Heating Degree Days  

   Heating engineers who wanted a way to relate each day's
temperatures to the demand for fuel to heat and cool buildings
developed the concept of degree days.
    Cooling degree days are based on the day's average temperature minus 65. They relate the day's temperature to the energy demands of air conditioning. For example, if the day's high is 90 degrees and the day's low is 70 degrees, the day's average is 80 degrees. 80 minus 65 is 15 cooling degree days.
    To calculate the heating degree days for a particular day, find the day's average temperature by adding the day's high and low
temperatures and dividing by two. If the number is above 65, there are no heating degree days that day. If the number is less than 65, subtract it from 65 to find the number of heating degree days.
    Heating and cooling degree days can be used to relate how much more or less you might spend on heating or air conditioning if you move from one part of the country to another. Of course, you'd have to take into account how well insulated your new home will be in comparison to your old one and the different costs of electricity, gas or heating oil. You could also use records of past heating degree days to see if the money you've spent on insulation or a new heat pump is paying off. To do this, you'd also needs records of past energy use.
    Engineers know that degree days don't account for all of the weather factors affecting heating and cooling energy demand. One of the efforts to create a better index is the CNS Energy Index, which takes humidity and sunshine as well as previous days' weather into account.