A combined experimental and numerical investigation was carried out to explore the structures of the moving transverse and mixed vortex rolls in a low Reynolds number air flow through a bottom heated horizontal plane channel for Re less than or equal to 50. The appearance of the intermittent how patterns characterized by the oscillatory thermal plumes in the upstream core region ahead of the longitudinal rolls at Gr/Re-2 congruent to 35 was experimentally found to be an early indication of the formation of the transverse rolls. At a high Gr/Re-2 there is a high possibility for the plumes to link with their spanwise neighbors to form transverse rolls. The severing of the returning flow, resulting from the blocking of the main flow by the existing transverse rolls, by the thermal plumes produces new transverse rolls. The how visualization revealed that at decreasing Re the vortex flow transformed from the longitudinal to transverse rolls in the sequence of stable longitudinal rolls, unstable longitudinal rolls and a mixture of the longitudinal and transverse rolls. For Re greater than or equal to 7.5 the transverse rolls occupied the core region and the longitudinal rolls exist near the side walls. For Re less than or equal to 5.0 the entire duct is filled with the moving transverse rolls before they become distorted at very high Ra. A correlation equation for the wave speed of the transverse rolls was also proposed. Additionally, a flow regime map delineating different vortex flow patterns was given,The results from the numerical prediction qualitatively supported the experimental observation.