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Limitations of the Winkler Index and GDD in Assessing Mesoclimates for Viticulture

Authors: Patrick Shabram*, Front Range Community College
Topics: Wine, Agricultural Geography, Climatology and Meteorology
Keywords: wine, viticulture, growing degree days, mesoclimates
Session Type: Paper
Presentation File: No File Uploaded


A long used index in the wine industry is the Winkler Index, a climate classification system using Growing Degree Day (GDD) heat summations to place growing areas in one of five “regions.” Developed in the mid-20th Century by M.A. Amerine and A.J. Winkler, the system was intended for regional descriptions specific to California. The system has been used to describe smaller American viticultural areas (AVAs) and even single vineyards and has been applied to districts outside California with more humid growing seasons. These classifications offer greater uncertainty than the widespread usage of the system would suggest. The methodology used to calculate these regions has evolved. Better meteorological records allow for more accurate classifications than those under which the Winkler Index was developed. Different application of the methodologies, often combined with limited long-term records, can produce inconsistencies in regional classifications. Shifts in growing season, especially with changing climatic conditions, also are not taken into account. Research from Gregory Jones, Roslyn Francine MacCracken, and others has shown that GDD has not been the best system for delineating climatic variations, or in the case of the MacCracken study, the best indicator of viticultural success. While GDD offers value at the sub-regional scale in applying climatic variations, temperature related growing conditions are not necessarily captured in these calculations. Further, applying Winkler Index regional classifications to conditions at the mesoclimate scale is not appropriate given variations in methodology and the overall intent of the original climate classification system.

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