Hi Susan: Some 34 grape varieties grow in the Napa Valley, but as you point out, cabernet sauvignon leads, comprising 50% of all plantings. Cabernet emerged as the dominant variety in 1992. Part of the reason was phylloxera which had been discovered in the valley in the 1980s. In the large scale replantings that followed, vintners not only planted on better (resistant) rootstocks, but also planted grape varieties more perfectly suited to Napa’s climate and soils. In particular, cabernet sauvignon achieves a very high level of quality when planted in warm areas with significant diurnal temperature fluctuation and where the soils are well drained. Napa Valley fits the bill perfectly. The climate is warm with little to no rain during the growing season. Nighttime temperatures are often 40 degrees cooler than daytime temperatures and the volcanic and sedimentary soils drain water quickly.