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Learn More about Climate in the Napa Valley

Growing Degree Days
Min, Max, Avg Temperature

Climate: Past and Present Trends for the Napa Valley

Historical climate data can give insight on current and future trends for phenological events such as bud break, flowering and veraison. In addition, through historical climate trends we can predict times of targeted pest development, emergence, and applications for sprays.

Here we present weather station data from St. Helena and Oakville, CA which have been collecting data such as temperature and precipitation since 1951 and 1989, respectively.

Vineyard in Napa County Video: S. Vengco

Growing Degree Days (GDD)

Growing Degree Days (GDD) is a tool used by viticulturists to predict the growth of vines and insects over the growing season. GDD is a unit of heat measurement that uses temperature to gauge how much a plant or insect will grow in a given day. Phenological events can be predicted based on the accumulated GDD over the growing season.


It's assumed that when the GDD is zero (or daily temperature is less than the base temperature) growth does not occur. To find the GDD of the entire season add daily GDD counts together. 

Screen Shot 2020-10-02 at 10.04.02

The base temperature needed for grapes grow is 50ºF. Various vineyard pests have different base temperatures needed to grow. 

Click the arrows to view more graphs

St. Helena Average GDD 1951-2019
St. Helena GDD Deviation 1951-2019
Growing degree days St. Helena, California UCCE Napa County
St. Helena Minimum, Maximum, & Average Temperature

Click left or right to view more graphs

St. Helena Average Temperature 1951-2019
St. Helena Average Temperature Deviation 1951-2019
St. Helena, California Average Temperature UCCE Napa County
St. Helena Precipitation

St. Helena precipitation data was taken from 1951-2019. On the left in purple is the average monthly precipitation (in) from 1951-2019. On the right are the monthly deviations from the average precipitation.

St. Helena Average Precipitation 1951-2019
St. Helena Average Precipitation Deviation 1951-2019
Average precipitation St. Helena, California UCCE Napa County
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