Heating degree days (HDD) and cooling degree days (CDD) are climatalogical metrics used to express the magnitude of the heating or cooling load in a given location. These metrics are expressed in terms of a "base temperature" (e.g., "Heating Degree Days Base 65 F"). HDD are calculated by taking the daily average temperature; if it is colder than the "base temperature," the difference is calculated. This difference is summed for the period (week, month, year) in question.