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What is a Degree Day?

Vineyard in St Helena

Vineyard in St HelenaWhat is a degree day?

A degree day can mean a few different things, but for wine here’s the break down:

Degree days take into account only days that fall into the average vines growing season, April 1st through October 31st.  While we can certainly nitpick those days, especially since bud break is happening closer and closer to the beginning of March each year, the idea is that we should measure temperature between bud break and harvest.

Calculating degree days is pretty easy from there.  Take the high temperature for the day, subtract 50 and you have your degree days for that specific day on the calendar. Add up all the days during the growing season and you have an imperfect, but good, estimate for what grapes might grow well in your region.

There are of course, plenty of other factors that help decide if a grape is going to do well in a region, but here’s a guide based solely on degree days:

Region I: Below 2,500 degree days; Chardonnay, Pinot Noir, Gewürztraminer, Riesling

Region II: 2,500-3,000 degree days; Cabernet Sauvignon, Merlot,
Sauvignon Blanc

Region III: 3,000-3,500 degree days; Zinfandel, Barbera, Gamay

Region IV: 3,500-4,000 degree days; Malvasia, Thompson Seedless

Region V: Over 4,000 degree days; Thompson Seedless, other table grapes

I live in a small town in San Francisco’s east bay.  We’re about a mile from the bay itself.  Fog is a constant companion during at least two seasons and my kids are sweating profusely if it hits 75 degrees.  Oh and given I grew up in San Diego, I’m still positively amazed that restaurants don’t have air conditioning. But, it isn’t cold during the winter so my friends who moved from the east coast always remind me, we pretty much get fall twice and spring twice, winter and summer never show up in large part because of the huge body of water next to us that helps to control temperatures.

Month Avg Tempertature Degree Day per Day # of Days Total Degree Days
April 55 5 30 150
May 58 8 31 248
June 60 10 30 300
July 61 11 31 341
August 61 11 31 341
September 62 12 30 360
October 60 10 31 310

In any case, over the upcoming months I’ll be publishing a degree day chart for the 2015 vintage and comparing both regions that we serve as part of our monthly wine clubs, but also comparing what people say about the vintage and it’s relative heat, to what the numbers tell us.

So if you’re wondering what is a degree day? Quite simply it is meant to be a quick estimate of the heat available for a vine to grow during its normal growing season.


1 thought on “What is a Degree Day?

  1. […] grape.  The grape has a thinner skin than Cabernet or Merlot and generally need significantly more degree days to ripen fully, making it a bad fit in the cooler climates of its French […]

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