A heating degree day (HDD) is calculated by adding the high and low temperatures of each day and dividing by two. If this average daily temperature is below 65°F, a heating degree day is recorded for each degree below 65°F. An example would be a day with a high of 77°F and a low of 50°F—this day would be recorded as having 1.5 HDDs. Any day with an average temperature above 65°F has zero HDDs. HDDs are an excellent resource for evaluating the climate and estimating heating costs in any location. Note that HDDs in the “Redbook” are given in degrees Celsius.