These are the climate zones of the Western United States as defined by the Sunset Western Garden Book. They are in widespread use in horticulture, gardening, and landscaping as determinants to which plants can survive where. The western portion of the United States is partitioned into 24 climate zones, which take into account rainfall, winter minimum temperatures, summer maximum temperatures, length of growing season, and humidity. The zones range from Zone 1, the colder areas and mountaintops of the west, to Zone 24, the frostless marine areas of Southern California. In between, many coastal, valley, mountain, foothill, and desert areas are defined. Although these zones have been criticized as too vague for extremely detailed work in the area, they are definitely thorough enough for all home gardeners. One should keep in mind that, especially in mountain areas, climate is quite locally variable, and the zone shown in your area may not be exactly correct.