obtaining degree-day data
Two units, the heating degree-day and the cooling degree-day, used by utility companies and heating, ventilating and air conditioning personnel to express the demand for heating or cooling created by the weather over a given period of time. When the term “degree-day” is not qualified, the heating degree-day is meant.
Degree-day data are often weighted by population or by the area experiencing that climate. Such weighting is sensible for utilities because it can predict the total energy demand for a region. For the individual homeowner or farmer, however, this data is not as valuable as unweighted degree-day statistics.
The concept originated with the observation that demand for natural gas for heating does not pick up until the average daily temperature falls below 65°F. Instead of the average daily temperature, in practice the highest and lowest outside temperatures during a 24-hour day are averaged. The result, subtracted from 65, is the number of heating degree-days for that day. The degree-days for longer periods are found by adding the degree-days for the individual days.
In Great Britain, where interior temperatures are not kept as high as they are in North America, the heating degree-day is based on 60°F instead of 65°F.
The cooling degree-day is less used and less firmly defined. Abbr., CDD. In National Oceanic and Atmospheric Administration statistics, the number of cooling degree-days for a day is the average of that day's high and low temperatures minus 65.
The corn growing degree-day (GDD), introduced by NOAA in 1969, is an index predicting the maturity of the corn (maize) corp. To calculate it, the day's maximum and minimum temperatures are first adjusted on the principle that the corn mainly grows between 50 and 86 degrees Fahrenheit. So any minimum temperatures below 50 are converted to 50, and maximum temperatures above 86 are changed to 86. The day's adjusted maximum and minimum temperatures are averaged, and 50 is subtracted. Each variety of corn requires a particular number of corn growing degree-days to reach maturity. For example, in Kansas, Dekalb DKC58-19RR2 requires 2735 GDD.
Growing degree-days are also calculated for other crops, using different baseline temperatures. A good example of such growing degree-days in action can be seen at
Select your state in the pull-down menu. Choose whether you wish a pdf or ascii file. The download is quite large but contains a wealth of climate data.
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Last revised: 5 June 2007.