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The Grape-Growing Climate of Maryland
by Kenneth E. Pickering - State Climatologist - University of Maryland


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 50F) 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.

Climatological Indices for Grape-Growing at Locations in MD
STATION MEAN DAILY
TEMP IN JULY
Max     -     Min
RECORD LOW DAYS OVER 90 FFP* GDD** UCD*** MTWM****
  Abedeen 87 66 -12 19 200 3,640 IV Very Hot
  Annapolis 88 68 -8 32 204 3,700 IV Very Hot
  Baltimore City 89 72 -7 33 231 3,640 IV Very Hot
  BWI Airport 87 67 -7 31 200 3,640 IV Very Hot
  Beltsville 87 64 -15 28 176 3,625 IV Very Hot
  Chestertown 87 67 -7 30 209 3,630 IV Very Hot
  College Park 88 67 -9 40 191 3,620 IV Very Hot
  Conowingo Dam 86 64 -10 24 199 3,500 III Very Hot
  Cumberland 88 63 -14 33 178 2,800 II Very Hot
  Denton 88 65 -11 37 187 3,650 IV Very Hot
  Emmitsburg 86 62 -27 21 162 3,250 III Very Hot
  Glenn Dale 88 63 -11 37 166 3,675 IV Very Hot
  Hagerstown 86 64 -17 25 187 3,200 III Very Hot
  La Plata 86 66 -8 24 188 3,700 IV Very Hot
  Laurel 87 67 -12 33 206 3,610 IV Very Hot
  Mechanicsville 87 65 -9 24 199 3,740 IV Very Hot
  Oakland 79 56 -27 2 128 2,400 I Warm
  Owings Ferry 87 66 -8 26 200 3,700 IV Very Hot
  Parkton 84 62 -12 12 170 3,470 III Hot
  Rockville 86 64 -13 25 190 3,590 IV Very Hot
  Royal Oak 87 68 -6 26 215 3,700 IV Very Hot
  Salisbury 86 67 -8 25 196 3,690 IV Very Hot
  Unionville 85 61 -22 22 152 3,330 III Hot
  Woodstock 87 64 -18 27 172 3,500 III Very Hot
STATION Max     -     Min
MEAN DAILY
TEMP IN JULY
RECORD LOW DAYS OVER 90 FFP* GDD** UCD*** MTWM****
* Average Frost-Free Period     ** Medium Growing Degree Days April-October (50 degree F base)
*** Grape region classification number based on UC Davis classification system (Winkler et al, 1974)
**** Mean Temperature of the Warmest Month (July) classification system of grape growing regions (Smart and Dry, 1980)

Winter extreme minimum temperatures were compiled for the same set of stations as used in the GDD analyses. Large variations in the frequency distributions of the minima are seen between stations. At Royal Oak (Talbot Co.) only four winters out of the 1961-1990 period had below zero temperatures, while at Oakland (Garrett Co.) only one winter out of this 30-year period did not have a zero or below temperature. Additional variables presented include mean maximum and minimum temperatures for July, the record low temperature, the number of days over 90 degrees F, and the length of frost-free period for each available station.


GRAPHIC PRESENTATION DONE BY DAVID SAVORY - GLOBAL GIS CONSULTING
Savory@GISconsulting.com

Future analyses could possibly include more detailed mapping of growing degree-days using more stations and with consideration of topography. Temperature trends in Maryland have been upward during the 1990s, but at many stations around the state the trend is slightly downward over the period from the late 1940s through 2000. However, warming of about 0.5 to 1 degree F occurred over the entire 20th century. Future climate projections show continued warming, pushing much of the region that is now UCD Class IV to Class V by 2030. Cooler parts of the state may also increase by one UCD class.



Maryland Grape Growers Association © 2012