“Warm days, cool nights.” This descriptive phrase appears on such a variety of wine labels and brochures that one would believe that the entire state of California has these ideal grape growing conditions. Coastal climates do have warm days with cooler nights while the inland climates can be very hot during the day and stay that way well into the night. The advantage of the cooling marine influence is that grapes grown under such conditions mature slowly, develop a higher degree of acidity, a lower pH, good color, and mature varietal fruit qualities.
The coastal influence is even visual in Sonoma County. The thick blanket of coastal fog rises up and out of the Tomales Bay in the late afternoon and moves up the Russian River Valley. By early evening, the cooling fog has curled north of Santa Rosa to Windsor, fading away by the time it reaches Healdsburg. When the fog reaches Sonoma Mountain, it divides, wrapping around both the northern and southern sides of the mountain. The northern arm reaches through Santa Rosa, descending down the Valley of the Moon. The southern arm moves along Highway 116 into the Sonoma Valley. By following the fog, the viticultural areas of Sonoma County can be described as either coastal warm or coastal coosee note at end of article). This classification is based on the degree to which the fog affects the area’s average daily temperature, the photosynthesis of the vine and the rate of grape maturity.
The areas which are affected by the cooling blanket of coastal fog are considered coastal cool. The fog lowers the daily average temperature considerably, allowing grapes to mature slowly. The areas without the cooling fog, such as those north of Healdsburg, are considered coastal warm. The reasoning behind this classification is to guide winegrowers on what varieties to plant in which viticultural area. In coastal cool areas, such as the Russian River Valley, it is advisable to plant early ripening varieties such as Chardonnay, Pinot Noir and Gewürztraminer, for these varieties will reach sill maturity before the threat of late autumn rains and cooler weather conditions. Cabernet Sauvignon, a late ripening grape, may be a bit risky in a cool area, and would be better suited to a coastal warm area such as the Alexander Valley.
While from a climatic standpoint, Sonoma County can be divided into coastal warm and coastal cool, these classifications are only generalizations. There are a numerous exceptions and variations depending on microclimate, soil conditions, and individual vineyard management.
The terms coastal warm and coastal cool were established by Robert Sissons of the University of California Cooperative Extension in Sonoma County. The terms supplement the commonly used climatic classification system devised by Professors Amerine and Winkler (1944) of Regions I – V which uses a method of average daily temperature within each 24 hour period. 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) 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.
The reason Sissons prefers to use the terms coastal cool and coastal warm as opposed to Region I or II us that the heat summation system used by Winkler and Amerine bases the average daily temperature of these areas on the extreme high and extreme low temperature readings. This does not fully take into account the amount of time a vine is actually exposed to a certain temperature. The terms coastal cool and coastal warm incorporate a method of heat summation which takes into account not only the highs and lows but the number of hours at which temperatures remain in the highly effective photosynthesis range of 70 degrees to 90 degrees F.
Courtesy of Sonoma County Wineries Association