The phenomenon of the thermal plume above a heat source has been reported in the literature as being influenced by a large number of factors. The objective of the present study is to identify the impact of the boundary conditions on the characteristics and development of thethermal plume above a sitting occupant. CFD predictions were performed to explain the reason for a skewness in the thermal plume above a sitting thermal manikin with realistic body shape, size, and surface temperature distribution, measured in a climate chamber with mean radiant temperature equal to the room air temperature, no radiant temperature asymmetry, and air velocity lower than 0.05 m/s. The results of the CFD predictions showed that even a small non-uniformity in the temperature field (±0.01ºC) or in the velocity field (±0.005 m/s) of the surrounding environment affects the development of the thermal plume above a sitting person and causes a skewness of the plume cross-section.