Average Annual Number of Growing Degree Days from Historic Observations

Average Annual Number of Growing Degree Days from Historic Observations

This tool is excerpted from Chesapeake Bay Watershed Climate Impacts Summary and Outlook for Summer 2021.

Tool Background

Growing Degree Days (GDDs) measure the number of degrees that accumulate over a given year, as a metric of the heat available for various needs (i.e. crops, insects) and are an important metric for agricultural management.1 GDDs help estimate when different plants and insects will arrive at various life stages, which is useful for determining when to plant crops and when to protect plants from various pests.2 Generally, a larger number of GDDs indicates warmer growing conditions.3 Research has shown that GDDs are increasing across the continental United States, which may reduce crop yields across the United States, particularly for corn, soybean, sorghum and wheat.4

GDDs can be calculated in a number of ways, but for this analysis we use the following equation:

Growing Degree Days Gained = Average Daily Temperature – Baseline Temperature

with a baseline temperature of 50 degrees F. This number is calculated for each day of the year and each positive number is added together to get the total annual GDDs. If GDDs are negative for a given day, which occurs when the average daily temperature is below 50 degrees F, it is not included in the annual total. To calculate the GDDs for a specific crop, a different baseline temperature is usually selected. For example, a baseline temperature of 40 degrees F is typically used for wheat, barley or rye, while 50 degrees F is used for corn, sorghum or rice.5

The number of GDDs varies with geography, with more southern or lower elevation areas typically having more GDDs in a given year and northern or higher elevation areas having fewer GDDs.

Increases in GDDs over time indicate either more days with average temperatures above 50 degrees F, or an increase in the average temperatures on days that were already above 50 degrees F, or some combination of the two. Therefore, increases in GDDs could potentially indicate an increase in the length of the growing season.

Changes in the accumulation of GDDs over the course of a year can also shift the timing of various crops and insect cycles. This could create a situation where crops reach vulnerable stages in their development at a time of year where frosts are still possible or when precipitation is less available.

Key Findings

  • Across all observed historic observations, the southeastern portion of the region (Eastern Maryland and Central and Eastern Virginia) experiences the greatest number of GDDs.
  • The average annual number of GDDs has increased from 1981 to 2020 according to historic observations.

How to Use the Tool

Selecting Time Periods
Use the Time Period slider bar to adjust the decade used to calculate the average annual growing degree days.

Viewing Variability Within a Location
Hover or tap over a point of interest. A window will pop up that displays changes in growing degree days by decade. You can also use the Geography filter to the right of the map to zoom into a location of interest.

Technical Notes

NOTE: In this analysis, growing degree days are calculated for each day using the following formula: Growing Degree Days Gained = Average Daily Temperature – Baseline Temperature. In our calculations, the baseline temperature is 50 degrees F. Positive numbers from each day of the year are added together to create a total Growing Degree Days number for the year. For the historical data, we plot the average of this number over a 10-year period, using 1981–1990 to represent the baseline period.

These maps were generated with gridded temperature estimates from the PRISM Climate Group at Oregon State University. Parameter-Elevation Regressions on Independent Slopes Model (PRISM) daily mean temperature data are available at a 4-km resolution for the coterminous United States. Data were processed by the Northeast Regional Climate Center. More information on PRISM data can be found at http://www.prism.oregonstate.edu.

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