Russian River, SONOMA COAST & Carneros
Comparing the climatic influences affecting the Russian River, Petaluma Gap and Carneros growing conditions. Weather analyses are based on regional data obtained from the California Irrigation Management Information Systems (CIMIS) Stations and only characterize general growing conditions for the region.
The Petaluma Gap is an average of 2.5 degrees cooler during the growing season than both the Russian River and Carneros AVAs. The monthly average maximum temperature in the Petaluma Gap is 4.6 degrees cooler than the Russian River AVA and 2.7 degrees cooler than the Carneros AVA. The difference in monthly average minimum temperatures in all three AVAs are negligible.
Another interesting temperature metric is Diurnal Temperature Variation. Diurnal temperature variation has the effect of producing high sugar content while retaining high levels of natural acidity due to the high sunlight exposure during the day and the sudden drop in temperature in the evening, respectively. The Russian River AVA experiences the greatest diurnal temperature swings (33.1˚F) followed by the Carneros AVA (30.2˚F) and then by the Petaluma Gap region (28.0˚F).
What does this all mean? If we were taking only temperature into account, we could expect to find the wines from the Russian River to be riper, bigger and more powerful than either of the wines made from Carneros or the Petaluma Gap. Wines from Carneros would be fruitful but less direct in their approach than Russian River wines with potentially lower acidity. Finally, wines from the decidedly cooler Petaluma Gap would be subtler, more nuanced and restrained than either the Russian River or Carneros and would tend to be higher in acidity than either of its warmer neighbors.
The development of sugar in grapes comes from carbohydrates stored in the vines roots and trunks and from the sunlight dependent process of photosynthesis (Remember 5th grade biology?). Besides specific vineyard practices designed to regulate sun exposure (row orientation, training, pruning, etc), overcast conditions through clouds or fog reduce the amount of sunlight reaching the vines.
If you are familiar with the ‘clock-work’ fog conditions often mentioned when describing the Petaluma Gap region, you might expect to find diminished sunlight in the area compared to either the Russian River or Carneros regions. However, what we find when we look at the data is that the Petaluma Gap, in total, is ever so slightly sunnier than the surrounding areas.
How can this be? One, fog in the north coast is not just confined to the Petaluma Gap. The Russian River and Carneros AVAs also experience their own degrees of foggy conditions.
Two, although not analyzed here, the rate that temperature changes above (or below) the dew point regulates how rapidly the fog dissipates. Without constructing temperature histograms of each region (assignment for the reader for extra credit), we can only speculate that perhaps the temperatures in the Petaluma Gap concentrate on the dew point more often but spend less overall time around the dew point (+/-4˚F), resulting in more days of fog (generating the perception of a foggier area) but for a smaller duration during the day.
Lastly, the break in the coastal range allowing the build up of heat in the inland valley to create the fog in the Petaluma Gap by drawing cool, moist air from the Pacific Ocean also creates windy conditions. The wind that is responsible for bringing the fog in is also responsible for clearing it away.
Besides its effects on fog and its importance to combat fungal pressure, wind can also effect grape berry composition. Significant wind conditions can result in smaller berries with thicker skins (read: thicker skin == more color, tannins, aroma and flavor).
Another defining trait of the Petaluma Gap is its consistent wind conditions which remain steady between 4.5-3.8 mph. The Petaluma Gap is bracketed by the slightly calmer and windier regions of the Russian River (-0.8mph) and Carneros (+0.4mph) respectively.
I debated including humidity in this post as it does not alone directly effect wine style, but it is important to include when considering how levels may effect vineyard decisions (spraying, leafing, ...) when combined with the previously detailed climatic conditions.
As we see here, the average humidity in the Petaluma Gap and Carneros regions (73.2%) have considerably higher humidity levels than the Russian River (68.6%). We also saw that these areas are considerably windier than Russian River potentially abating the increased fungal pressure associated with higher humidity.