Calculating Degree Days
Ashley Font
National Science Foundation Graduate STEM Fellow in K-12
Education
Sugar Creek Watershed K-12 Education Program
The Ohio State University-Ohio Agricultural Research and
Development Center
The Degree Day
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FIRST OF ALL!
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Degree DAY is not a literal DAY
It is just CALLED a Degree DAY
The Degree Day
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Unit of Heat
Average amount of heat in degrees
that have occurred in a single day
1 Degree Day results when the
Average daily temperature is 1
degree above the lower threshold
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EX: Average 51 degrees, threshold is
50 degrees… 51-50= 1 dd
What is a lower threshold?
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Just means the lowest temperature that
something can grow at.
Some insects cannot develop until the
temperature is 50°F
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Any lower, and the bug stops growing!
Therefore, 50°F would be that bug’s
threshold
Why do we use it?
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Monitoring these values of heat can help
us predict pest activity
Since insects are cold blooded, daily
temperatures have A LOT to do with their
development
Temperature can be more precise than
just using calendar dates.
Calculations
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Use the daily maximum
temperature and the daily minimum
temperature
Get an Average for the day
Subtract a lower threshold (we use
50F unless otherwise instructed)
The Formula
(Max. Daily Temp. + Min. Daily Temp.)
2
__
Lower
Threshold
Examples (pg 2)
#1
June 1st, 2008
MAX: 74.1°F
Calculation
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MIN: 50.2°F Lower Threshold: 50°F
(74.1 + 50.2 ) - 50 = 12.15
2
So, on June 1st, 12.15 Degree Days have
accumulated
Examples cont. (pg 2)
#2
 When the minimum daily temperature is
BELOW the lower threshold, REPLACE the
Minimum Daily Temp in the formula with the
THRESHOLD.**
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
May 28th,2008
MAX: 66.5°F MIN: 38.6°F LT: 50°F
( 66.5 + 50 )
2
-
50 = 8.25
Examples cont. (pg 2)
#3
** There are not NEGATIVE degree days. If
the formula results in a negative, it is
ZERO degree days… meaning no heat
accumulated on this day
January 1st
50°F
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Max:41.6 Min:19.0
LT:
(41.6 + 50) - 50 = -4.2
2
So, ZERO DD.
No development occurred this day.
Cumulative Degree Day
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Just adding up each day’s DD units to
get the total for multiple days
Usually start adding at January 1st.
This can help us predict what day of
the year something will occur by
following how much heat is
accumulated each day.
Assignments
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Do in class
In pairs
Use Calculators