The grape-growing climate of Maryland is defined in terms of two primary climate indices, the University of California - Davis (UCD; Winkler et al, 1974) system and the Mean Temperature of the Warmest Month (MTWM; Smart and Dry, 1980) method. Maps of the distribution of Growing Degree-Days (GDD; Base 50°F) for Maryland are shown for median conditions as well as for the 10th (cool summer) and 90th (hot summer) percentile conditions. Corresponding maps of the UCD classes are also presented.
Under median conditions all of the Eastern Shore, Southern Maryland, and the Washington - Baltimore - to north end of the Chesapeake Bay region is Class IV. The northern tier of counties from Washington Co. to Cecil Co. comprise a Class III region. Points further west are in Class II, and the high terrain of Garrett Co. is Class I. However, for 10th percentile conditions, only St Mary's Co., lower Calvert Co. and portions of the lower Eastern Shore are in Class IV. The remainder of the Eastern Shore and all of Central Maryland are in Class III. Classes I and II expand eastward compared with median conditions. In an extremely hot summer (90th percentile conditions), St Mary's Co., lower Calvert Co., and portions of the lower Eastern Shore move into Class V.
At least two-thirds of the state is in Class IV and Washington Co. and part of Frederick Co. are Class III. Even Garrett Co. becomes Class II. The UCD system proved to be a better discriminator of climate differences within the state than the MTWM method. One must keep in mind that these maps were prepared using data from stations on average 25 miles apart. Therefore, these analyses represent relatively large-scale conditions. Certainly large variations occur within the UCD class regions due to topography and proximity to the Chesapeake Bay.