Explanation of Sunset zones
Sunset magazine has developed its own system of climate zones. You can find your Sunset zone from the link:
The Sunset Western Garden book (copyright 2001) explains the method behind determining its climate zone classification:
"The U.S. Department of Agriculture employs a climate zone scheme based on winter minimum temperatures. It provides a useful plant hardiness index, but it has some important drawbacks. Its focus on cold-tolerance alone, for example, places the Olympic rain forest into a zone with parts of the Sonoran Desert. The American Horticultural Society has developed a heat zone system based on the average number of ays above 86 degrees F . . . . Used with the USDA system, it targets optimal climates for particular plants. But this information is largely irrelevant to many Wetern gardeners particularly those who live in mild coastal areas where it seldom gets that hot. Our method of zoning considers a broad range of factors, including winter minimums, summer highs, elevation, proximity to coast or mountains, rainfall, humidity and aridity, and growing season. It gives gardeners a more acurate picture of what will grow where."