In this paper an analysis of natural convection in an enclosure filled with air and with opposite heated and cooled walls is performed; four different tilts of the cavity are investigated: 0 = 0, pi/6, pi/3 and pi/2. A complete definition of the velocity field in the cavity is obtained through PIV (particle image velocimetry) and is shown in terms of velocity contour maps and streamlines; the same results are obtained through numerical analysis and compared with the experimental ones: a very good agreement is found. In addition the thermal field is calculated and the value of the Nusselt number (Nu) next to the how wall is reported. A brief overview on a new PIV aspect is also discussed: since very small velocity fields are investigated, the effect of heating by the pulsed laser light becomes relevant, thus introducing a disturb on the measuring field that has never been considered for Ply, since it is generally treated as a completely non-intrusive technique. (C) 2012 Elsevier Ltd. All rights reserved.