The thermal impedance Zth (j*omega) has been measured for a transistor fixed on a cooling fin. We have put the setup inside a cilindrical chimney with an inner diameter around 5cm and various heights H ranging from 20 to 60cm. The enhancement of the natural convection cooling by the chimney is clearly demonstrated by means of Nyquist plots of the thermal impedance. Depending on the insertion distance of the setup into the chimney, the case-to-ambient thermal resistance Delta Rth could be reduced by more than 15%. We also propose a simple hydraulic model for the chimney. The theoretical analysis predicts that 1/Delta Rth is proportional to H^1/4 . The experimental results agree quite well with this tendency.