Heat transfer from a rotating circular cylinder immersed in a spatially uniform, time-dependent convective environment is investigated numerically including the effects due to buoyancy force. The flow equations, based on the vorticity and stream function, are solved along with the energy equation by a hybrid spectral scheme that combines the Fourier spectral method in the angular direction and a spectral element method in the radial direction. Several cases are simulated for Grashof numbers up to 2 x 10(4), Reynolds numbers up to 200, and a range of speed of rotation from -0.5 to +0.5. The results show that vortex shedding is promoted by the cylinder rotation but is vanished by the presence of the buoyancy force. In opposing flows, the counter flow currents cause a large expansion of the streamlines and isotherms in the direction normal to the free stream velocity. These changes in the structure of the flow and the temperature fields greatly modify the heat flux along the surface of the cylinder and consequently, the heat transfer rate is strongly dependent upon Reynolds number, Grashof number, rotational speed, and the gravity direction. Effects due to how pulsation are also reflected in the Nusselt number history in the form of periodic oscillations.