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 between late June and early 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 in altitude, the desert has a slightly milder climate in the summer (though usually daytime June temperatures are in the range of ) and cool or cold winters with occasional frosts. The average annual temperature in the desert is , 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 with a range of approximately , although it receives more precipitation than other warm desert ecoregions. Nearly two-thirds of the arid zone stations have annual totals between . Snowfall is scant except at the higher elevation edges. The desert is fairly young, existing for only 8000 years.