Heating Degree Day

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A method of measuring how cold a region is. A heating degree day (HDD) is any day when the mean temperature outside is lower than 65°F.

65°F is the temperature when indoor heating is generally needed to feel comfortable. If you add up all the daily mean temperatures in a year that were less than 65°F, you get the total HDDs in a year.

Ie, if a region had only three days in a year that fell below 65°F, where the mean temperatures were 55°F, 50°F and 45°F:

(65°F – 55°F = 10°F) + (65°F – 50°F = 15°F) + (65°F – 45°F = 20°F) = 45 Heating Degree Days

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