Used during warm weather to estimate the energy needed to cool indoor air to a comfortable temperature. Mean daily temperature is converted to cooling degree-days by subtracting the reference temperature from the mean. For example, a day with a mean temperature of 80F and a reference temperature of 65F would correspond to (80-65), or 15 cooling degree-days. http://www.ems.psu.edu/PA_Climatologist/index2.html
A measure of how hot a location was over a period of time, relative to a base temperature. In this report, the base temperature is 65 degrees Fahrenheit, and the period of time is 1 year. The number of cooling degree-days for a single day is the difference between that day's average temperature and the base temperature, if the daily average is greater than the base; it is zero if the daily average temperature is less than or equal to the base temperature. The number of cooling degree-days for a longer period of time is the sum of the daily cooling degree-days for the days in that period. Annual cooling degree-days averaged over 30 years from 1961 to 1990 are called Normal Cooling Degree-Days. Average daily temperature is the mean of the maximum and minimum temperatures for a 24-hour period. Cooling degree-days can also be calculated using a base temperature other than 65 degrees. The computation is performed in an analogous manner. (See Heating Degree-Days [HDD] and Climate Zone.)