Growing Degree Days in Ontario
- A Description

Table of Contents

1. Heating Degree Days
2. Cooling Degree Days
3. Growing Degree Days
4. Modified Growing Degree Days
5. Related Links

Heating Degree Days

Heating degree days, or HDD, are used primarily by oil and gas companies and utilities to estimate the amount of energy (oil, natural gas, etc.) required for home and commercial heating during the cold months. To calculate the HDD they first determine the mean temperature for the day. This is usually done by taking the maximum and minimum temperatures for the day, adding them together, and dividing by two. This mean temperature is then subtracted from a base temperature to give a daily HDD. If the daily HDD calculates to a negative number it is made equal to zero. The daily HDD is then accumulated over the heating season. Generally the larger the accumulated HDD is the more energy we use to heat our homes and businesses.

Cooling Degree Days

Cooling degree days, or CDD, are used primarily by electrical utilities to estimate the amount of heat that has accumulated and then the amount of electricity required for home and commercial cooling during the warm months. CDD are calculated in much the same manner as HDD are. First the mean temperature for the day is determined. This is usually done by taking the maximum and minimum temperatures for the day, adding them together, and dividing by two. The base temperature is then subtracted from the mean temperature to give a daily CDD. If the daily CDD calculates to a negative number it is made equal to zero. The daily CDD is then accumulated over the cooling season. Generally the larger the accumulated CDD is the more energy we use to cool our homes and businesses.

Growing Degree Days

Growing degree days, or GDD, are used to estimate the growth and development of plants and insects during the growing season. They are calculated in much the same manner as CDD but with different base temperatures. The basic assumption is that development will only occur if the air temperature exceeds some minimum developmental threshold or base temperature. The increase in development is also assumed to be close to linear in relation to an increase in temperature or heat energy accumulation. The base temperatures are determined experimentally and are different for each organism.

To calculate GDD you first determine the mean temperature for the day. This is usually done by taking the maximum and minimum temperatures for the day, adding them together, and dividing by two. The base temperature is then subtracted from the mean temperature to give a daily GDD. If the daily GDD calculates to a negative number it is made equal to zero. The daily GDD is then added up (accumulated) over the growing season. Generally the larger the accumulated GDD is the greater the plant or insect development.

The GDD equation used by OMAFRA can be written as follows:

dailyGDD = ((Tmax +Tmin)/2) - Tbase

where:
Tmax = the daily maximum air temperature
Tmin = the daily minimum air temperature
Tbase = the GDD base temperature for the organism being monitored

OMAFRA uses the above GDD calculation and accumulation to help determine the progression of the growing season and to make comparisons to previous years and long term averages (normals). There are four factors to consider when comparing GDD accumulations from various sources or regions.

1. Are the base temperatures used the same?
   • OMAFRA uses various base temperatures and produces reports for three basic ones; 3, 5 and 10 degrees Celsius.
2. Are the start dates for the accumulations the same?
   • OMAFRA uses April 1 as a start date for accumulations.
3. Are the equations used to calculate the daily GDD the same?
   • Many modifications to the simple GDD calculation have been developed over the years. Many American States use more complex ones but still call them simply GDD or Degree Days.
4. Are the temperatures used in degrees Celsius or Fahrenheit?
   • OMAFRA uses the Canadian degrees Celsius standard for temperatures while the United States uses the degrees Fahrenheit scale.

Another problem that has caused some confusion over the years is with the terminology used. Here in Ontario we use the terms Growing Degree Days (GDD) and Crop Heat Units (CHU), which represent two different, temperature dependent, development models. The Crop Heat Units calculation now used in Ontario is identical to the traditional Corn Heat Units calculation with the name change reflecting its now broader application to other crops. OMAFRA has developed a CHU Factsheet with a full description of the CHU calculation and it's applications. Growing Degree Days are some times referred to just as Degree Days or Degree Days Averaging Method. Some jurisdictions also use the term Heat Units interchangeably with Degree Days.

Modified Growing Degree Days

Modified GDD are similar to GDD but with several temperature and/or calculation adjustments. A common modified GDD calculation used in the United States to monitor corn development places an upper limit of 30°C (86°F in the States) on the maximum daily temperature. Other Modified GDD systems include:

1. Baskerville-Emin (BE) Method
   • This method uses a sine curve, fitted to the maximum and minimum temperature profile, to simulate how the daily temperature would vary. The area under the curve and above the base temperature is then calculated. This gives a better estimate of the heat accumulation, but uses a more complex calculation. This method is commonly used in Michigan, USA.
2. Hourly Average Integration Methods
   • These methods are usually implemented on automated dataloggers. They use average hourly temperatures to calculate a hourly heat accumulation. These are then summed up over the day to give a daily GDD. Some systems use 15 minute and even 5 minute average temperatures.

The University of California, Integrated Pest Management Project, has put together a good description of various modified GDD calculation methods, particularly those in use across the United States. See Degree-Days and Phenology Models for more information.

Related Links

• CHU Factsheet
• Degree-Days and Phenology Models