Heating degree days are defined as?

• A. The sum of daily high minus low temperatures
• B. The sum of days above 65°F
• C. The average temperature above 65°F
• D. The sum of daily mean temperature minus 65°F for days below that threshold

Answer: (D)

Heating degree days measure how much heating is needed by looking at how far daily mean temperatures fall below a baseline, typically 65°F. For each day, if the mean temperature is below 65°F, you count the shortfall as 65 minus the daily mean; if the mean is 65°F or higher, that day adds nothing. Add up these shortfalls over the period to get the HDD, which reflects the total cold exposure and thus the heating demand.

For example, a day with a mean of 60°F contributes 5 HDD, while a day at 66°F contributes 0 HDD. Over multiple days, you sum all the daily shortfalls.

The other options don’t capture this idea: one refers to a temperature range, another to days above 65°F, another to the average temperature being above 65°F, and the last proposes the difference in the opposite order. The concept you’re testing is the cumulative deficit below the base temperature on cold days.