Campbell Scientific VisualWeather Software - Manual (Page 67)

Appendix B. Growing Degree Days (GDD)
B.1 Growing Degree Days
This section explains the concept of growing degree-days (GDD) and methods
used in VisualWeather to calculate its hourly values.
Growing degree-days (GDD) is a measure of temperature condition that is
favorable to plant growth. A range of temperatures is defined by entering
lower and upper temperature limits. Temperatures lying within these limits are
assumed to be conducive for growth of a given plant. The lower and upper
limits of temperatures favorable to plant growth vary from plant-to-plant and
are provided by the user. Generally, temperatures above 50°F (10°C) are
considered favorable for plant growth.
B.1.1 Method
1. An hourly average temperature value, Ta, is compared to the upper and
lower temperature limit.
2. If Ta > upper temperature limit, then Ta is set equal to upper temperature
limit, and
If Ta < lower temperature limit, then Ta is set equal to lower temperature
limit.
3. If upper temperature limit >Ta > lower temperature limit, then
GDD = Ta- lower temperature limit is calculated for every hour.
4. The 24 hourly GDD values are summed.
The GDD value calculated per day is added to the previous day's GDD value.
This process is continued and a cumulative GDD value is reported. When the
cumulative GDD value reaches a threshold for a given plant value, as judged
by a user, then it is assumed that it has grown to an extent where it must be
trimmed.
B-1