Skip to main content Skip to Search Box

Definition: degree–day from Merriam-Webster's Collegiate(R) Dictionary

(1832) : a unit that represents one degree of difference from a given point (as 65°) in the mean daily outdoor temperature and that is used esp. to measure heat requirements


Summary Article: degree-day
from The Columbia Encyclopedia

a unit of measure used to estimate the fuel and power requirements in heating and cooling a building; it is equal to a difference of 1 degree between the outdoor daily average temperature (the mean of the maximum and minimum daily dry-bulb temperatures) and a reference temperature. Degree-days are an indicator of how far the average temperature departs from a human comfort level called the base. In the United States the base is generally 65 degrees Fahrenheit (18 degrees Celsius), although in very warm or cold locations an alternative may be used, while in Great Britain the base is 15.5 degrees Celsius (60 degrees Fahrenheit).

Each degree of outside average temperature below the base is one heating degree-day (HDD), and each degree above the base is one cooling degree-day (CDD). To calculate the number of heating degree-days in a month, for example, the outdoor average temperature for each day is subtracted from the base, and the results for each day are added (with negative remainders being treated as 0).

Heating degree-days are a measure of the severity and duration of cold weather; the colder the weather over a given period the higher the cumulative heating degree-day value. Similarly, the warmer the weather over a given period, the higher the cumulative cooling degree-day value. The ability to compare one week, month, or other period with another using degree-days permits the analysis of seasonal patterns of energy consumption, enables the setting and tracking fuel and power budgets, and can be used to verify that projected economies are achieved by energy-saving measures.

The growing degree-day (GDD), an extension of the degree-day concept, is defined as a day on which the mean daily temperature is one degree above the minimum temperature required for the growth of a particular crop. The GDD is used as a guide to planting times and for determining the approximate dates when a crop will be ready for harvesting.

The Columbia Encyclopedia, © Columbia University Press 2017

Related Articles


Full text Article Degree-day
Dictionary of Environmental Science and Technology

The difference, expressed in degrees, between the mean temperature of a given day and a reference temperature used as a predictor for fuel...

Full text Article TEMPERATURE
The AMA Handbook of Business Writing

When writing about temperatures, use figures for temperatures except zero. Example: The high temperature today was 55. Example: It looks like the te

Full text Article Celsius degree
The Penguin Dictionary of Mathematics

Symbol: °C. A division of a temperature scale in which the melting point of ice is taken as 0 degrees and the boiling point of water is taken as...

See more from Credo