SE-76-02-2 -- Converting Heating Degree Days from Below 65F to Below 18C PDF

Current values of heating degree days published by the meteorological services in North America will probably be expressed as degree days below 18°C, starting sometime in 1975 and will continue for a considerable time.before tables of normal degree days below 18°C are available. To compare current values with the normal, it will be necessary to convert the published, normal degree days below 65 F to degree days below 18°C. Because 65 F and 18°C are slightly different temperatures, the degree-day conversion is not simply a matter of mUltiplying by a constant. In addition to the multiplication there is an adjustment, which in a 3D-day month may amount to 18 F degree days or 10 C degree days. For-any month in which the daily mean temperature exceeds 65 F for one or more days the conversion cannot be exact, but the method outlined in this note will give adequately accurate results.

The heating degree days for a single day are defined as the number of degrees that the mean temperature for the day is below the base temperature. If the mean temperature for the day is above the base temperature then the degree days are zero. The heating degree days for a month are the sum of the daily values. The normal heating degree days for a particular month are the average of the values for that month over some standard period, usually 30 years.