Bolivia's glaciers, situated between lat 14°37' and 18°23' S. on the southern edge of the tropical zone of the Southern Hemisphere, are affected by the change between intertropical circulation in the summer and southeast trade winds in the winter. During the southern summer, this generally means precipitation that decreases in amount and duration from north to south. This author believes that the term "summer" is appropriate for the rainy season in Bolivia, in contrast to the central tropics of Venezuela, Colombia, and Ecuador, even though Schubert (1992) gives a different view. Inhabitants of these countries, however, seldom refer to summer or winter; they speak of dry and rainy seasons. The dual climatic situation and the orientation and elevation of its mountain ranges are the determining factors in the occurrence and distribution of Bolivia's glaciers (fig. 1).
In contrast to extratropical glaciers, the fundamental difference in glacier formation lies in the fact that the tropical snow reserves must be established during the summer. They cannot be established during the coldest period of the year because during the winter, as a rule, little to no precipitation falls. Ablation, on the other hand, takes place during the interseasonal periods and the winter when solar radiation is intense, as well as during summertime dry periods. This results in a completely different kind of mass-balance situation over the budget year, which is further complicated by irregularities in the annual precipitation cycle (Jordan, 1979). During the summer, the maintenance of a glacier is a delicate balance between the accumulation of snow reserves and the ablation from radiation at an increased temperature. Data from mass-balance measurements give more exact information (Jordan, 1992; Francou and others, 1995). Ribstein and others (1995) discuss the results of a 2-year study of the hydrology of a 3-km2 basin in the Cordillera Real that is 77 percent glacierized. In this area, accumulation and melt periods coincide during the rainy season, but the amount of melt often exceeds precipitation, which is resulting in the rapid recession of the glacier termini.
Because of the year-round high position of the Sun in the tropics, north-south exposure differences are less apparent than they are outside the tropics. In the Southern Hemisphere, the effect of the Sun increases in importance toward the south, however, and the cycle of cloud formation during the day must then be taken into consideration with respect to the exposure differences that affect the mass balance of a glacier. Because cloud cover descends very regularly at night to a level of 3,500 to 4,000 m, the glaciers are fully exposed to the morning Sun even during the rainy season. The cloudiness that develops during the forenoon protects the glaciers from radiation during the rest of the day (see fig. 10). Because the Sun shines on the eastern slopes in the early morning and because the northern slopes have greater solar radiation in the Southern Hemisphere, the east-to-north slope exposures have comparably smaller glacierization. The snowline is lower on the western and southern slopes and rises substantially (100 to 300 m) on the eastern and northern slopes (see figs. 11-13; Jordan, 1985). The solar-radiation effect increases toward the arid regions to the south. Combined with the extreme dryness of the air, solar radiation produces a peculiar phenomenon on firn and glacier surfaces, the intensified development of snow and ice penitents (Troll, 1942). The penitents phenomenon is also very dependent on the slope and radiation exposure and the annual climate cycle; these factors produce large differences, both with respect to time and space, in shaping the penitents (fig. 14). As a result of this differential surface ablation, which is typically a subtropical-tropical phenomenon, glaciers and snow patches become especially difficult to traverse toward the end of the dry season.
A further manifestation of the distinctive daytime climate of high mountains on the southern edge of the tropics is the presence of glaciers with a high accumulation of debris. In addition, rock glaciers are found at elevations of 4,800 m and above, south of the actual occurrence of glaciers (fig. 15). Their exact classification is the subject of scientific controversy (see section on Rock Glaciers in this volume), and their exact areal distribution is not elaborated in this section because the author is inclined to characterize them, on the basis of their typical slope, as a periglacial permafrost phenomenon. Also, because of their modest size, they are not discernible on satellite images.
U.S. Geological Survey, U.S.Department of the Interior