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  1. Generally, a measure of the departure of the mean daily temperature from a given standard: one degree-day for each degree (°C or °F) of departure above (or below) the standard during one day.

    Degree-days are accumulated over a "season" at any point during which the total can be used as an index of past temperature effect upon some quantity, such as plant growth, fuel consumption, power output, etc. This concept was first used in connection with plant growth, which showed a relationship to cumulative temperature above a standard of 5°C (41°F). Recently, degree-days have been more frequently applied to fuel and power consumption, for example, heating degree- day, cooling degree-day. In the life sciences, the standard is often referred to as the base temperature or upper threshold, depending on whether the standard is used as a lower or upper limit, respectively. As used by the U.S. Army Corps of Engineers, Fahrenheit degree-days are computed as departures above and below 32°F, positive if above and negative if below. To avoid confusion, it might be well to call this a "freezing degree-day." The advantages and disadvantages of the latter concept are discussed well by Sakari Tuhkanen (1980).
    Compare degree-hour.

  2. Common contraction for heating degree-day.

    Tuhkanen, S. 1980. Climatic parameters and indices in plant geography. Acta phytogeographica Suecica. 67. Svenska Vaxgeografiska Sallekapet, Uppsala, . 105 pp.

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