From Glossary of Meteorology
A measure of the departure of the mean daily temperature from a given standard: one degree-day for each degree (°C or °F) of departure above or below the standard during one day.
Degree-days are accumulated over a "season" at any point during which the total can be used as an index of the effect of past temperature upon some quantity, such as plant growth, fuel consumption, or power output. Cooling degree-days are used to estimate energy requirements when the mean daily temperature is higher than a given standard, while heating degree-days are used when the mean daily temperature is lower than the standard. For example, if a standard/threshold of 65°F is used and a location has a mean daily temperature of 75°F, then there were 10 cooling degree-days for that day.
Term edited 1 September 2019.