An experimental investigation of air natural convection between heated parallel plates has been carried out by the visualization of the flow and the measurement of air temperature. Grashof numbers, based on the gap between the plates and the longitudinal aspect ratio (L/b), varied in the 3.8·102–1.1·105 range. The investigated distance between the plates, b, were 20.00, 32.25 and 40.00 mm, corresponding to aspect ratios L/b of 20, 12.4 and 10 and plate width/gap between the plates ratio of 22.5, 14 and 11.25. The Ohmic heat fluxes have been changed in the 60–240 W/m2 range. Furthermore, three heating modes have been tested in order to analyze their peculiar patterns and the onset of secondary motions. The typical main flow pattern resembles a C shape (C-loop). In fact, the flow penetrates inside the channel close to the leading edge of the lower plate and exits from the upper part, by reversing its motion inside the channel. The flow visualization shows that secondary flows add to the C-loop main flow, when the lower plate is heated. Such secondary structures start in the form of thermals and, then, they change into longitudinal vortex, whereas in the upper region of the open-ended cavity a chaotic motion is detected. The existence of these structures is confirmed by the measurement of istantaneous temperature values. They show that the greater the Grashof number the more chaotic the turbulent flow in the outflow region of the C-loop, when the lower plate is heated and the upper plate is unheated.