Numerical study on transient to steady state natural convection inside an enclosure filled with air has been performed where periodic temperature profile is applied from bottom wall. Periodic condition arises from left to right. To understand the thermal characteristic stream function, isotherm and Nu have been plotted against Ra and with respect to time. A typical formation of stream function has been found from study that indicates the formation global circulation at high Ra (10⁶) and at lower period of heating (λ/H=0.5). Penetration also drastically alters at high Ra.