The Sonoran Desert as currently defined covers approximately 100,000 square miles (260,000 sq.km.) and includes most of the southern half of Arizona, southeastern California, most of the Baja California peninsula, the islands of the Gulf of California, and much of the state of Sonora, Mexico. It is lush in comparison to most other deserts. Two visually dominant life forms of plants distinguish the Sonoran Desert from the other North American deserts: legume trees and columnar cacti. It also supports many other life forms encompassing a rich spectrum of some 2,000 species of plants.
The amount and seasonality of rainfall are defining characteristics of the Sonoran Desert. Much of the area has a biseasonal rainfall pattern, though even during the rainy seasons most days are sunny. From December to March frontal storms from North Pacific Ocean occasionally bring widespread, gentle rain to the northwestern areas. From July to mid-September, the summer monsoon brings surges of wet tropical air and frequent but localized violent thunderstorms.
The Sonoran Desert prominently differs from the other three North American Deserts in having mild winters; most of the area rarely experiences frost. About half of the biota is tropical in origin, with life cycles attuned to the brief summer rainy season. The winter rains, when ample, produce huge populations of annuals (which comprise half of the species in our flora).
