DEGREE DAY DESCRIPTION

## Heating Degree Days

Heating Degree Days (HDD) is an index used to estimate the amount of energy required for heating during the cool season. When the daily mean temperature falls below 65°F, most buildings require heat to maintain a comfortable interior temperature. By monitoring heating fuel usage and heating degree day accumulation over a period of time, a building's energy consumption per HDD can be calculated and this value used, for example, in fuel consumption monitoring, energy efficiency evaluation or future fuel supply estimates. The daily mean temperature is found by adding together the high and low temperature for the day and dividing by two. When the mean temperature is above 65°F, the HDD total is zero. If the mean temperature is below 65°F, the HDD amount is the difference between 65°F and the mean temperature. For example, if the high temperature for the day was 68°F and the low 52°F, the mean temperature for the day would be 68 + 52 = 120 / 2 = 60°F. The Heating Degree Day total would then be 65 - 60 = 5 HDD.

In equation form:

HDD = TBASE – TMEAN, if TMEAN is less than TBASE
HDD = 0, if TMEAN is greater than TBASE

Where:
TBASE  =   Heating Degree Day base temperature, usually 65
TMEAN  =   mean temperature, (TMAX + TMIN)/2

## Cooling Degree Days

Cooling Degree Days (CDD) is an index used to estimate the amount of energy required for cooling during the warm season. When the mean temperature rises above 65°F, many buildings use air conditioning to maintain a comfortable indoor temperature. By monitoring air conditioner energy usage and cooling degree day accumulation over a period of time, a building's energy consumption per CDD can be calculated and this value used, for example, in energy use monitoring, energy efficiency evaluation or future energy usage estimates. The daily mean temperature is found by adding together the high and low temperature for the day and dividing by two. When the mean temperature is below 65°F, the CDD total is zero. If the mean temperature is above 65°F, the CDD amount is the difference between the mean temperature and 65. For example, if the high temperature for the day was 92°F and the low was 68°F, the mean temperature for the day would be 92 + 68 = 160 / 2 = 80°F. The Cooling Degree Days would then be 80 - 65 = 15 CDD.

In equation form:

CDD = TMEAN – TBASE, if TMEAN is greater than TBASE
CDD = 0, if TMEAN is less than TBASE

Where:
TBASE  =   Cooling Degree Day base temperature, usually 65
TMEAN  =   mean temperature, (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 development threshold, or base temperature (TBASE). Base temperatures are determined experimentally and are different for each organism.

The tables below show Tbase for many crops, sorted by crop name and by base temperature.

By Crop Name

Crop Base Temperature
(Tbase) (°F)

Asparagus

40

Barley

40

Bean, snap

50

Beet

40

Broccoli

40

Carrot

38

Collards

40

Corn

50

Cucumber

55

Eggplant

60

Flaxseed

40

Lettuce

40

Muskmelon

50

Oats

40

Onion

35

Okra

60

Pea

40

Pepper

50

Potato

40, 45

Rice

50

Rye

40

Sorghum

50

Soybeans

50

Squash

45

Strawberry

39

Sunflower

45

Sweet corn

50

Sweet potato

60

Tomato

50

Watermelon

55

Wheat

40

By Base Temp

Base Temperature
(Tbase) (°F)
Crops

35

Onion

38

Carrot

39

Strawberry

40

Asparagus, Barley, Beet, Broccoli, Collards, Flaxseed, Lettuce, Oats, Pea, Potato, Rye, Wheat

45

Potato, Squash, Sunflower

50

Bean (snap), Corn, Muskmelon, Pepper, Rice, Sorghum, Soybeans, Sweet Corn, Tomato

55

Cucumber, Watermelon

60

Eggplant, Okra, Sweet Potato

To calculate GDDs, you must first find the mean temperature for the day. The mean temperature is found by adding together the high and low temperature for the day 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 75°F, then the GDD amount equals 25 for a TBASE of 50°F. You can think of Growing Degree Days as similar to Cooling Degree Days, only the base temperature can be something besides 65°F.

In equation form:

GDD = TMEAN – TBASE, if TMEAN is greater than TBASE
GDD = 0, if TMEAN is less than TBASE

Where:
TBASE  =   Growing Degree Day base temperature
TMEAN  =   mean temperature, (TMAX + TMIN)/2

Source: Oregon State Small Farms

## Modified Growing Degree Days

Modified Growing Degree Days are similar to Growing Degree Days with several temperature adjustments. If the high temperature is above 86°F, it is reset to 86°F. If the low is below 50°F, it is reset to 50°F. Once the high / low temperatures have been modified (if needed), the mean temperature for the day is computed and compared with a base temperature, which is usually 50°F. Modified Growing Degree Days are typically used to monitor the development of corn, the assumption being that development is limited once the temperature exceeds 86°F or falls below 50°F. For example, if the high for the day was 92°F and the low 68°F,  the mean temperature used in the modified GDD calculation would be 86 + 68 = 154 / 2 = 77°F.  The modified GDD would then be 77 – 50 = 27.

## Stress Degree Days

All plants have an optimum range of temperatures for growth (Hartfield, et al 2001).  While it may be obvious that temperatures too cold will inhibit growth (if not damage the plant), temperatures that are too high can cause stress for the plant.  Stress Degree Days (SDD) are a way of tracking how much stress a type of plant has been subjected to within its growing season.

There are various ways in which to calculate SDD.  However, a very simplified approach is to calculate the difference between the daily maximum temperature and the peak optimum temperature for that plant.  For example, non-irrigated corn’s optimum maximum temperature is 86°F (Water Stress in Corn).  If the daily maximum temperature on a particular day is 92°F, then SDD for that day are 92 – 86 = 6 SDD.

In equation form:

SDD = THIGH – TBASE, if THIGH is greater than TBASE
SDD = 0, if THIGH is less than TBASE

Where:
THIGH  =  daily maximum temperature
TBASE  =  Stress Degree Day base temperature, 86°F for corn

## Handling of Missing Data

When the maximum temperature or minimum temperature is not available for a day, degree days cannot be calculated for that day.  On the graphs, this is indicated by a spike in the yellow line. In the data it is shown by an “M” in the temperature and degree day columns.  When this occurs, the seasonal accumulation is not increased.  In the data, once a missing value occurs in the season, the accumulation is marked with a "*" for the rest of the season to denote that it includes days with missing data.

## References

Water Stress in Corn:  Estimating from Stress Degree Days (SDD), Iowa State University, http://www.extension.iastate.edu/CropNews/2012/0703taylor.htm, Accessed February 2013

Hartfield, J. L., K. J. Boote, B. A. Kimball, L. H. Ziska, R. C. Izarralde, D. Ort, A. M. Thomson, and D. Wolfe, 2011:  Climate Impacts on Agriculture:  Implications for Crop Productions, Agronomy Journal, 103 (2), 351-370.