Growing Degree Days

Here are the Growing Degree Days (GDDs) for SeaTac airport for the last 7 years. GDDs are an easy way to calculate how much heat has occured during a growing season. Starting April 1st and going to October 31st you take the high and low of each day and add them together, then divide by 2 and then minus 50 and anything above zero is recorded. Then we take all the GDDs for each day of the growing season and add them together.

Example:

70 + 50 = 120 / 2 = 60 - 50 = 10 GDDs

It's an easy way to find out how your site compares to others and also a good way to track how the current year is progressing compared to past years.

Here are some numbers for Seatac airport near Seattle:

Year	Degree Days
57 year average	2050
1998	2205
1999	1866
2000	1955
2001	1720
2002	1822
2003	2192
2004	2220

The blue line is the year to year GDDs.
The Black line is a regressive analysis trendline projected out 20 years to the year 2024.
The red line is the 57 year average of 2050 GDDs.

Comparison of Puget Sound Growing Regions

Comparison of Cool Climate growing Regions

*From "The Production of Grapes and Wine in Cool Climates", by Jackson and Schuster, except the U.S. districts.