This Matter of Climate

A general knowledge of grape growing is valuable to those who would like to understand wine. Considering the great number of grape varieties and the many types of wine, it is fortunate for the stu­dent of wines that it is not possible to grow every variety of grapes and produce every type of wine ef­ficiently in every area. Certain guiding principles govern the choice. Weather is one of the most important factors.

Much research has been done in “climatology” (that branch of science which deals with climate and climatic conditions) as it ap­plies to grape growing. An impor­tant concept to grasp here is some­thing called “degree days”.

Grapes do not ripen below the temperature of 50 degrees Fahren­heit. So, if on a given day the temperature averages 65 F., that day is considered to have a 15 de­gree “credit” (65 – 50 = 15) to­wards the ripening process. If in a particular district the average tem­perature is 65 F. in June, the dis­trict earns 450 “degree days” for the month (15 x 30 = 450). Tal­lying up all “credits” throughout the four month growing season re­sults in a “heat summation” (rang­ing from 1,700 to 5,200) for each locale evaluated.

There are five climatal zones which support vitculture:

Region I…62 – 65 F.average
1,700 – 2,500

Region II…65-68 F.average
2,501 – 3000

Region III…68 -70 F.average
3,001 – 3,500

Region IV…70 -74 F.average
3,501 – 4,000

Region         V…74 -80 F.average
4,001 – 5,200

Southern Napa Valley and Santa Rosa as well as Europe’s Rhine and Champagne districts typify cool Region I. Middle Napa Valley, Sonoma Valley and Bor­deaux are classified Region II. Northern Napa Valley and the Ital­ian Piedmont are Region III. Da­vis, Lodi, Sicily and Central Spain are examples of Region IV. Fres­no, Bakersfield, Algeria, and hot areas in Australia are rated Region V.

In Region I, due to low aver­age temperatures and short growing seasons, only early ripening varieties can mature. In these areas obtaining sufficient sugar in the grapes can be a problem. Acidity in the juice tends to be high. Char­donnay and Pinot Noir do best in cool Regions I and II. Sauvignon Blanc and Cabernet Sauvignon fare better in Region II or III. At the other end of the spectrum, in warm Region V vineyards, grapes have a high sugar content with low acidity. These regions are ideal for dessert wine grapes.

Microclimates are also a factor. These are variations within a cli­mate region that have their own weather characteristics. Establishing vineyards with maximum sun exposure or in valleys protected from the wind would be examples of microclimate conditions.