The extensive smoke pall shown in this high-oblique, northeast-looking photograph of southern Bolivia is typical of palls that can extend for thousands of square miles (thousands of square kilometers) in Brazil, Paraguay, and Bolivia. The smoke palls are most widespread during the dry winter months when vegetated areas are cleared for slash-and-burn agriculture—crop cultivation in recently cut and burned tropical vegetation clearings that are soon abandoned in favor of newly burned land. The darker, vegetated parallel ridges (western edge of the photograph) are the foothills of the Andes Mountains that give way to the higher elevations (brown area) further west, which act as a barrier to the westward movement of smoke throughout South America. The Pilcomayo River is visible (center of photograph) as it emerges from the foothills of the Andes Mountains near the town of Villa Montes. Large cultivated field patterns appear in some of the valleys in the foothills and in the lowlands adjacent to the eastern flank of the foothills.