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 Degree Days

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  • Degree days give a measure of the effect of the seasons' temperatures on crop growth and fuel requirements, especially in the case of a building which is continually heated. For each day that the average temperature is one degree above the base temperature, one degree day has accumulated.
  • Probably the most widespread application of the degree day concept is the management of industrial and domestic heating. Heating degree days are indicators of household energy consumption for space heating. The air temperature in a building is on average 2° C to 3° C higher than that of the air outside. A temperature of 18° C indoors corresponds to an outside temperature of about 15.5° C. If the air temperature outside is below 15.5° C , then heating is required to maintain a temperature of about 18° C. 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.
  • Because of the relationship between temperature and the rate at which plants grow and develop, the concept of accumulated degree days has been found useful in quantifying the accumulated energy of a region or season. Such heat units have widespread use in agriculture, particularly in the vegetable processing industry to predict the date of harvest and the timing of successive harvests.
  • Monthly and daily Degree Day values are available from our Synoptic and Climate stations. The base temperature for monthly values is 15.5° C.

Base temperatures for daily values are 15.0, 15.5, 17.0, 18.0 and 18.5 ° C and include:
  • Daily maximum and minimum temperatures
  • Daily degree days above and below the base temperatures
  • Monthly degree day values and the mean maximum, mean maximum, and mean air temperatures
The method used in calculating Degree Days is as follows, 4 cases are considered:

Case I
The minimum air temperature (Tmin) above the base temperature (Tbase). In this case there are no degree-days below the base temperature and the degree-days above are given by 1/2(Tmax + Tmin) - Tbase.

Case II
The maximum air temperature (Tmax) below the base temperature. In this case there are no degree-days above the base temperature and the degree-days below are given by Tbase - 1/2(Tmax + Tmin).

Case III
The maximum air temperature above and the daily minimum air temperature below the base temperature but the mean temperature which equals 1/2(Tmax + Tmin), greater than the base temperature. In this case degree-days above are given by 1/2(Tmax - Tbase) - 1/4(Tbase - Tmin) and degree-days below by 1/4(Tbase - Tmin).

Case IV
The maximum air temperature above and the minimum air temperature below the base temperature but the mean temperature which equals 1/2(Tmax + Tmin), less than the base temperature. In this case degree-days above are given by 1/4(Tmax - Tbase) and degree-days below are given by 1/2(Tbase - Tmin) - 1/4(Tmax - Tbase). If the mean temperature is equal to the base temperature the same results obtain using either the formula for Case 111 or Case IV
 

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