This paper reports experiments on a vertical buoyant round jet of air-helium mixture impinging on an horizontal wall. For Reynolds number from 47 to 22,780 and Richardson number from 3.57.10^-4 to 2.91.10^3, flow visualizations were made and the density was measured at the impinging point. It was found that, under particular conditions, this density can decrease whereas the injected mixture density is increased by adding air. Flow visualizations point out that this result can be linked to a transition from a "puff-like" flow pattern to a "jet-like" one. Then, this transition was quantified by a condition on the initial Richardson number, Ri, and the dimensionless source diameter, phi, yielding the criterion: Ri = 100.phi^3/2.