Sonoma Valley

The Sonoma Valley lies to the west of and parallels the Napa Valley. It is separated from the Napa Valley by the Mayacamas Mountains and from the Cotati and Petaluma Valleys by the Sonoma Mountains. The Sonoma Mountains are not as high nor as extensive as the Mayacamas. The valley is 22 miles long and narrow. The valley floor is no more than 5 miles wide at its southern end, and quickly shrinks to 2 miles and then less than a mile wide further north.

The climate is like Napa's. Even though no long term wind measurements are known to exist in the valley, available information suggests that the valley has the same basic wind characteristics as other valleys; strongest up valley winds in the afternoon during the summer, and best down valley winds during clear, calm cold winter nights. Prevailing winds would follow the longitudinal axis of the valley, northwest/southeast, while some upslope flow during the day and downslope flow during the night may be observed near the base of the mountains. At the town of Sonoma summer average maximum temperatures are in the high 80's reaching 90 degrees in July with extremes in the mid 90's. Summer minimum are 49 to 50 degrees F. Winter maximum are in the high 50's to the mid 60's with minimum ranging from the mid 30's to 40 degrees F. Winter minimum extremes range from the high 20's to the low 30's. Sunshine is plentiful and annual precipitation averages 29 inches.

As in the Napa Valley, air pollution potential is high. Prevailing winds can transport non-local and locally generated ozone precursors northward into the narrow valley which can effectively trap and concentrate the pollutants under stable conditions. The local upslope and downslope flows set up by the surrounding mountains may also recirculate pollutants adding to the total burden. Also, with light to calm winds and associated stable conditions during the late fall and winter, particulates and carbon monoxide concentrations from automobiles, agricultural burning and fireplace burning can become elevated.