount of heating/cooling needed for 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. The graphic below shows normal heating degree day accumulations over the full heating season (Source: NESDIS, NOAA).