Napa Valley

Napa Valley

The Napa Valley is nestled between the Mayacamas Mountains to the west and the Vaca
Mountains to the east. These mountains are effective barriers to the prevailing northwesterlies with an
average ridge line height of about 2000 feet, some peaks approaching 3000 feet and 4344 foot Mount
Saint Helena. The valley is 27 miles long with Napa and Calistoga defining its southern and northern
ends, respectively. It is widest, 4 3/4 miles, at its southern end and narrows northward to less than a
mile at Calistoga. A minor pass, Knight's Valley, links the northern end of the valley to the Alexander
Valley north of Healdsburg.

An upvalley wind frequently develops during warm summer afternoons drawing from air
flowing through the San Pablo Bay. During the evening, especially in the winter, downvalley drainage
flow can occur. At the AIR DISTRICT 's Napa station, the prevailing winds are upvalley, southwest
through south southeasterly, and occur about 53% of the time. The second most common winds are
down valley drainage winds, north northwesterly through northeasterly, which occur 26% of the time.

Wind speeds are low with almost 50% of the winds between calm and 4 mph and an average speed of
about 5 mph. Only 5 % of the winds are between 16 and 18 mph which represent strong summer time
up valley winds and winter storm winds. Summer average maximum temperatures at the southern end
of the valley are in the low 80's with extremes in the high 80's, and at the northern end are in the low
90's with extremes in the high 90's. Summer minima are in the low 50's. Winter maxima are in the
high 50's and low 60's with minima in the high to mid-30's with the slightly cooler temperatures
favoring the northern end. Winter minima extremes range from the high 20's to the mid 20's.
Sunshine is plentiful and annual precipitation averages range from 43 inches at Angwin in the
mountains at 1820 feet, 38 inches at Calistoga to 24 inches at Napa.

Air pollution potential is high. Summer and fall prevailing winds can transport non-local and
locally generated ozone precursors northward where the valley narrows, effectively trapping and
concentrating the pollutants under stable conditions. The local upslope and downslope flows setup by
the surrounding mountains may also recirculate pollutants adding to the total burden. Also, the high
frequency of light winds and associated stable conditions during the late fall and winter, contributes to
the buildup of particulates and carbon monoxide from automobiles, agricultural burning and fireplace
burning.