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 600 to 1,675 m (1,969 to 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.