The results of two-year monitoring of the Schumann-resonance signals are processed and the diurnal and seasonal variations in the activity of the global thunderstorm centers are estimated. We calculate the field in the model of thunderstorm activity localized in a small circular area (the model of a single source). Comparison between the experimental and model data allowed us to find the position of the maximum of the global thunderstorm activity. It is shown that a more exact interpretation of the experimental data is achieved if additional background radiation, which remains approximately constant during a day and is approximately equal to the intensity of a compact source, is included into the model.