(For cooling degree days, the process works in reverse: the base temperature is subtracted from the average, and if this value is positive, that number represents the CDD.) This method works satisfactorily if the outside air temperature does not exceed the base temperature. But if the value is positive, that number represents the number of HDD on that day. If the value is less than or equal to zero, that day has zero HDD. National Weather Service, is to take the average temperature on any given day (the mean of the high and low temperature) and subtract it from the base temperature. One popular approximation method, that used by the U.S. HDD are often calculated using simple approximation methods that use daily temperature readings instead of more detailed temperature records such as half-hourly readings, the latter of which can be used to estimate an integral. There are a number of ways in which HDD can be calculated: the more detailed a record of temperature data, the more accurate the HDD that can be calculated. The variation in choice of base temperature implies that HDD values cannot always be compared – care must be taken to ensure that only HDDs with equal base temperatures are compared. Base temperatures of 16 ☌ and 19 ☌ (61, 66 ☏) are also used.
The base temperature does not necessarily correspond to the building mean internal temperature, as standards may consider mean building insulation levels and internal gains to determine an average external temperature at which heating will be required. In the latter case, building standards or conventions may exist for the temperature threshold. Base temperatures may be defined for a particular building as a function of the temperature that the building is heated to, or it may be defined for a country or region for example.
Heating degree days are defined relative to a base temperature-the outside temperature above which a building needs no heating.
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