The Sierra Nevada mountains are a roughly north-south range, about 400 miles (644 km) in length. The mountains consist of granite which formed deep underground, beginning some 100 million years ago. Just four million years ago, tectonic movement caused uplift, driving the granite first to the surface, then well above to form the mountain range.
Weather and glacial movement eroded surface earth, exposing the granite. Erosion continued to work on the granite. The “decomposed” granite washes down the mountains and is carried further by rivers. The soils of the Placer County growing vineyards are combinations of decomposed granite and alluvial deposits of topsoils.
Mountains/River/Other Key Influences
Placer County winegrowing is in the foothills, well west of the mountains themselves. Though vineyards range between 500 and 1,500 feet (152-456 m) above sea level, altitude does not have a significant climatic influence. Vineyards do experience some cooling in the evening as cold air “slides down” from the mountains while hot air rises from lower altitudes. The mountains are too distant to provide any shading. And, as the mountains lie east of Placer County, and not west, they don’t create a rain shadow effect for the vineyards either.
The American River, and other streams, have contributed to the soils, but don’t have a significant impact on climate. So, the main climatic influence is the Pacific Ocean. However, because Placer County is so far inland and separated from the ocean by several mountain ranges, the ocean’s moderating effect is minimal. Thus, Placer County’s wine region has a hot-summer Mediterranean climate, rather than the warm-summer Mediterranean climate common in regions closer to the ocean.
The difference between those climate types is the shift between summer and winter average temperatures. The hot-summer Mediterranean climate is, as the name suggests, hotter during the summer. However, as with a warm-summer Mediterranean climate, the vast majority of precipitation is concentrated during the winter months. In Placer County, that may include not just rain, but snow. However, the growing areas don’t get cold enough during winter to kill vines.
During the growing season, there is little to no rain. Humidity is also low. Those factors, coupled with the hot days means there’s little threat from moisture-related vine maladies, such as rot and mildew.