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 heating degree-day

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.

map of contiguous United States indicating by colors the annual heating degree days

Mean Annual Heating Degree Days

National Atlas of the United States

The cooling degree-day

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.

map of contiguous United States indicating by colors the annual cooling degree days

Mean Annual Cooling Degree Days

National Atlas of the United States

bar chart showing annual HDD and CDD for each year between 1895 to 2017 in the United States

Data: NOAA.

The corn growing degree-day

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. If you live in Ohio, this explanation includes a calculator giving the what's happening, based on GDDs, for each zipcode

A PDF file relating GDD to annual developments, such as bloom, of various species of plants and pests.

Obtaining degree-day data for the United States

Go to

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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