Napa Valley is located within the rare dry
Mediterranean climate zone, which encom-
passes just 2% of the earth’s surface. The
long growing season is marked by warm
summer days and cool evenings – ideal for
wine grapes to ripen slowly and evenly, with
great balance between sugar and acid de-
velopment. Lack of summer rainfall helps to
contribute to consistency of vintages and re-
duces the risk of vineyard disease.
Most areas of the appellation can dry farm
or tightly manage their irrigation practices.
Compared to most growing regions in the
world, Napa Valley is unique in this regard.
When it does rain in the winter, the southern
reaches of Napa Valley AVA receive between
18 and 23 inches of rain each year. The moun-
tains at the northern end can record nearly
60 inches annually.
During the summer growing season, condi-
tions exist for creating a recurring pattern of
marine fog from the Pacific Ocean. As hot air
in California’s interior valley rises, it creates
a vacuum effect which draws in moist, cool
air from the Pacific Ocean, forming fog. This
pattern repeats most days during the warm-
er months of the year.
Daytime temperatures can also vary dramati-
cally throughout the AVA during the grow-
ing season. The southern part of the valley,
particularly the Los Carneros AVA, is closer
to San Pablo Bay and is cooled by marine
breezes. In summer, there can be as much
as a 10-15°F difference between Carneros
and St. Helena, to the north. On many days,
due to the cooling effect of the fog, there
is a large diurnal (day to night) temperature
swing throughout the valley.
Adding to the diversity of Napa Valley is its
topography. It changes with its length, from
the flat estuaries in the south near San Pablo
Bay to flat river valley with some rolling hills
leading to nearly 3,000 foot mountains on
either side of the valley floor to the valley’s
sentinel, 4,339-foot Mount St. Helena in the
north. There is a strong correlation between
topography and climate: slope, aspect, and
elevation all influence the many growing con-
ditions found throughout the Napa Valley.