Tropical glaciology includes investigation of climate variability in poorly documented regions of large surface-atmosphere energy exchanges. This study examines the surface energy fluxes of the Bolivian Zongo Glacier (16¿S, 68¿W, 6000--4900 m asl) in order to identify the atmospheric variables that control melting. Measurements from 1998 to 2000 taken from two meteorological stations in the ablation area are analyzed. During the progressive development of the wet season from September to January, melting energy was high: Solar irradiance was close to its summer solstice peak, clouds were sporadic, and albedo was low. During the core of the wet season from January to April the magnitudes of the net short-wave (+) and net long-wave (-) radiation fluxes were reduced by frequent clouds and snowfalls so that melting energy was moderate. In the dry season from May to August, melting energy was small because of the energy losses essentially in long-wave radiation but also in sublimation. The turbulent sensible heat flux to the ice (+) generally offsets the energy loss in latent heat (-), except in the dry season, when sublimation prevailed because of strong wind and dry air. Solar radiation was the main source of energy, but the seasonal changes of the melting energy were driven by long-wave radiation. In particular, clouds sharply increased the emittance of the thin high-altitude atmosphere. Closely linked to clouds and humidity, the main seasonal variables of low-latitude climates, long-wave radiation is a key variable in the energy balance of tropical glaciers.