The Pacific Northwest Cooperative
AgriMet Growing Degree Days AlgorithmAgriMet uses an agronomically accepted algorithm for computing growing degree days that provides a means of evaluating the progress of the growing season. The formula is essentially:
(Max Air Temp + Min Air Temp) / 2 - Base Temperature (50 F)
where Max Air Temp is capped at 86 degrees F and Min Air Temp is capped at 50 degrees F
Temperatures above 86 degrees are not always beneficial to plant growth (and can be stressful) while temperatures below 50 degrees do not "detract" from any heat energy available to the plant. Let's take an example where the Max Air Temp is 90 for the day and the Min Air Temp is 45. Since these values are outside of the upper and lower limits, they are reset to 86 and 50.
GDD = (86+50)/2 -50 = 18
TIP: For a quick summation of growing degree days by month, go to Archive Data Access: Water Year Report Format and select "TG" as the Parameter.
For more information about Growing Degree Days, visit the Oregon State University online weather and Degree Day Calculator at http://pnwpest.org/wea/