The Chihuahuan Desert is a desert, and an ecoregion designation, that straddles the U.S.-Mexico border in the central and northern portions of the Mexican Plateau. It is bordered on the west by the extensive Sierra Madre Occidental range, along with overlaying northern portions of the Sierra Madre Oriental. On the United States side, it occupies much of southwestern Texas and small parts of New Mexico and Arizona. On the Mexican side, it covers the northern half of the state of Chihuahua, along with the majority of Coahuila, north-eastern Durango, the extreme northern part of Zacatecas, and small western portions of Nuevo León. With an area of about 362,000 km 2 (139,769 sq mi), it is the third largest desert of the Western Hemisphere and the second largest in North America, after the Great Basin Desert. Climate The desert is mainly a rain shadow desert because the two main mountain ranges covering the desert, the Sierra Madre Occidental to the west and the Sierra Madre Oriental to the east block most moisture from the Pacific Ocean and the Gulf of Mexico respectively. Climatically, the desert has a dry climate with only one rainy season in the summer and smaller amounts of precipitation in early winter. Most of the summer rains falls in June and October, during the North American Monsoon when moist air from the Gulf of Mexico penetrates into the region. Owing to its inland position and higher elevation than the Sonoran Desert to the west, mostly varying from 600–1,675 m (1,969–5,495 ft) in altitude, the desert has a slightly milder climate in the summer (though usually daytime June temperatures are in the range of 35 to 40 °C or 95 to 104 °F) and cool or cold winters with occasional frosts.The average annual temperature in the desert is 24 °C (75 °F), which varies with altitude. The hottest temperatures in the desert occur in lower elevation areas and in valleys. Northern areas have more severe winters than the southern portion and can receive snowstorms. The mean annual precipitation for the Chihuahuan Desert is 235 mm (9.3 in) with a range of approximately 150–400 mm (6–16 in), although it receives more precipitation than other warm desert ecoregions. Nearly two-thirds of the arid zone stations have annual totals between 225 and 275 mm (8.9 and 10.8 in). Snowfall is scant except at the higher elevation edges. The desert is fairly young, existing for only 8000 years. Geography Several larger mountain ranges include the Sierra Madre, the Sierra del Carmen, the Organ Mountains, the Franklin Mountains, the Sacramento Mountains, the Sandia-Manzano Mountains, the Magdalena-San Mateo Mountains, the Chisos Mountains, the Guadalupe Mountains, and the Davis Mountains. These create "sky islands" of cooler, wetter, climates adjacent to, or within the desert, and such elevated areas have both coniferous and broadleaf woodlands, including forests along drainages and favored exposures. There are a few urban areas within the desert: the largest is Ciudad Juárez with almost two million inhabitants, neighboring El Paso; then the city of Chihuahua and Torreón. Las Cruces, Albuquerque, and Roswell are among the other significant cities in this ecoregion. Saltillo and Monterrey are located near the Chihuahuan desert. According to the World Wide Fund for Nature the Chihuahuan Desert may be the most biologically diverse desert in the world as measured by species richness or endemism. The region has been badly degraded, mainly due to grazing. Many native grasses and other species have become dominated by woody native plants, including Creosote Bush and Mesquite, due to overgrazing and other urbanization. The Mexican Wolf, once abundant, has been extirpated. Flora Creosote Bush (Larrea tridentata) is the dominant plant species on gravelly and occasional sandy soils in valley areas within the Chihuahuan Desert. The other species it is found with depends on factors such as the soil, altitude, and degree of slope. Viscid Acacia (Acacia neovernicosa), and Tarbush (Flourensia cernua) dominate northern portions, as does Broom Dalea (Psorothamnus scoparius) on sandy soils in western portions. Yucca and Opuntia species are abundant in foothill edges and the central third, while Arizona Rainbow Cactus (Echinocereus polyacanthus) and Mexican Fire-barrel Cactus (Ferocactus pilosus) inhabit portions near the US-Mexico border. Herbaceous plants, such as Bush Muhly (Muhlenbergia porteri), Blue Grama (Bouteloua gracilis), Gypsum Grama (B. breviseta), and Hairy Grama (B. hirsuta), are dominant in desert grasslands and near the mountain edges including the Sierra Madre Occidental. Lechuguilla (Agave lechuguilla), Honey Mesquite (Prosopis glandulosa), Opuntia macrocentra and Echinocereus pectinatus are the dominant species in western Coahuila. Ocotillo (Fouquieria splendens), Lechuguilla, and Yucca filifera are the most common species in the southeastern part of the desert. Candelilla (Euphorbia antisyphilitica), Mimosa zygophylla, Acacia glandulifera and Lechuguilla are found in areas with well-draining, shallow soils. The shrubs found near the Sierra Madre Oriental are exclusively Lechuguilla, Guapilla (Hechtia glomerata), Queen Victoria's Agave (Agave victoriae-reginae), Sotol (Dasylirion spp.), and Barreta (Helietta parvifolia), while the well- developed herbaceous layer includes grasses, legumes and cacti. Grasslands comprise 20% of this desert and are often mosaics of shrubs and grasses. They include Purple Three-awn (Aristida purpurea), Black Grama (Bouteloua eriopoda), and Sideoats Grama (Bouteloua curtipendula). Early Spanish explorers reported encountering grasses that were "belly high to a horse;" most likely these were Big Alkali Sacaton (Sporobolus wrightii) and Tobosa (Pleuraphis mutica) bottomlands. Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply.Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.