The impact of the external thermal stratification on the flow behavior andthe mass flow rate in an open ended vertical channel with one side uniformlyheated is investigated experimentally and numerically. Both experimentaland numerical studies show that the increase of the external stratificationlowers the velocity and the mass flow rate but also displaces the transitionheight at a lower location in the channel. The numerical model was usedto study the cases of weak and negative thermal stratifications which arebarely achievable in non-controlled experimental laboratory conditions butare common in the atmosphere. Different flow regimes are identified with flowbeing fully laminar when the stratification is weak or negative and flow beingtransitional to turbulent for higher stratifications. Finally, the numericallyobtained mass flow rates with the various external thermal stratifications areshown to be in excellent agreement with a theoretical model developed in aprevious work.