Judgement of Paris: Napa Wine Climate

What was the judgement of Paris?

The “Judgement of Paris” was a 1976 wine tasting organized by Steven Spurrier that pitted Napa Valley wine against the creme de la creme of French wine. As documented in a Time magazine article about the wine-tasting event, Napa Valley was not known at the time as the wine-producing powerhouse it is today. In fact, virtually everyone who participated assumed the French wines would win the tasting contest. If you were not previously aware of the Judgement of Paris, you’re probably guessing what happened. You guessed it! Napa Valley produced wines won the competition.

The cabernet sauvignon tasting was won by Stag Leap Wine Cellars. The chardonnay tasting was won by Chateau Montelena. While the monumental importance of this result to the wine world took some time to be realized (press presence at the event was a single Time magazine reporter), this event put Napa Valley on the global wine map. Heck, there is even a movie about it.

Weather and Wine Quality

It’s generally understood by wine critics that weather conditions during the growing season affects wine quality. The first quantitative study relating weather conditions to wine quality was published by Orely Ashenfelter in 2008. The paper compared the price of Bordeaux Grand Cru (i.e., the premier wineries of Bordeaux) wine producers to the weather conditions that evolved during the growing season of each vintage using linear regression. One of the weaknesses the original analysis is that the weather data was not normalized. This means the relative importance of temperature and precipitation during different times of growing season to wine quality can’t be easily compared.

The First Quantitative Weather vs. Wine Quality Study

World Wine Weather published an updated analysis of Ashenfelter’s wine quality paper. We normalized the weather data to allow intercomparison of the variables. The updated results show the most important variable to Bordeaux wine quality is summer temperatures. A warmer-than-normal summer produces better cabernet sauvignon. The amount of rainfall during winter months is important as well. Winter precipitation recharges the soil moisture in preparation for the warm summer months. And temperature and precipitation during harvest is important. Warmer-than-normal harvest temperatures helps the grape concentrate it’s water content intensifying the flavor. Lower-than-normal precipitation has the same effet.

The Most Recent Weather vs. Wine Quality Study

An interesting peer-reviewed paper made the wine news in October of 2023. The study was published by Andew Wood, of the University of Oxford. The paper’s title is “Seasonal weather impacts wine quality in Bordeaux.” This study is similar to the Ashenfelter paper but, instead of using the price of wine as the quality indicator, it uses a published history of Bordeaux wine critic wine ratings and weather data. The weather data is the ECMWF Reanalysis Version 5 (ERA5), which is itself interesting because ERA5 is a modeled weather data in which the solution is forced to known weather observations.

The results of this paper (see this image – a summary of results) show that greater than normal winter precipitation and summer temperature are beneficial to wine quality. Likewise, it shows that below-normal temperatures and precipitation during Autumn (i.e., harvest time) are beneficial to wine quality. This autumn result is different than what the Ashenfelter paper and our rework of the study show.

The Ashenfelter and Wood studies have conflicting results. Both agree that lower than normal precipitiation during Autumn improve wine quality. However, Ashenfelter suggests warmer-than-normal temperatures during Autumn improve wine quality while Wood’s suggests the opposite. Our rework of the Ashenfelter paper agrees that warmer temperatures are better and suggests that above normal precipitation before harvest improves wine quality. In our study, however, the coefficient is quite small. Because the data is normalized, the magnitude of the coefficient indicates its relative importance. In other words, the importance is relatively small, which is supported by Woods figure 2B graphic as well.

A Napa Valley Cabernet Sauvignon Quality Index

Using the research information about how weather during the year impacts wine quality, I propose two wine quality indices. These indices will be used to compare the weather that occurred in Napa Valley from 1965 to 2023. The index combines each of the wine season weather parameter into a single number (the index) allowing years to be inter compared.

Wines of the Judgement of Paris

The wines tasted and tested in the Judgement of Paris are shown below. I take no credit for this table. It’s a screenshot directly from Wilkipedia. As can be seen, the Napa Valley cabernet Sauvignon spanned years from 1969 to 1973

Wines of the Judgement of Paris

Napa Wine Climate During the Judgement Years

The graphic below shows the normalized seasonal averaged precipitation and temperature in Napa Valley from 1965 to 1974. The years highlighted in red are the vintage years of the different Napa Valley cabernet sauvignons.

Napa Valley Climate during the vintage years of the napa valley selected years.

Values above zero mean the temperature or precipitation was above normal (normal is defined as 1965 to 2020 in this case). Values below zero mean the temperature and precipitation were below normal.

The Napa Valley Wine Quality Index

The Napa Valley wind quality index is evaluated in two ways. The first is an equal weighted index.

Index = 0.25 * Winter Precipitation + 0.25 * Summer Precipitation + 0.25 * Summer Temperatures + 0.25*Harvest Temperatures.

The index uses normalized data from each of the “seasons” used in the equation. In this case:
Winter = November, December, January, February, and March contiguously. The November and December are from the prior year.
Summer = July, August, September
Harvest = October, which is generally when cabernet sauvignon is harvested in Napa Valley.

The second version of the index uses the information from the two aforementioned studies. In this case, the index is:

Index = 0.2 * Winter Precipitation + 0.3 * Summer Precipitation + 0.4 * Summer Temperatures + 0.1*Harvest Temperatures.

The second version of the index gives more importance to summer time (i.e., the core of the wine grape growing season) than the other seasons.

Climate of the Judgement of Paris Napa Valley Wines

The index is calculated for each year from 1965 to 2023 using the proposed weights above. The results are shown below.

The equal-weighted index shows the 1969 to 1973 years were decent but not exceptional years when compared to the full history of data. According to this index, 1986 was the best Napa Valley wine climate year since 1965. Wine-searcher.com say the 1986 Napa Valley vintage was “spectacularly good.” This suggests the index is valid.

The seasonal weighted index shows some differences for the Judgment years but 1972 and 1973 are still high index value years. 1986 is still rated as the best year in the history. The ranking of each index and the Judgement years is shown below. The 1973 seasonally weighted index, which according to the research referenced, should be a better indication of wine quality, is ranked 15th out of the 58 years.

It’s interesting to note the winning Cabernet Sauvignon wine is the 1973 Stags Leap Cellars. How much of the perceived quality of that wine was due to the excellent growing weather conditions of 1973? How good was 1970 relative to other years in Bordeaux? That question will be answered in a separate post.

The wine quality index results suggest the outcome of the tasting may have been different if differnt Napa Valley vintages were selected.