Deserts are defined by a lack of water. They are arid places that generally receive less than 10 inches (25 cm) of rain per year and where water is severely limiting. Deserts can be cold or hot, and their plants, animals, and people have adapted to scarce and unpredictable rainfall events. Three major Deserts fall partially within the area covered by the Science of the American Southwest. The Sonoran Desert occurs in southern Arizona and extends into California, and well into Mexico. It is a subtropical desert considered by some to be the biologically richest desert in the world. The Chihuahuan Desert occurs mainly in Mexico but extends north into Arizona, New Mexico, and Texas. The Great Basin Desert covers most of Nevada and portions of Arizona, New Mexico, Colorado, Utah. Wyoming, Idaho, and Oregon.