Heating degree and cooling degree days

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Dear Tom,
There used to be a weather-related metric called “heating degree” and “cooling degree” day units. Are they still used?
— Larry Lewandowski, Countryside
Dear Larry,
Both are alive and well. Heating degree and cooling degree days indicate the departure of a day’s average temperature from 65 degrees. When the average temperature (sum of the high and low divided by 2) equals 65 degrees, engineers have determined that neither heating nor cooling is needed to maintain a comfortable indoor environment. When the day’s average temperature is above 65, each degree of departure is one cooling degree day (CDD), and when below 65, one heating degree day (HDD). Running totals are kept for both to give a quick one-number snapshot for comparing seasonal heating and cooling costs. The HDD season runs from July 1 to June 30 while CDD totals are kept on a calendar-year basis.


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