An experimental investigation on natural convection of air in horizontal channels with well-insulated lower wall and a heated upper wall is carried out. Flow visualization and air temperature measurements are employed to obtain a phenomenological description of air natural convection inside a horizontal, open-ended cavity with a heated upper plate and an unheated lower one. A laminar flow, with a C-loop shape, is observed inside the open-ended cavity. The penetration length is dependent upon the Grashof number. The penetration length increases particularly by increasing either the distance between the walls or the heat flux. The temperature measurements confirm the flow visualization observations. The air temperature profiles inside the open ended cavity indicate that the temperature gradients along the gap cavity are weak for low heat fluxes values. Scale analysis is carried out and shows that penetration length depends on Ra-1/2, in accordance with other authors. Furthermore, the estimation of the penetration length depends on the values of the thickness of the boundary layer, evaluated in terms of distance between the walls. Monomial correlations for average Nusselt numbers and dimensionless wall temperatures are proposed in a range of Rayleigh number from 2.78 x 10(3) to 3.90 x 10(5) and for an aspect ratio between 2L/b = 10 and 2L/b = 20. The equations agree highly with the experimental results. (c) 2006 Elsevier Ltd. All rights reserved.