Growing degree days (GDD) measure accumulated heat during the growing season, starting April 1. GDD are calculated daily as the average temperature minus 50°F (the base temperature below which vines don't accumulate heat). The Winkler scale classifies wine regions by season total: Region I (<2,500 GDD) produces cool-climate varieties like Pinot Noir; Region II (2,500–3,000 GDD) suits Merlot and Chardonnay.