How to Measure Growing Degree Days

1. Measure the Daily High and Low Air Temperatures
You can find the daily high and low readings in your newspaper. Make sure you use the day's actual tempeartures, not the temperatures predicted in the forecast. The actual temperatures for a day are published in the next day's newspaper. If you have a weather station at your school, you can measure the temperatures yourself with a maximum/minimum thermometer.

2. Find the Average Daily Temperature
To calculate the average daily temperature, first add the high and low temperatures. Then divide by two. Here is the equation:

(High Temp. + Low Temp. )/2 = Average Daily Temperature

3. Calculate the Amount of Heat Accumulated Each Day
Each day, use the Growing Degree Day equation to calcuate the amount of heat your plants have received. Subtract the base temperature from the average daily temperature. These daily units of heat are called "Growing Degree Days".

Average Air Temp. - Base Temp. = Growing Degree Days

4. Keep a Running Total of Heat Accumulated During the Season.

See Sample Chart for practice and details:

  • Notice that the Growing Degree Day value for 2/3/04 is zero. This is because the average daily temperature (39 degrees) is below the base temperature (40 degrees). (39 - 40 = -1, so no heat is added.) That is, it was not warm enough on 2/3/04 for any growth to occur.
  • Growing Degree Days are cumulative. This means you add them up from day to day. In the sample chart: 2/3/04 has zero. 2/4/04 has 2. We add Day 1 and Day 2 together to get 2 GDD. We continue to add the amount of heat the plants receive each day. After 5 days they have received 17 Growing Degrees Days.
  • The sample chart is completed through 2/7/04. Have students fill in the last 2 days, for practice.