ABOUT DEGREE DAYS Back to the Data Page

What is a heating degree day? The definition provided by the National Weather Service is: Early this century heating engineers developed the concept of heating degree days as a useful index of heating fuel requirements. They found when the daily mean temperature is lower than 65 degrees, most buildings require heat to maintain an inside temperature of 70 degrees. The daily mean temperature is obtained by adding together the maximum and minimum temperatures reported for the day and dividing the total by two. Each degree of mean temperature below 65 is counted as one heating degree day. Thus if the maximum temperature is 70 degrees and minimum 52 degrees, four heating degree days would be produced. (70 + 52 = 122; 122 divided by 2 = 61; 65-61 = 4.) If the daily mean temperature is 65 degrees or higher, the heating degree day total is zero. The cooling degree statistic is the summer counterpart of the heating degree day. Suppliers of heating fuels use this measure to estimate their customers' fuel requirements. Each home will use a different amount of fuel per degree day, depending on factors such as size of home, thermostat settings, amount of insulation, wind exposure, and efficiency of furnace. By monitoring a customer's pattern of usage during a short period, a supplier can estimate the home's consumption per degree day and use this index, along with the heating degree day data to estimate when they need to re-supply the home with fuel. Many Upper Valley suppliers rely on data from the National Weather Service for their calculations. The two nearest stations that provide this type of information are Rutland VT and Springfield, MA and thus their figures will be somewhat different than Montshire's data which is taken at the Museum building in Norwich, VT. The Montshire weather station employs a more refined approach to calculating the mean temperature than that described above. Rather than averaging the high and low for the day, multiple samples are taken each hour of the day and night and an average is calculated. This approach more closely reflects the true heating requirements of a given day. Monitoring heating degree day information over time, relative to your fuel consumption, can provide a method of evaluating the impact of energy conservation practices that you may choose to try.