Weather, Degree Days & the Quality of Wine

People have been asking me how this year’s wet weather will affect the quality of the 2017 vintage wine. We must have had at least 70 in. of rain, more than double the average yearly rainfall.  The rains kept coming down all the way into the month of June.  While at times, some parts of the vineyards were flooded, I do not see that this will have a lasting effect on the quality of the wine this year.  Erosion has been minimal even on some of the steeper slopes mainly because of the well-maintained cover crop.  Bottom line, the excessive rains had no significant effect on the quality of the crop, so far.  Nevertheless, it is not time to rejoice. We must keep in mind that this is only the beginning of the growing season and a lot can happen between now and harvest time.

In the following paragraphs I will try to give you an insight how the weather, during the growing season, may affect the quality of the wine.  A.J. Winkler, a professor at Davis determined empirically that it takes so many degree-days during the growing season to get the grapes of a specific variety to an adequate level of ripeness to make good quality wine. This is called Growing Degree Days (GDD).  He assumed that the grapes would not ripen at a temperature below 50 degrees.  Therefore, he calculated the GDD by subtracting 50 from the average temperature for the day.  The average temperature for the day is the highest temperature plus the lowest temperature divided by 2.  Thus, the GDD for the day that has a high of 95 degrees and a low of 60 degrees (typical weather in the Sierra Foothills in August)  will be (95+60)/2 – 50 = 37.5. If this weather persisted for 31 days during the month of August, the cumulative GDD for August would be 1,162.5.  Winkler determined the “growing season” to be between April 1 to September 30.  He developed the Winkler Index, which is based on the Growing Degree Days summed up over the “growing season” in different regions.  Based on the average GDD he classified five different growing regions and identified different grape varieties that can be fit to each of these regions. I will only show the data for three of these regions since they are the ones that would apply to the regions in California and the varieties that we grow.

  • Region I: Below 2,500 degree days;

Varietals:  Chardonnay, Pinot Noir; Region: Burgundy

  • Region II: 2,500-3,000 degree days;

Varietals:  Cabernet Sauvignon, Merlot, Sauvignon Blanc; Regions: Bordeaux, Napa

  • Region III: 3,000-3,500 degree days;

Varietals:  Zinfandel, Barbera, Tempranillo, Grenache, Syrah; Regions:  Sierra Foothills,    Piedmonte, Barossa Valley

Unfortunately, the Winkler Index is not very accurate or reliable since it does not bring into consideration all the weather factors that play a role in the ripening of grapes.  It does not take into account the effect of the temperature variations between day and night on the ripening.   Hypothetically, a region that would have an identical day and night temperature, say of 77.5 degrees would have the same GDD as one that has a day maximum of 95 and a night minimum of 60 or one that would have a day temperature of 85 and a night of 70. All three would have the same Index and would be classified as the same region. In reality, they would be worlds apart in the way grapes would grow and ripen.

Conclusion – Take it from me!  Don’t use GDD’s for decision-making!  Use them for information only!

Cheers!

Chaim

July 2017