Cooling degree days
Cooling Degree Days (CDDs) are measures of temperature deviations above 65° Fahrenheit. A CDD value is a measure of how warm a day is in terms of how many degrees that day’s average temperature is above 65° Fahrenheit. The energy industry uses the Cooling Degree Day index to gauge how much energy customers are likely to use for cooling based on how warm it is in terms of CDDs for given months or a season.  For example, an average daily temperature of 80° would generate a CDD value of 15 (65 + 15 = 80). If the temperature were below 65°, the value of the CDD would be zero. A monthly or seasonal CDD value is simply the sum of each daily CDD value recorded during a given month or season. For example, if there were seven CDD daily values recorded in July in Chicago, the 2008 July CDD index would be the sum of the seven daily values. Thus, if the CDD values were 25, 4, 2, 5, 18, 12 and 33 the July CDD index value would be 99 .