Degree days is used as a measurement of the accumulation of heat over the summer growing season. It is a guide to grape ripening since the higher the units equates to a warmer location which facilitates the ripening of grapes. This measurement serves as a means of comparing the potential of different locations as grapegrowing regions. For example, The Upper Willamette Valley in Oregon averages about 2150 degree days. This compares with 1750 in the Rhinegau, 2400 in Burgundy, and 3100 in St. Helena, California. Different grape varietals have varying degree day requirements to attain their maximum and optimal levels of ripeness.
Technically speaking, a degree day is based on the average daily temperature within each 24 hour period where the number of degrees above 50 degrees F are totaled. The total number of degrees above 50 degrees F over a certain period of time (usually recorded between the summer months during grape maturity) when combined are called “degree days.” Based on these degree days, grape growing areas are segregated into five climatic regions. The coolest region, Region I, ranges 2,500 degree days or less while the hottest region, Region V ranges between 4,001 to 5,200+ degree days. This scale was devised by Professors Amerine and Winkler in 1944.