(redirected from degree day)
Also found in: Dictionary, Thesaurus, Wikipedia.


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°F; (18°C;), although in very warm or cold locations an alternative may be used, while in Great Britain the base is 15.5°C; (60°F;).

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 Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved.


Measure of how cold or warm a location is over a period of time relative to a base temperature, typically 65 degrees F (although other base temperatures, such as 75 degrees F, can be used for cooling). To calculate the number of heating degree-days (HDDs) of a given day, average the maximum and minimum outdoor temperatures and subtract that from 65 degrees F. The annual number of HDDs is a measure of the severity of the climate and is used to determine expected fuel use for heating. Cooling degree-days (CDDs), which measure air-conditioning requirements, are calculated by subtracting the average outdoor temperature from an indoor base temperature.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved


[di′grē ‚dā]
(mechanical engineering)
A measure of the departure of the mean daily temperature from a given standard; one degree-day is recorded for each degree of departure above (or below) the standard during a single day; used to estimate energy requirements for building heating and, to a lesser extent, for cooling.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


A unit used in estimating energy requirements for building heating and, to a lesser extent, for building cooling. It is applied to all fuels, district heating, and electric heating. Origin of the degree-day was based on studies of residential gas heating systems. These studies indicated that there existed a straight-line relation between gas used and the extent to which the daily mean outside temperature fell below 65°F (18°C).

The number of degree-days to be recorded on any given day is obtained by averaging the daily maximum and minimum out-side temperatures to obtain the daily mean temperature. The daily mean so obtained is subtracted from 65°F and tabulated. Monthly and seasonal totals of degree-days obtained in this way are available from local weather bureaus.

A frequent use of degree-days for a specific building is to determine before fuel storage tanks run dry when fuel oil deliveries should be made. Number of Btu which the heating plant must furnish to a building in a given period of time is where “Btu required” is the heat supplied by the heating system to maintain the desired inside temperature. “Heat rate of building” is the hourly building heat loss divided by the difference between inside and outside design temperatures. When the estimating procedure is applied to buildings with high levels of internal heat gains, as in a well-lighted office building, then degree-day data on other than a 65°F basis are required. See Air conditioning

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.


A unit used in estimating the fuel consumption for a building; equal to the number of degrees that the mean temperature, for a 24-hour day, is below the base temperature; the base temperature is taken as 65°F (18.3°C) in the US and as 60°F (15.6°C) in Great Britain.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
References in periodicals archive ?
The discrepancy between daily accumulation of degree days and monthly accumulation of degree days is most pronounced in early-season and late-season numbers, when mean daily temperatures may be below 50[degrees] F.
Following Munoz and Sailor (1998), the present study employs the Degree Day method for incorporating temperature variations.
Keywords: Degree day factor, Snowmelt runoff modelling; Naran watershed; Time Step; Z-curve
3) You're not sure which weather station to use for degree day data or can't find the time to obtain weather data on a regular basis.
LOGAN, IA: Monthly total heating degree days, [online] [cited 05 October 2012].
The first method correlates energy use to average outdoor temperature, referred to as the temperature-based method, and the second correlates energy use to heating degree days, referred to as the degree day-based method.
Laboratory estimation of degree day developmental requirement of Plebotomus papatasi (Diptera: Psychodidae).
A degree day (also called heat unit or thermal unit) is a measure of the amount of heat that accumulates above a specified base temperature during a 24-hour period, and cumulative degree days (DD) are the total number of DD that have accumulated since a designated starting date, and they are calculated simply by adding the number of DD that accumulate each day: and if maximum temperature for the day never rises above the base temperature then no development occurs, and zero DD accumulate (Herms, 2004).
At 300 East 46th Street, ES Energy Group recorded fuel usage prior to installation of the BEMS, during the 2009-2010 heating season, was 14.03 gallons per degree day. Following installation, during the 2010-2011 heating season, it decreased to 10.93 gallons per degree day.
Because of a later planting date in 2002, the crop matured under a slower rate of degree day accumulation, which may be the driving force behind the year differences for boll maturation period (weighted mean of 49.5, 57.4, and 51.7 d in 2001, 2002, and 2003, respectively).