Growing Degree Days in My Back Yard, 2000
This document gives Growing Degree Day (GDD) calculations for my back yard in East Norriton, Pennsylvania, a northwestern suburb of Philadelphia. Temperatures have been collected at 15-minute intervals, using an Onset Computer Corporation datalogger with a built-in temperature sensor. This datalogger is installed in a standard louvered, white-painted wooden thermometer enclosure mounted on a fence about 1-1/2 m from the ground. Each 15-minute interval yields a growing degree incremental value of (T-50)/96 when T is greater than 50 and 0 otherwise. With a time interval this small, the calculated GDDs are essentially "exact" values, as they do not depend (as is often the case) on modeling the daily temperature cycle based just on daily minimum and maximum values.
GDDs based on hourly temperatures from Philadelphia International Airport (PHL) have been included for comparison. Each one-hour interval yields a growing degree incremental value of (T-50)/24 when T is greater than 50. The temperatures have been obtained from the weather data published daily in the Philadelphia Inquirer. In most cases, the hourly values are not complete and missing values at the end of the day have been linearly interpolated to the measurement at 1am the following day. There is no evidence that this linear model causes a significant error in calculating GDDs.
Note that, by the end of the 2000 growing season (the end of October), the accumulated GDD value from PHL is higher by almost 300 than the value in my back yard. The difference can be attributed mostly to the fact that the air temperature at PHL, which is in a heavily urbanized area, typically does not cool off as rapidly at night as it does in my suburban location.
| Month | Back Yard (East Norriton) | Philadelphia Airport |
| March | 98 | 97 |
| April | 233 | 236 |
| May | 644 | 681 |
| June | 1268 | 1371 |
| July | 1930 | 2070 |
| August | 2878 | 2809 |
| September | 3016 | 3302 |
| October | 3223 | 3505 |