Term referring to the number of units (degrees) that the average outdoor temperature falls below or exceeds a base value (usually 65° F) in a given period of time. Each degree that the mean daily temperature is above the base value is called a cooling degree day (CDD) unit. Each degree that the mean daily temperature is below the base value is called a heating degree-day (HDD) unit. The monthly CDD & HDD values are simply the sum of the individual CDD and HDD units for the days in the month. Degree-days are reasonably good indices of the heating and cooling requirements of buildings.