This paper provides an experimental analysis of the natural convection in a differentially heated H side square enclosure. The cavity is full of air and is heated by a hot strip with a height of H/2. The effect of the position of this source on the dynamic structures generated by the natural convection heat transfer was analyzed at the steady state and under laminar conditions. The experimental apparatus is a 2D-PlV system while the experimental data consist of vector maps, velocity maps and streamlines at different Rayleigh numbers. During the study the presence of two small vortexes was noted on the upper surface of the source. These are dependent both on the Rayleigh numbers and on the position of the strip. (c) 2008 Elsevier Ltd. All rights reserved.