What are heating and cooling degree days?

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Dear Tom,
What are heating and cooling degree days?
Colin McIntyre,  Chicago
Dear Colin,
Heating and cooling degree days are measures that indicate the departure of a day’s average temperature from a base of 65 degrees. When the day’s average temp (the sum of the midnight-to-midnight, CST, high and low temperatures, divided by two) is below 65 degrees, the number of degrees of departure is called heating degree days; when above, cooling degree days.

Degree day units are a useful guide for operations in which temperature is an important factor — in heating and air conditioning, for example, and in agriculture. A base of 65 degrees is used because heating engineers have determined that when air temperatures average below 65 degrees, heating is generally required in a building; when above, air conditioning is helpful.

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