In this paper, an experiment is carried out to investigate the buoyancy driven vortex how at slightly subcritical and supercritical Rayleigh numbers in a mixed convective air flow through a bottom heated horizontal flat duct. Particular attention is paid to the flow at a very low Reynolds number for 3.0 less than or equal to Re less than or equal to 5.0. Results from the flow visualization have revealed two new vortex flow patterns in addition to those often seen in literature such as longitudinal rolls (L rolls), moving transverse rolls (T rolls), and mixed longitudinal:transverse rolls (M rolls). The newly observed vortex flow patterns include the stable longitudinal rolls near the duct sides along with nonperiodic traversing transverse waves in the duct core, and the mixed longitudinal and transverse rolls as well as irregular cells. Moreover, steady longitudinal rolls and nonperiodic traversing transverse waves are noted even at subcritical Rayleigh numbers. A flow regime map delineating various vortex flow patterns is given. Temporal characteristics of the flow are also inspected. Furthermore, a correlation is given to estimate the local onset locations of the longitudinal rolls. Meanwhile, the oscillation frequency and convection speeds of the transverse rolls are correlated from the present data. Finally, many complicate processes during the vortex flow formation are noted, such as the generation of the L and T rolls and transverse waves, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the moving and bending of T rolls. (C) 2001 Elsevier Science Ltd. Ail rights reserved.