Growing Degree Days (Daily Mean Method)

Growing degree days measure how much heat a day adds toward the development of a plant or an insect. The day's mean temperature is taken as the mean of the daily maximum and minimum, and the amount by which it is above the base temperature, below which the organism does not grow, is the day's contribution. The degree days of a season are the sum of the daily values.

In $\mathrm{GDD}=\max(0,\,(T_{max}+T_{min})/2-T_{base})$ the base is set by the organism: 5 degrees for many cool-season crops and trees, 10 for corn and many others. The $\max(0,\ldots)$ means a cool day adds nothing instead of a negative amount, so it never cancels heat counted on earlier days. Because the maximum and minimum enter only through their mean, a hot afternoon can make up for a cold night. The result is a count of degrees for one day, not a temperature, so the page shows it as a plain number. The page also solves for the base or for either extreme, given the others.

The published examples show how it behaves. A day with a high of 23 C and a low of 12 C over a base of 10 C contributes 7.5 degree days. A day with a high of 13 C and a low of 5 C over a base of 10 C has a mean under the base, so it contributes 0 by the variant of the method the page uses (variant A); variant B, which first raises a minimum below the base to the base, would give 1.5. A warmer case, 25 C and 10 C over a base of 5 C, gives 12.5, and running the first case backwards from 7.5 degree days returns the base of 10 C.

This page is for one day's contribution and for the daily mean method. The sine-wave and horizontal-cutoff methods, which treat a day's temperature cycle more closely, are not in it. For plants and insects that stop developing in heat the maximum temperature is often capped, at 30 degrees Celsius, which the page does not do for you. The reference given for the equation is McMaster and Wilhelm (1997).