Winkler Index

Definition - What does Winkler Index mean?

The Winkler Index also referred to as the “heat summation method” uses temperature to organize the different climates of vineyard regions. This index helps differentiate which variety of grape would grow best in each region and guides the viticulturist to practice methods based on the region’s classification. The Regions are divided into five groups with Region I representing cooler temperatures and Region V describing the hottest climates.

WineFrog explains Winkler Index

Developed in the 1940’s by A.J. Winkler and Maynard Amerine from the University of California, the Winkler Index has been used to divide grape growing regions into temperature classifications. The determining factor is the number of “degree days” in which the average daytime temperature of the region goes above the limit of 50 degrees Fahrenheit, and one “day” equals one degree over 50. The degree days are only measured between April 1st through October 31 to include only days in which grapevines grow and mature.

The index was originally used in California and spread to other parts of the world to form a classification system that determines which grapes and types of wine are produced in different climates.

  • Region I - The coldest region, it is comparable to the Côte d’Or/Champagne regions of France or Oregon’s Willamette Valley where the degree days are less than 2,500, wines produced there include Pinot Noir, Chardonnay, Champagne and Riesling.
  • Region II - is defined with degree days between 2,501-3,000 degrees and reflect climates like Bordeaux which produce Cabernet Sauvignon, Merlot, Cabernet Franc and Sauvignon Blanc.
  • Region III - This middle index is responsible for Sauvignon Blanc, Zinfandel or Syrah wines and is comparable to the French region of Rhône with 3,001-3,500 degree days.
  • Region IV - represents the range of degree days between 3,501-4,000 and comparable to Spanish or southern Californian climates which create Port and Barbera wines.
  • Region V - The warmest region, Region V is comparable to Morocco or North American climates like the San Joaquin Valley where the degree days are higher than 4,000 and produce Muscat or Verdelho wines.