National Weather Service United States Department of Commerce

The NWS method of determining heating and cooling degree days is based off a mean temperature of the day by taking the average of the daily maximum and minimum temperatures, or (Tmax + Tmin)/2. Other statistical methods have been shown to be more accurate, utilizing either hourly degree days or the average temperature of the day. As a demonstration of the potential value of using such methods, the plots below compare differences in daily degree days between the current NWS method and a scheme using the average temperature of the day, Tavg (where Tavg = (T1 +T2....+T24)/24). While differences on any given day are relatively small, prior research has shown that correctly forecasting these differences may contribute to substantial revenue savings to regional utility companies over longer time scales. The maps below show observed degree days using the current NWS method (Tnws), calculated degree days using the daily averaging scheme (Tavg), and resultant differences between each (Tdif). For a full description, see here


NWS Method (Tnws)

Daily Averaging Method (Tavg)

Difference (Tdif)


Maps shown are 2.5km resolution and incorporate 24 hour observational data from quality controlled data sources. Slight differences of up to 1 degree day may be noted between the Tdif plots and the actual difference values represented in the two methods. Differences are due to several contributing factors, including numerical rounding and background mapping techniques. These factors also contribute to the apparent "herringbone" pattern in the Tdif maps. Heating (Cooling) degree days are depicted as negative (positive) values in Tnws and Tavg maps. Tdif values are always positive, as they express the absolute value in the difference between Tnws and Tavg.


Archived data is available since January 22, 2012 upon request.