Generally, 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.
Heating degree day (HDD) is a measurement designed to quantify the demand for energy needed to heat a building. HDD is derived from measurements of outside air temperature. The heating requirements for a given building at a specific location are considered to be directly proportional to the number of HDD at that location. Related measurements include the cooling degree day (CDD), which quantifies demand for air conditioning.
