Classic versus integral mean temperature calculations in the estimation of the Winkler index

Abstract

The use of bioclimatic indexes is a common practice to evaluate the suitability of regions for specific crops or cultivars, particularly in viticulture. Among these, the Winkler Index (also known as the Degree-Day Index) is one of the most widely used worldwide. Developed in California by Winkler and Amerine (1944) and later updated by Cook, Kliewer and Lider (1974); the index is based on the accumulation of degree days above a base temperature of 10 °C throughout the grapevine’s vegetative cycle, from April to October in the Northern Hemisphere. This cumulative value allows the classification of wine-growing regions into five climatic zones (Regions I–V), ranging from cool to very warm viticultural areas, each associated with suitable grape varieties.

Despite its widespread use, the Winkler Index is sensitive to the method used to calculate daily mean temperature (Tmean). Traditionally, Tmean value is estimated using both the maximum and minimum daily temperature, Tmax and Tmin respectively, as in the formula (Tmax + Tmin) / 2. However, modern automatic weather stations, now common in many viticultural regions, often compute an integral value for Tmean calculated from frequent interval readings (e.g., data aggregated every 30 minutes which comes from several measurements taken every few seconds), which can yield different results. This discrepancy raises questions about the consistency and comparability of Winkler Index values across regions and data sources.

To explore this issue, we analysed meteorological data from three public networks operating in two Spanish Appellations of Origin: La Rioja AOC and Rueda AO. We calculated the Winkler index using both the classical Tmean formula and the integral mean temperature provided by the networks. Results reveal systematic differences between methods and among networks, suggesting that the choice of temperature averaging procedure can significantly introduce biases in viticultural climate classification, especially in regions near the boundaries between Winkler zones. This finding has broader implications for viticultural regions worldwide, where diverse data infrastructures and averaging protocols may introduce inconsistencies in climate zoning. As precision viticulture and climate adaptation strategies become increasingly important, harmonizing temperature calculation methods is essential to ensure reliable bioclimatic assessments and informed decision-making.

References

Kolyvas, C.; Missiakoulis, S.; Gofa, F. Mean Daily Temperature Estimations and the Impact on Climatological Applications. In Proceedings of the 16th International Conference on Meteorology, Climatology and Atmospheric Physics—COMECAP 2023, Athens, Greece, 25–29 September 2023.

Winkler, A. J., & Amerine, M. A. (1944). The effect of temperature on the composition and quality of grapes and wines. American Journal of Enology and Viticulture, 1(1), 1–19.

Winkler, A. J., Cook, J. A., Kliewer, W. M., & Lider, L. A. (1974). General Viticulture (2nd ed.). University of California Press.

Acknowledgments

This work was supported by the Regional Government of La Rioja (Spain) and FEDER funding (project PR-18-25).