Fact Sheet - Heating Degree Days
Glen Conner
State Climatologist Emeritus for Kentucky
Heating Degree Day Definition
Heating degree days are an index of "cold" useful in energy consumption calculations. The number of heating degree days is calculated for each day by subtracting the day's mean temperature from a base temperature (usually, 65F). The daily totals are accumulated for each month and the monthly totals are accumulated for the "heating year" from July through June. The amount of energy consumed for heating is closely correlated to these heating degree days.
Mean Annual Heating Degree Days in Kentucky
The mean heating degree days for the heating year vary from 3751 in the Gilbertsville area of the extreme southwest to over 5200 in the Covington to Ashland area in the northeast. The difference is significant because Gilbertsville would need two additional Decembers each year to equal Covington's annual total. Yet, Covington is relatively mild compared to cities in other states. For example, Covington would need an additional January each year to equal Moline, Illinois or Wabash, Indiana
Mean Annual Heating Degrer Days
Expected Monthly Accumulation
The monthy heating degree days, expressed as a percentage of the winter's total, anticipate heating costs. January accounts for about 25 to 27 percent of the season's total, December and February each contribute about 20 to 23 percent, and November and March each provide about 14 to 17 percent of the winter total. In each case, the amounts are lower in the western and higher in the northeastern areas.
Monitoring Heating Degree Days
The graph below depicts the accumulation of heating degree days at Danville during a heating year. The plotted curve represents the cumulative percentage of heating degree days as they would accumulate during a climatologically average heating year. The graph is representative of much of Kentucky. To prepare the graph for use, multiply the percentage indicated on the left Y Axis by the average energy used for heating (kilowatts, cubic feet, etc.). Label the right Y Axis opposite the percentage. To use the graph, plot the actual accumulated energy consumption above the data date. Plots that appear above the curve indicate consumption excessive to the average. Read vertically to the curve to determine the amount of the excess. From the plot, read horizontally to the curve to determine the number of days at zero consumption that would be required to equalize actual and average consumption. It will indicate the severity of the excess usage. The graph offers a basis for decisions to implement conservation measures.
Annual Graph

Owenby, J.R. and D.S. Ezell, 1992: Climatography of the United States No. 81: Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days, Kentucky, 1961-1990. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC.

Owenby, J.R., D.S. Ezell, and Richard R. Heim Jr., 1992: Climatography of the United States No. 81, Supplement No.2: Annual Degree Days to Selected Bases Derived from the 1961-90 Normals. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC.

Thom,H.C.S. 1954: The Rational Relationship Between Heating Degree Days and Temperature. Monthly Weather Review, Vol 82, pp 1-6.