A strong west wind drift brings cold water to the California Coast. It also drives water westward from below the Aleutian Islands toward the shore of Southern British Columbia. This current splits as it hits the North American coastline. The current that heads north is called the Alaskan Current. The current that heads south is called the California Current. The water in the California Current is icy and impacts the summer weather in the coastal valleys of Northern California.
Spring Mountain is only 30 miles from the Pacific ocean. Lying between the Spring Mountain appellation and the cold ocean currents are the low coastal hills between around Bodega Bay.
Summer heat in the interior of California creates a low pressure area that draws the cold air from the coast over these low hills and across the broad Santa Rosa plain. This air movement is bumped north by Sonoma Mountain pushing the cold air flow towards Santa Rosa Creek and the Mayacamas mountain range.
Spring Mountain lies in the Mayacamas range and its ridge is lower than Bald Mountain to the south and Diamond Mountain to the north. This gap allows this cool, moist air to enter the Napa Valley and spill down over the vineyards planted on the slopes of Spring Mountain.