The thermal impedance has been measured for the junction of a transistor mounted on a cooling fin. The setup was put inside a chimney in order to enhance the natural convection cooling. From the thermal impedance the influence of the chimney on the reduction in thermal resistance is clearly visible. A maximum improvement of almost 20% has been observed experimentally. Measuring the thermal impedance as a function of the angular frequency provides specific information about the thermal path being influenced by the chimney.