What are Degree Days (Heating, Cooling, Growing)?

Degree days come in several flavors:

  • Heating Degree Days
  • Cooling Degree Days
  • Growing Degree Days
  • Modified Growing Degree Days
  • Heating Degree Days


    Heating degree days (HDD) are used to estimate the amount of energy required for residential space heating during the cool season. To calculate the HDDs you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or above 65F, then the HDD amount is zero. If the mean temperature is below 65F, then the HDD amount equals 65 minus the mean temperature. For example, if the mean temperature was 55F then the HDD amount equals 10.

    In equation form:

    HDD = Tbase - Ta    if Ta is less than Tbase
    HDD = 0             if Ta is greater or equal to Tbase
    
    Where:   Tbase = temperature base, usually 65F
             Ta = average temperature, Ta = (Tmax + Tmin) / 2
    

    Cooling Degree Days


    Cooling degree days (CDD) are used to estimate the amount of air conditioning usage during the warm season. To calculate CDDs, you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or below 65F, then the CDD value is zero. If the mean temperature is above 65F, then the CDD amount equals the mean temperature minus 65. For example, if the mean temperature was 75F then the CDD amount equals 10. You can think of cooling degree days as the flip side to heating degree days.

    In equation form:

    CDD = Ta - Tbase    if Ta is greater than Tbase
    CDD = 0             if Ta is less than or equal to Tbase
    
    Where:   Tbase = temperature base, usually 65F
             Ta = average temperature, Ta = (Tmax + Tmin) / 2
    

    Growing Degree Days


    Growing degree days (GDD) are used to estimate the growth and development of plants and insects during the growing season. The basic concept is that development will only occur if the temperature exceeds some minimum developmental threshold, or base temperature (Tbase). The base temperatures are determined experimentally and are different for each organism (Tables 1 and 2).


    Table 1. Reported base temperatures for GDD computations for different crops.

    BASE TEMPERATURE                        CROP
         40 F           wheat, barley, rye, oats, flaxseed, lettuce, aparagus
         45 F                           sunflower, potato
         50 F             sweet corn, corn, sorghum, rice, soybeans, tomato
    


    Table 2. Reported base temperatures for GDD computations for different insects.

    BASE TEMPERATURE                 INSECT
         44 F                    Corn Rootworm
         48 F                    Alfalfa Weevil
         50 F             Black cutworm, European Corn Borer
         52 F                   Green Cloverworm
    



    To calculate GDDs, you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or below Tbase, then the growing degree day value is zero. If the mean temperature is above Tbase, then the growing degree day amount equals the mean temperature minus Tbase. For example, if the mean temperature was 75F then the growing degree day amount equals 10, using a Tbase = 65F. You can think of growing degree days as similar to cooling degree days, only the base temperature can be something besides 65F.

    In equation form:

    GDD = Ta - Tbase    if Ta is greater than Tbase
    GDD = 0             if Ta is less than or equal to Tbase
    
    Where:   Tbase = temperature base
             Ta = average temperature
    
    

    Modified Growing Degree Days


    Modified growing degree days are similar to growing degree days with several temperature adjustments. If the high temperature is above 86F, it is reset to 86F. If the low is below 50F, it is reset to 50F. Once the high and/or low temperature has been modified if needed, the average for the day is computed and compared with the base temperature (usually 50F). Modified growing degree days are typically used to monitor the development of corn, the assumption being that development is limited once the temperature exceeds 86F.