A degree day is a measure of

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

ing or cooling. Total degree days from an appropriate starting date are used to plan the planting of

crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropon ...

s and management of pests and

pest control Pest control is the regulation or management of a species defined as a pest; any animal, plant or fungus that impacts adversely on human activities or environment. The human response depends on the importance of the damage done and will range ...

timing. Weekly or monthly degree-day figures may also be used within an

energy monitoring and targeting Energy monitoring and targeting (M&T) is an energy efficiency technique based on the standard management axiom stating that “you cannot manage what you cannot measure”. M&T techniques provide energy managers with feedback on operating practic ...

scheme to monitor the heating and cooling costs of

climate control Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...

led buildings, while annual figures can be used for estimating future costs. A degree day is computed as the

integral In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along wit ...

of a function of

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...

that generally varies with

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

. 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 The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

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

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. Degree days are a useful metric for estimating energy consumption required for household heating and cooling, and in this context are formally referred to as

heating degree day Heating degree day (HDD) is a measurement designed to quantify the demand for energy needed to heat a building. HDD is derived from measurements of outside air temperature. The heating requirements for a given building at a specific location are ...

s. Since the escape or ingress of heat due to conduction is proportional to the difference between the indoor and outdoor temperature, the amount of energy needed to maintain the base temperature indoors for some period of time is roughly proportional to the number of degree days. For example, if the base temperature is and the outdoor temperature is constant at for one day, this counts as 8 degree days (14 degree days in Fahrenheit). Note that the base temperature used for these calculations is lower than a typical indoor temperature setting, since a building will naturally be slightly warmer than the surrounding air due to body heat of its occupants and absorption of solar radiation.

Canada

Growing degree days

Growing degree days are based on 5 °C, given that typical plant growth stops below that temperature. Plant growth has been observed to be correlated to the number of degree days. Many plants will fruit after a certain number of degree days. Common examples are strawberries and raspberries. In contrast, deterministic plants will fruit based on the time of year, which the plant determines from the number of consecutive hours of dark. Examples of these include Easter lilies or Christmas cactus. The paper referenced shows the complexity of plant growth and degree days.

Black Fly

In Canada, Growing Degree Days are used in order to predict when

mosquitoes Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small Diptera, flies within the family Culicidae (from the Latin ''culex'' meaning "gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish ...

and blackflies emerge and vanish. But in order to chart the actual growth of insects, degree days above freezing are used.

United States

In the United States, the mean () daily temperature in Fahrenheit and a temperature of is 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.

Standardization

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 kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...

, the SI base unit). Expressed as a proper SI unit, a quantity of kelvin second is four

orders of magnitude An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...

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).

References

External links

{{Commons category, Degree days maps
Free degree day downloads for 77 UK weather stations and various building base temperatures

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