Degree days are computed from each day's mean temperature (max+min/2). Each degree that a day's mean temperature is below or above a reference temperature is counted as one degree-day. The amount of fuel necessary for home or industrial heating is indicated by the mean temperature for that day. Estimates are that most people use their furnace when the mean daily temperature drops below 65F.
Heating degree-days are determined by subtracting the mean temperature for the day from the reference temperature. Thus, if the mean temperature for a day is 50F and the reference temperature is 65F, there would be 15(65-50) heating degree-days on this day. On days when the mean temperature is above the reference temperature, there are no heating degree-days. Therefore, the lower the average daily temperature, the more heating degree-days and the greater the consumption of fuel.
Cooling degree-days are 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. Higher values indicate warm weather and result in a high power production for cooling. Knowledge of the number of cooling degree-days in an area in the summer gives power companies a way of predicting the energy demand during peak energy periods. A summary of heating and cooling degree-days can give a practical indication of the energy needed over the year.
Growing degree-days are used as a guide to planting and for determining the approximate dates when a crop will be ready for harvesting. A growing degree-day is defined as a day on which the mean daily temperature is one degree above the base temperature-minimum temperature required for growth of a particular crop. For sweet corn, the base temperature is 50F and, for peas, the base temperature is 40F.
The mean temperature on a summer day in Iowa might be 80F. If the base temperature for beans was 50F, then the beans would accumulate 30 growing degree-days. Theoretically, beans can be harvested when it accumulates a total of 1200 growing degree-days. So, if beans are planted in early April and each day thereafter averages about 30 growing degree-days, the beans would be ready for harvest about 40 days later, or around the middle of May. Although moisture, variation of temperature and other factors are not taken into account, growing degree-days serve as a useful guide in forecasting approximate dates of crop maturity.
C. Donald Ahrens, 1985. Meteorology today. West Publishing Company, 104-105