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How is a "heating degree day" defined?

Early in the twentieth century, engineers developed the concept of heating degree days as a useful index of heating fuel requirements. They found that when the daily mean (average) temperature is lower than 65° Fahrenheit (18° Celsius), most buildings require heat to maintain a 70° Fahrenheit (21° Celsius) temperature. Each degree of mean temperature below 65° Fahrenheit (18° Celsius) is counted as "one heating degree day."

For every additional heating degree day, more fuel is needed to maintain a 70° Fahrenheit (21° Celsius) indoor temperature. For example, a day with a mean temperature of35° Fahrenheit (1.5° Celsius) would be rated as 30 heating degree days and would require twice as much fuel as a day with a mean temperature of 50⁰ Fahrenheit (10° Celsius ; 15 heating degree days).

Source: Bair, Frank E. The Weather...

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