Winkler Index
The Winkler index is a system for identifying the suitability of an area for grape growing and winemaking by the amount of heat received from the sun during the growing season. The heat units are summed up as average "degree-days," then used in comparison with degree day summations from the known wine regions of the world. Five major regions are used:

Region I receives up to 2,500 degree days of heat each year.
Region II receives from 2,501 to 3,000 degree days per year.
Region III receives from 3,001 to 3,500 degree days per year.
Region IV receives from 3,501 to 4,000 degree days per year.
Region V receives above 4,000 degree days of heat per year.

Most of the world's best table wines come from regions I, II and III. However, any given variety usually is found to prefer only one narrow range of heat summation for optimum wine quality. For example, the best sparkling wines are grown in the coolest of climates (region I), Cabernet Sauvignon produces its best wines when the grapes are grown in Region II vineyards but high alcohol dessert wines are best in the warmer climates of Regions III and IV. Table grapes are often best in Regions IV and V. Each local vineyard or wine organization can tell you their area's average heat summation. (Wine/Other)