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Understanding Degree Days

Heating degree days were developed as a way to relate each day's temperature to the demand for fuel to heat buildings.
 
Heating degree days are calculated by adding the high and low temperatures of a given day and then dividing by two (to obtain the average temperature).  Each degree the average daily temperature falls below 65 degrees represents a heating degree day. 
 
Take a look at the following example:

High temperature: 38
Low temperature: 24
  1. Add the day's high and low temperatures: 38 + 24 = 62
  2. Divide by two: 62 / 2 = 31
    The average temperature for the day is 31 degrees.
  3. Subtract the average temperature from 65: 65 - 31 = 34
There are 34 degree days for this day.


Since your individual gas use and your bill can be affected by weather, your Nicor Gas bill shows "total degree days" for both the current billing period and the same period last year so you can compare them.
 
View the most recent two months' degree day information.
 
The National Weather Service has detailed degree day information for various regions of the country, including the Chicago area.