The temperature distributions arising in a low-loss dielectric sample in the process of microwave heating have been studied by means of numerical simulation. The convective heat removal has been demonstrated to play the determining role in the energy balance of the sample. The obtained temperature distributions have been compared with the results of a purposely designed microwave heating experiment with multi-channel temperature measurement. The temperature on the surface of the sample agreed well to the simulation accounting for convective heat removal. The temperature measured inside the sample was higher than predicted by simulation. The electrodynamic calculations have shown that due to diffraction effects the electromagnetic field inside the sample is inhomogeneous even if the sample is irradiated by microwaves isotropically. Thus, it is concluded that in order to simulate the microwave heating process accurately, it is necessary to account for air convection and calculate the structure of electromagnetic field inside the sample.