The number of Heating Degree Days (HDDs) during a period is an index that is intended to relate to the amount of heating used during that period. Because heating is most often used during winter, HDDs are most often calculated over some or all of the winter period, and are designed to reflect how cold that period is. The number of HDDs on one day is defined to be the number of degrees by which the Average Temperature on that day is below a threshold, usually taken to be 65F. The number of HDDs over a period is just the sum of the number of HDDs on each day during that period. For instance, a single day of 60F corresponds to 5HDDs. Two days of 60F correpond to 10 HDDs, and so on. Days during which the Average Temperature is greater than 65F do not contribute any HDDs. This somewhat complicated formula is an attempt to create a linear relationship between a meteorological index and the consumption of power. Contrast with Cooling Degree Days (CDDs).