Final answer:
Western South America has a unique climate feature of altitudinal zonation, resulting in varying climates along the elevation of the Andes Mountains, as well as extremely arid areas like the Atacama Desert due to the rain shadow effect.
Step-by-step explanation:
One unique climate feature of western South America is the presence of altitudinal zonation, where different climate zones exist due to elevation changes. Specifically in the Andes Mountains, climate can change from humid and tropical at the base (Type A climate) to cold and snowy at higher altitudes, exhibiting Type H highland climates. In addition, the rain shadow effect creates extremely arid areas like the Atacama Desert, which sees very little precipitation due to the moisture being blocked by the high mountains.