The inland part of Asia is a zone of great aridity extending from the Caspian Sea in the west to northwestern China and Mongolia in the east and from southern Siberia in the north to northern Iran and Afghanistan in the south. Throughout the region, rainfall never exceeds 300 mm in an average year, far too little to grow grain; parts of Central Asia receive less than 100 mm of rain per year. The prevailing winds blow from west to east, losing much of their moisture before reaching the inland parts of Asia.
There are four major natural, or ecological, zones of Central Asia: steppe, steppe-desert, desert, and mountain. Only in the 20th century have human beings disturbed the ecological processes in the area. These ecological zones extend in broad bands from east to west across Eurasia. The northernmost zone is the steppe, a belt of grassland covering Mongolia and northern Kazakstan and extending into southern Siberia and eastern Europe. The terrain is flat, almost featureless, with few rivers and mountains. The grass cover is denser toward and within the European portions, where there is somewhat more rainfall; in eastern Kazakstan the grass cover thins out, becoming the dry steppe. Mongolia is a high-altitude steppe averaging about 1 mile (1.6 kilometres) above sea level; Kazakstan is a low-altitude steppe lying at sea level. Steppe-desert (semidesert) is grassland drier than the steppe, lying at low altitude and not extending into Europe or Siberia. There the grass cover is thinner than in the steppe, with patches bare of plant cover increasing in size and number toward the south. It is likewise flat terrain with few watercourses and no mountains. In the desert, grasses give way to dryland vegetation. There lie the innermost parts of Asia, south of the steppe and steppe-desert, farthest from the moisture-bearing winds and the seas, with most intensive water evaporation. The Central Asian deserts are vast, spreading from the Karakum Desert of Turkmenistan to the Kyzylkum of Uzbekistan and southern Kazakstan, to the Takla Makan Desert of western China, to the immense Gobi of China and Mongolia. The mountain zone lies in southeastern Central Asia, the Pamirs rising almost five miles above sea level as part of the highest mountain system in the world.