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Sara N. Schaffer
Accumulated Growing Degree Day Products
What are Growing Degrees?
In many plants and animals, phenological transitions – especially those in spring - happen when enough warmth has accumulated. This warmth is often measured using growing degree days (GDDs). Growing degrees days are defined as the number of degrees the average daily temperature exceeds a base temperature, or the temperature below which the organism will remain in dormancy. Growing Degree Days are calculated as:
GDD = ((Tmax + Tmin)/2) - Tbase
If the average temperature for a day is lower than the base temperature, then no Growing Degree Days are counted.
Growing Degrees are accumulated daily, following a specified start date, by adding each day’s total to all previous days’ totals.
What is the value of Growing Degree Day calculations?
For many plants and animals, there is a specific number of growing degree days that must be accumulated to trigger a change in phenological status such as budburst in plants or egg hatching in insects. These are referred to as growing degree thresholds. If a growing degree threshold for a phenological transition in a particular organism is known, it is possible to assess how soon that transition is likely to be reached, by calculating accumulated growing degree days (AGDDs) over the course of the season.
How does this year's growing degree day accumulation compare to what's "normal"?
On any day of this year, we can assess whether we are ahead or behind what is average for this day of year by comparing the number of growing degree days accumulated on the day of interest in this year to the number of accumulated growing degree days for the same day of year averaged over a 30-year period. The resulting map is called an “anomaly” map. Whether the number of growing degree days that has been accumulated is higher or lower than what is average for a particular day will vary by location. Learn more about AGDD anomalies.
Available Growing Degree Day Products
The USA-NPN is currently generating provisional daily Accumulated Growing Degree Day (AGDD) maps using a January 1 start date and two base temperatures, 32°F and 50°F.
Accumulated Growing Degree Day maps can be explored in the USA-NPN Visualization Tool.
Raster data files can also be accessed via the USA-NPN Geoserver instance. Available products include:
- Contemporary (daily, current year) and 6-day forecast maps of Accumulated Growing Degree Days (AGDD)
- Daily anomaly maps of AGDD, generated by comparing current day maps to 30-year (1981-2010) averages
- Daily 30-year (1981-2010) average temperature accumulations
- Daily Tmin and Tmax values for the current year
How to Cite Map Products
USA National Phenology Network. Year of dataset access. Name of data product, USA-NPN, Tucson, Arizona, USA. Data set accessed YYYY-MM-DD at http://dx.doi.org/10.5066/F7SN0723
Estimating Uncertainty in AGDD Products
The USA-NPN is committed to reporting measures of uncertainty, where possible, to support the use of these data products in decision making.
To assess the level of uncertainty in the USA-NPN AGDD products, we compare our URMA/RTMA-based calculations of Accumulated Growing Degree Days to those made using measurements from U.S. Climate Reference Network stations, accessed via the Applied Climate Information System, in the AGDD Uncertainty Assessment Dashboard.