Ice melts, leaves emerge and tulips bloom--the winter world comes back to life as the earth warms. Scientists have discovered that you can actually measure the amount of heat it takes to make these things happen. This accumulated heat is measured in units called "Growing Degree Days". This lesson will help your students understand this concept and see the important role temperature plays in setting the pace of spring's journey.
Classroom Activity
1. Ask students to think about the steps in baking a cake. The oven must be set at a certain temperature--and the cake must remain in the oven for a certain length of time--in order for it to bake properly. A cake placed in a cold oven won't bake no matter how long it remains there. Ask students how the requirements for baking a cake might be similar to the needs of growing plants. See if they can develop the analogy to explain that it takes a certain amount of heat, accumulated over time, in order for plants to grow.
Also, just as a cake requires a certain temperature, there's a minimum temperature at which plants begin to grow. This is called the "BASE TEMPERATURE". The base temperature we will use for leaves is 40 degrees.
2. Show students how to calculate Growing Degree Days using the example below. In order to find out how much heat leaves need to emerge and grow, students will add up the heat the tree receives every day--as long as it is warmer than 40 degrees, the base temperature.
Here's an Example:
A) Find the Average Daily Temperature
Use your max/min thermometer to measure air temperature each day. Calculate the daily average according to this equation.
High Temp. + Low Temp. / 2 = Average Daily Temperature
B) Calculate the Heat Accumulated Each Day
Each day, use this equation to calcuate the amount of heat your trees have received. These heat units are called Growing Degree Days.
Average Air Temperature- Base Temperature = Growing Degree Days (GDD)
C) Keep a Running Total of Heat Accumulated During the Season.
This chart shows the running total calcluated over a 5 day period.
Day High/Low Average GDD Equation Growing Degree Days
Day1 45/33 39 39-40 = -1 0
Day 2 50/34 42 42-40 = 2 2
Day 3 50/36 43 43-40 = 3 5
Day 4 52/38 45 45-40 = 5 10
Day 5 54/40 47 47-40 = 7 17
Comments:
* In this example, the high and low temperatures are taken each day and averaged. This gives us the Average Daily Temperature.
* Next, the Base Temperature is subtracted from the Average Daily Temperature. This gives us the GDD value for that day.
* Notice that the GDD value for Day 1 is zero. This is because the average daily temperature (39 degrees) is below the base temperature (40 degrees). Thus, it was not warm enough on Day 1 for any growth to occur.
* Growing Degree Days are cumulative. Day 1 has zero. We add Day1 and Day 2 together to get 2. We continue to add the amount of heat the plant receives each day. After 5 days it has received 17 GDD. Remind students that GDD does not equate to actual days of growth, but is an index of the amount of heat plants have received during the season.
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