During the Dynamique et Chimie de l'Atmosphère en Forêt Equatoriale (DECAFE) campaign, dynamical and thermodynamical measurements were made at Impfondo (1°37′N, 18°04′W), over the dense rain forest of northern Congo during the dry season (February 1988). During the measurement period the experimental site was located south of the intertropical convergence zone ground track which manages the dynamics of the large scale. Above the experimental site, the atmospheric low layers are supplied by monsoon air coming from the Guinean gulf; the upper layers (>1500 m) are supplied by warm and dry air (trade winds) coming from the northern desert region and the savanna. Our experimental approach consists of analyzing the heat and moisture content in the low troposphere from vertical soundings made by a tethered balloon (0–400 m) and an aircraft (0–4000 m). The analysis of the evolution of the observed planetary boundary layer (PBL) is made with a mixed layer one‐dimensional model which is forced to represent correctly the observed PBL height growth. The simulated and observed budgets of the heat and moisture in the PBL are balanced by adding dry air to the simulated PBL in the afternoon. This drying out can be maintained only by high levels of entrainment flux at the PBL top. An entrainment velocity of 3 cm s⁻¹ enables the balancing of the moisture budget. This entrainment velocity seems compatible with physicochemical transfers as those of methane and ozone.