General harmonic analysis (FFT) and maximum entropy spectral analysis (MEM) are used in order to study annual and monthly values of air temperature, precipitation and insolation in a number of weather stations in Duero Hydrographic Basin. The corresponding spectra estimates were prepared and compared. MEM estimates show better power resolution and a better spectral definition than FFT estimates. Based on the similarity behavior of the spectra, four main climatic zones were clearly defined. Finally, physical interpretation of the main features of the most representative spectra for the four regions was attempted.