I have read other questions about zero degrees or zero degree but why is it heating degree days and not degrees? It makes sense to say one degree but zero degrees holds true in until you use absolute zero.
closed as unclear what you're asking by Edwin Ashworth, Drew, Nigel J, David, jimm101 Jan 5 '18 at 11:52
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A heating degree day (HDD) is an energy measure. Its exact value depends on a reference temperature and the mechanism used to compute a temperature difference that can be applied to the entire day. Broadly speaking, a higher value of HDD means that more energy must be used to heat an occupied space to the reference temperature for a specified duration.
For energy trading, the following definition is given by the Chicago Mercantile Exchange Group:
The CME HDD Index is equal to the sum of the average degrees that the outside air temperature drops below the base temperature of 65°F (or 18°C for non-U.S. cities) in the specified city each day in the contract month.
The above definition is from the Settlement Procedures for CME Group Weather Products (Monthly Temperature Futures). There are also cooling degree days, and quite a few other definitions.
BTW, it's worth noting that the CME Group didn't just make this standard up out of thin air. The U.S. Federal Government defines most of these things in the U.S., and their terminology influences most everybody else.
A degree day is one degree times one day. It forms its plural in the same way as acre feet, ton miles, kilowatt hours and other compound units where the individual terms have physical meaning.