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A degree day is a measure of heating or cooling. Total degree days from an appropriate starting date are used to plan the planting of crops and management of pests and pest control timing. Weekly or monthly degree-day figures may also be used within an energy monitoring and targeting scheme to monitor the heating and cooling costs of climate controlled buildings, while annual figures can be used for estimating future costs.
A degree day is computed as the integral of a function of time that generally varies with temperature. The function is truncated to upper and lower limits that vary by organism, or to limits that are appropriate for climate control. The function can be estimated or measured by one of the following methods, in each case by reference to a chosen base temperature:
- Frequent measurements and continuously integrating the temperature deficit or excess;
- Treating each day's temperature profile as a sine wave with amplitude equal to the day's temperature variation, measured from max and min, and totalling the daily results;
- As above, but calculating the daily difference between mean temperature and base temperature;
- As previous, but with modified formulae on days when the max and min straddle the base temperature.
A zero degree-day in energy monitoring and targeting is when either heating or cooling consumption is at a minimum, which is useful with power utility companies in predicting seasonal low points in energy demand.
Heating degree days are typical indicators of household energy consumption for space heating. The air temperature in a building is on average 2–3 °C (4–5 °F) higher than that of the air outside. A temperature of 18 °C (64 °F) indoors corresponds to an outside temperature of about 15.5 °C (60 °F). If the air temperature outside is below 15.5 °C, then heating is required to maintain a temperature of about 18 °C. If the outside temperature is 1 °C (1.8 °F) below the average temperature it is accounted as 1 degree-day. The sum of the degree days over periods such as a month or an entire heating season is used in calculating the amount of heating required for a building. Degree Days are also used to estimate air conditioning usage during the warm season.
Growing degree days
Growing degree days are based on 5°C as typical plant growth stops below that temperature. Plants growth has been observed to relate stage of growth to the number of degree days. Many plants will fruit after a certain number of degree days. Strawberries for instance. Other plants however, called deterministic, will fruit based on the time of year which the plant determines from the number of hours of sunlight in a day. Think of Easter lilies or Christmas cactus.
The paper referenced shows the complexity of plant growth and degree days. 
In the United States, a simplified method is used to calculate both heating and cooling degree days. The mean (that is, max + min/) daily temperature in Fahrenheit and a nominal temperature of 65 °F (18 °C) are used. If the mean daily temperature is 65 °F, no degree days are counted. If the mean daily temperature is below 65 °F, the mean degrees Fahrenheit below 65 °F are counted as the heating degree day. If the mean daily temperature is above 65 °F, the mean degrees Fahrenheit above 65 °F are counted as the cooling degree day. The heating and cooling degree days are tallied separately to calculate monthly, seasonal, and yearly total heating and cooling degree days. Heating and cooling degree days closely correlate with heating and cooling demand.
The degree day is not an SI derived unit; although the day is acceptable for use in the SI, it is not a decimal multiple of its base unit, the second. (The degree, in Celsius and measured relative to a base temperature, is identical to the kelvin, the SI base unit). Expressed as a proper SI unit, a quantity of kelvin second is four orders of magnitude higher than the corresponding degree day (1 Celsius degree-day is 8.64×104 K·s; 1 Fahrenheit degree-day is 4.8×104 K·s).
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