Instructions
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.
