I came across an interesting page at the web site of the Chateau Montelena, located in Calistoga, CA.   Thier Vintage Summary page discusses the weather and climate conditions of each vintage year and provides a few notes regarding each vintage.    Clearly I had to find some weather data and apply a bit of analysis to it.

The California Department of Water Resources maintains the California Irrigation Management Information System, which includes a network of weather stations around the state.   It includes a weather station located in St Helena, which is in the Northern Napa Valley,  just south of Calistoga.    The data provided by CIMIS includes daily maximum and minimum temperaures and daily precipitation values.     There are a few details that I have glossed over currently regarding the data set, but as I continue my analysis over the coming months, I’ll address those issues.

Following the lead of the Ashenfelter et al., 1995 paper and earlier analysis of Bordeaux Wine Quality and Climate,  have constructed a simple index to predict the quality of St. Helena  and Northern Nappa Valley grown wine.

The index was created using the following steps:

1) Download CIMIS data for Daily Maximum Temperature, Daily Minimum Temperature, and Daily Precipitation

2) Calculate seasonal averages for the above values.  The seasonal averages are:

A)  The daily average temperature during the growing season between May and August.

B) The Winter season precipitation.  Winter in this case is the months between October to March, inclusive.

C)  Harvest season precipitation, which is the total accumulated precipitation during August and September

3) I then normalized each data based on averages for each of the above variables from 1952 to 2008.

4) Applied a weighting such that:

A) The daily average temperature during the growing season, has  a weight of one.   The warmer the average the temperature, the better the quality.

B) The Winter accumulated precipitation is given a weight of +0.5 .  More winter precipitation than average positively contributes to wine quality.

C) The Harvest season accumulated precipitation is given a weight of -0.5.    Above average Precipitation during harvest season is detrimental to wine quality.

A bar chart representation of the index is shown below.

A Northern Napa Valley Wine Quality Index

A Northern Napa Valley Wine Quality Index

I compare this to a few of the years from the Chateau Montelena Winergy page (look at the Blue bars,  red comes later):

Best Index Year:  According to the index, the1996 was the  best year for wine quality had an index value of 2.9.

According to Chateau Montelena,  1996 is described as:

Hot. Average rainfall. Mild winter, untimely spring rains and intense summer heat resulted in low vineyard yields. Ripening weather in September-October was excellent. Lightest crop size since 1988, very high extraction and very concentrated juice.

So,  based on the inputs my index, the siginficantly wetter than average winter, a warmer than normal growing peroid, and significantly less rain during harvest time all positively contributed to the quality.

Worst Index Year:  1989.  Index value: -2.33.

Chateau Montelena says:   Cool Variable. Record winter rains. Crop down 20% from 1982. Cool spring gave way to a heat spell in mid-July that caused damage. Early crush with a moderate sized crop. Good maturity, clean, balanced fruit.

Definitely a tough year.    Winter Precip was -2.17 standard deviations below normal.  Translation:  4.18 inches when normal precip during this time period is 26 inches.  Growing season temperatures were -0.26 standard deviations (or 0.3 degrees F cooler than normal,  not a big detrimental effect), and Harvest Precip was almost 2 standard deviation wetter than normal.
Second Worst Index Year:  1976.  Index value:  -2.09

Chateau Montelena says:

Hot. First drought year. Fruit had high sugars, low acids. Vineyards stressed. Early August – September crush.

Very low winter precipitation,   lower than average temperature,  and greater than normal harvest rain fall all contribute to the low index value.   Interestingly,  the data contradicts the “Hot” assertion indicating the growing season average temperatures were not “Hot”, but were in fact,  0.35 standard deviations below normal.

Summary:

Using the guidance of the Ashenfelter et al.  1995 paper a simple index is contstructed to indicate the quality of wine produced by Chateau Montelena based on CIMIS daily weather data.         A few things come to mind:

  1. The vintage discussion must be taken with a grain of salt.   Since the purpose of thier web page is to educate web visitors and, in the end, sell wine it is interesting to note that apparenetly they never have poor quality wine.    This objective analysis provides a framework to analyze the quaility of Chateau Montelena wine from a year to year basis.
  2. Thier 1996 description says:  ” intense summer heat resulted in low vineyard yields.”  While low yields do not necessarily indicate poor quaility,  statements like this are made several times.   An upcoming post will discuss the index with the addition of a “threshold variable” added.  Namely, the number of days each growing season in which temperature exceeds 95 degrees F.   The more days in a season, the lower the quality.
  3. The 2000s have been a generally excellent decade for wine quality.
  4. I’d like to track down some indicate of market prices for Chateau Montelena wines to allow me to conduct a direct regression of Napa Valley weather against wine quality, as measured by the market.