The Winkler Index, also known as the Winkler Scale, is a measure for classifying wine regions based on the heat summation or growing degree days over a season. The index was developed by professors Albert Winkler and Maynard Amerine at UC Davis in the 1940s after prohibition. The purpose of the Winkler Index was to encourage and guide grape growers in choosing the best-suited grape varietal for the region’s climate. Their classification of wine regions was published in the book California wine grapes composition and the quality of their musts and wines, in 1944.
How is the Winkler Index calculated?
The Winkler Index is calculated by adding the daily mean temperatures above 50°F (10°C) from April 1 to October 31. The measure is based on the principle that grapevines need a certain amount of heat to ripen their fruit. These growing degree-days (GDD) are summed and used to assign a region to one of five climate zones, Region or Category I-V, ranging from the coolest to warmest. The data for the Winkler Index is typically collected and analyzed by climate stations and agricultural organizations to determine the Winkler Index values for different regions, including American Viticultural Areas (AVAs).
How Wineries Use the Region’s Winkler Index?
The Winkler index is an important tool for winemakers to determine which grape varietals best suit their region’s climate. Different grape varietals have different heat requirements, so the Winkler index can help winemakers select the right grapes for their specific region. A winemaker in a cool region might want to use grape varietals that require less heat, while a winemaker in a warm region might want to use grape varietals that require more heat. It also helps winemakers predict the quality of their wines. For example, a higher Winkler Index creates wines with higher sugar content and lower acidity.
What are the limitations of the Winkler Index?
The Winkler Index was originally developed for California and does not always apply in other wine regions as well. The index is based on temperature, without taking into account soil type, precipitation, and latitude. For that reason, the Winkler Index is now primarily used in conjunction with other measures. There is also the variability of the index within regions. In a single climate zone, there may be significant temperature differences. Wine researchers have also criticized the index for inconsistencies in reporting and lack of variation within the levels. More complex climate indices have been created to address these issues but are not always available for the general user or the public.
What is the future of the Winkler Index?
Today’s most important use of the Winkler index is its ability to help wine regions, or AVAs, adapt to changing climate conditions. As global temperatures continue to rise, some regions that were once considered too cool for wine production may become more suitable. It’s important to remember that the Winkler Index was designed for regional purposes, not necessarily subregions or single vineyards. In 2021, UC Davis announced an update and expansion to the index to account for changing climate conditions and some limitations. At this time, the research has not been completed.
