The geography of Tibet consists of the high mountains, lakes and rivers lying between Central, East and South Asia. Traditionally, Western (European and American) sources have regarded Tibet as being in Central Asia, though today's maps show a trend toward considering all of modern China, including Tibet, to be part of East Asia. Tibet is often called "the roof of the world," comprising tablelands averaging over 4,950 metres above the sea with peaks at 6,000 to 7,500 m, including Mount Everest, on the border with Nepal. Description It is bounded on the north and east by the Central China Plain, on the west by Kashmir and on the south by Nepal, India and Bhutan. Most of Tibet sits atop a geological structure known as the Tibetan Plateau, which includes the Himalaya and many of the highest mountain peaks in the world. High mountain peaks include Changtse, Lhotse, Makalu, Gauri Sankar, Gurla Mandhata, Cho Oyu, Jomolhari, Gyachung Kang, Gyala Peri, Mount Kailash, Kawagebo, Khumbutse, Melungtse, Mount Nyainqentanglha, Namcha Barwa, Shishapangma and Yangra. Mountain passes include Cherko la and North Col. Smaller mountains include Mount Gephel and Gurla Mandhata. Regions Physically, Tibet may be divided into two parts, the "lake region" in the west and north-west, and the "river region", which spreads out on three sides of the former on the east, south, and west. The region names are useful in contrasting their hydrological structures, and also in contrasting their different cultural uses which is nomadic in the "lake region" and agricultural in the "river region". Despite its large size and mountainous nature, variation of climate across the Tibetan Plateau is more steady than abrupt. The "river region" has a subtropical highland climate with moderate summer rainfall averaging around per year, and daytime temperatures ranging from around in winter to in summer–though nights are as much as cooler. Rainfall decreases steadily to the west, reaching only at Leh on the edge of this region, whilst temperatures in winter become steadily colder. On the south the "river region" is bounded by the Himalayas, on the north by a broad mountain system. The system at no point narrows to a single range; generally there are three or four across its breadth. As a whole the system forms the watershed between rivers flowing to the Indian Ocean – the Indus, Brahmaputra and Salween and its tributaries – and the streams flowing into the undrained salt