Why Sonoma Valley? The big picture...
There were some things I got right in school, considering what I ended up doing for a living. Among the classes that are assets was meteorology. Making wine in Sonoma Valley (or anywhere for that matter) is dependent on grape quality. Grape quality is dependent on the weather and consistent grape quality is dependent on climate. Climate differs from weather in that it is weather over years and centuries. Climate is the big picture. Climate is site specific. Sonoma Valley is a unique spot on earth ideally situated to grow wine grapes. What is it about the location of Sonoma Valley on Planet Earth that provides this unique climate?
To begin at the beginning, Earth is a sphere orbiting the Sun, which warms the side that is facing it. To complicate matters, thankfully, the earth rotates. So instead of simply having a warm and a cold side, warm and cold air is constantly mixing in a complex dance that we do not completely understand. To complicate matters further, the earth is tilted, which creates the seasons. The earth is warmed most by sunlight shining directly down. If you have a finite number of rays of sunshine, it makes sense that they will heat more when concentrated on a smaller area such as the equator, and less when they are spread over a wide area as they are at the poles. Because of this tilt, or axis of 23.5 degrees, the Sun can shine straight down to 23.5 degrees north and 23.5 degrees south of the equator. On or near June 21 is that day for the northern hemisphere. It is the summer solstice and first day of summer.
So letís look at a typical June 21 in Sonoma Valley. It is warm and dry; why? At the equator the Sun heats the air and it expands, which causes it to rise. As it rises it cools and because cool air can carry less moisture, it rains heavily in the tropics (between the Tropic of Cancer to the north and the Tropic of Capricorn to the south). This rising dry air cannot simply fall back to earth because more warm air is following behind, so it must travel north and south. Eventually it finds a place to fall and that place is here and Italy and a few other wonderful grape growing areas. Warm dry summers lead to less mold and mildew, helping grape quality.
As winter comes, the direct sunlight moves south along with the falling dry air and Sonoma is subject to a completely different climate. Now we (hopefully) receive our 30 or so inches of rain. If the Earth were a cylinder, the rising air at the Equator would simply move north and south. With a sphere, however, there is less and less real estate as it moves north and south through the latitudes. (A person standing at the North Pole is just spinning around.) To look at it another way, the surface of the earth is spinning fastest at the equator because there is more ground to cover. Looking at the earth with the North Pole at the top, the earth is spinning to the right, or west to east. This rising air is spinning with the high velocity of the earth beneath it at the equator. But as it moves north and south it gains speed relative to the earth and makes an eastern turn; the earth is slowing down beneath it.
At our latitude in the winter this stream of air is moving quickly west to east directly over California. It is moving so fast that it is known as the Jet Stream.
But what makes it wet? Think of this stream of air like you would a stream of water coming from a garden hose. If you flick the hose the stream of water becomes wider as it travels through the air. At this elevation (30,000 feet or so) it is moving too fast to fill the void itself. It is now an area of low pressure and the void is filled from below. As the air rises it drops in temperature and is wrung of its moisture (sound familiar?) in the form of rain storms coming off the Pacific Ocean.
This is my take on the gigantic forces that influence the climate of Sonoma Valley. My hope is to focus on the details in my next column.
Mark Stupich is Cellar Master & Winemaker, Kenwood Vineyards