The enormous mass of the Himalayas creates a rain shadow, denying entry to the moisture-laden clouds of the Indian monsoon. Ladakh is thus a high altitude desert. The main source of water is the winter snowfall on the mountains. The regions on the north flank of the Himalayas—Dras, the Suru valley and Zanskar—experience heavy snowfall and remain virtually cut off from the rest of the country for several months in the year. Summers are short, although long enough to grow crops. The proportion of oxygen is less than in many other places at a comparable altitude because of lack of vegetation. There is little moisture to temper the effects of rarefied air.