Radiosonde, satellite data, Tropical Ocean--Global Atmosphere (TOGA) radar 2 km constant altitude plan position indicator (CAPPI), and rainfall collected from the TRMM-Wet Season Atmospheric Mesoscale Campaign (WETAMC)/Large-Scale Biosphere-Atmosphere (LBA) Experiment in Amazonia have been used to investigate the diurnal cycle of the tropical convection. Geostationary Operational Environmental Satellite (GOES 8) images were used to describe the diurnal modulation of the total/high/convective cloud fraction and the diurnal evolution of the size spectrum and initiation/dissipation of the convective systems. Radar 2 km CAPPI were used to describe the diurnal cycle of the rain fraction for different thresholds and the diurnal evolution of the size spectrum and initiation/dissipation of the rain cells. An average over the four rain gauge networks was applied to describe the average hourly rainfall. The upper air network data set was used to compute the thermodynamic variables: equivalent potential temperature (θe), convective available potential energy (CAPE), thickness of positive buoyancy, instability, and convective inhibition. High and convective cloud area fractions reach their maximum some hours after the maximum rainfall detected by rain gauge and radar 2 km CAPPI. The minimum cloud cover occurs only a few hours before the maximum precipitation and the maximum cloud cover occurs during the night. The maximum rainfall takes place at the time of the maximum initiation of the convective systems observed by satellite and rain cells. At the time of maximum precipitation the majority of the convective systems and rain cells are small sized and present the maximum increasing area fraction rate. The diurnal evolution of θe also presents a very clear diurnal variation, with maximum occurring in the early afternoon. The CAPE is well related to θe. When θe is high CAPE is high; the atmosphere is unstable and has a deep layer of positive buoyancy and small convective inhibition. These results suggest the following mechanism controlling the diurnal of convection: In the morning, cloud cover decreases as the solar flux reaching the surface increases and consequently increases θe. In the early afternoon, convection rapidly develops, high and convective cloud fractions increase rapidly, and the maximum precipitation and initiation is observed. After convection is developed the atmosphere profile is modified, reaching a nearly saturated state; the water vapor flux decreases in the boundary layer which becomes very stable, thereby inhibiting surface fluxes and consequently extinguishing the convection.