Flow visualization is conducted here to study the vortex flow patterns associated with the Rayleigh-Benard convection in a horizontal shallow cavity of air. The cavity is a rectangular enclosure characterized by the aspect ratios A(x) = 16 and A(z) = 20. Attention is focused on the convection rolls driven at slightly supercritical and subcritical buoyancies. The results show that at slightly subcritical Ra the induced vortex flow is in the form of rectangular rolls along the cavity sides and short straight parallel rolls in the cavity core. At slightly higher Ra near Ra-c proportional to (= 1708) more rectangular rolls appear and the short straight rolls in the cavity core merge together to form a serpentine roll. At slightly supercritical buoyancy with 2000 less than or equal to Ra less than or equal to 3000 the entire cavity is filled with the straight rolls all parallel to the short sides of the cavity. At an even higher Ra of 4000 the vortex rolls become irregular and time dependent. Moreover, the processes through which various vortex flow structures evolve during the transient stage are shown to be rather complicate and the vortex flow patterns during the how formation are significantly affected by the heating rate in raising the buoyancy force. Furthermore, the wavenumber reduction at higher Ra for the parallel vortex roll pattern was noted to mainly result from the splitting of some rectangular rolls into cells and the subsequent merging of the cells into bigger rolls at the intermediate stage of the flow formation.