The heat dissipation from two parallel plates vertically held in free air is experimentally studied. From the Schlieren photos taken under various conditions of S (spacing of two plates) and ΔT (temperature difference between the heated surface and environment), the temperature gradients over the surface are evaluated and the local heat transfer coefficients : α_x are calculated. An experimental formula representing the mean Nusselt number is obtained.[numerical formula] for [numerical formula] where [numerical formula] H : height of plate. The physical properties at the wall temperature are considered. A comparison is made between the result of this work and the experimental works of Kennard and Elenbaas.