Stretching in a broad arc across western Bolivia, the Andes define the country's three geographic zones: the mountains and Altiplano in the west, the semitropical Yungas and temperate valleys of the eastern mountain slopes, and the tropical lowlands or plains (llanos) of the eastern lowlands, or Oriente. The Andes run in two great parallel ranges or cordilleras. The western range (Cordillera Occidental) runs along the Peruvian and Chilean borders. The eastern range (Cordillera Oriental) is a broad and towering system of mountains stretching from Peru to Argentina. Between the two ranges lies the Altiplano, a lofty plateau 805 kilometers long and 129 kilometers wide
Climate: Although Bolivia lies entirely within tropical latitudes, climatic conditions vary widely from tropical in the lowlands to polar in the highest parts of the Andes. Temperatures depend primarily on elevation and show little seasonal variation. In most locations, rainfall is heaviest during the Southern Hemisphere summer, and yearly amounts tend to decrease from north to south.