The dry region formed on the downwind side of a mountain range is called a rain shadow. Moist air approaching the mountains is forced upward on the windward side, where it expands, cools, and often produces clouds and precipitation.
After crossing the ridge, the air descends on the leeward side. Descending air is compressed and warms, reducing its relative humidity and making further rainfall less likely. This process can create a much drier climate close to the wet mountain slope.
Rain shadows help explain why some deserts lie beside major mountain ranges. The Atacama Desert, for example, is influenced by the Andes, while parts of the Great Basin in the United States lie in the rain shadow of western mountain ranges. The effect depends on prevailing winds, mountain height, moisture supply, and seasonal circulation.