During the summer months, and especially in August, temperatures in the Napa Valley vary greatly from those in San Francisco. While the weather in San Francisco may feature chilly foggy mornings and nights, a short drive to the Napa Valley will reward you with average daytime temperatures in the 90's and very temperate nights
In August, the average temperature in Calistoga is 72 degrees, with an average high of 91, and an average low of 52 degrees. The average rainfall in Calistoga for August is 0.1 inches, with the last August rainfall recorded in 1997. Visitors from the East will immediately notice that the air is dry, and rarely humid.
In the town of Napa, the average August temperature is 68 degrees, while the average high is 81 degrees and the average low is 54. The average rainfall for August is 0.1 inches. Temperatures and rainfall in Yountville and Oakville, several miles north of Napa, would be roughly the same as in Napa.