Climate
Local topography and regional weather patterns make the Spring Mountain District the coolest and wettest appellation within the Napa Valley. These same factors create a diurnal fluctuation in temperature in the summertime that differs from other regions of the Napa Valley. Spring Mountain is only 30 miles (48 km) to the east of the Pacific Ocean. The coastal waters of northern California are strongly influenced by the California Current, an icy flow of water that originates near the Aleutian Islands. This cold current moderates the summer weather in the coastal valleys of Northern California.
Lying between the Spring Mountain appellation and the cold ocean current is a gap in the coastal mountains between Bodega and Tomales Bay and extending through the Santa Rosa Plain. Summer heat in the interior of California creates a low pressure area that draws cold air from the coast through this coastal gap and across the broad Santa Rosa plain. This on-shore air movement is bumped north by Sonoma Mountain pushing the cold air flow towards Santa Rosa Creek, the Northern Mayacamas Mountains, and directly at the Spring Mountain District AVA. The ridge of the Spring Mountain District is lower than Bald Mountain to its south and Diamond Mountain to its north. This lower ridgeline allows the cool, moist coastal air to enter the Napa Valley spilling down over forest and the vineyards that lie on the slopes of Spring Mountain and moderating peak daily temperatures.
A typical summer afternoon on Spring Mountain is cool, sometimes with “waterfalls” of fog tumbling over the western ridge and down through the canyons of the district. Into the evening, the cool air settles to the valley floor creating a cover of fog and warm air is lifted to the higher elevations. Nighttime temperature rise from this effect. Mornings warm more quickly on Spring Mountain than on the valley floor as most of the district lies above the morning fog line. The overall effect of this is moderately warm peak daily temperatures and moderately warm nighttime temperatures which keeps sugar accumulation in the berry in pace with flavor development.
The topography of the Spring Mountain District AVA also influences in the vineyards on the valley floor below it. While much of the Napa Valley depends on cooling from the San Pablo Bay, the vineyards in and around Saint Helena benefit from this second and direct source of coastal cooling that comes through the Spring Mountain District AVA. Also moderating summer temperatures in the appellation is the altitude of the vineyards; generally they are cooler at higher elevations. A final moderating influence is the district's predominately eastern exposure which shades the district from the harsh afternoon sun. The topography influences climate over the entire year. Spring Mountain District AVA receives 10 inches (25 cm) to 15 inches (38 cm) more annual rainfall than the Napa Valley floor or the eastern slopes of the valley. Total precipitation can range as high as 70 inches (178 cm) to 95 inches (241 cm) in some of the wettest years.
Read more about this topic: Spring Mountain District AVA
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