Monthly-averaged 10-year climatological data series of global solar irradiation, average ambient temperature and mean relative humidity in Ibadan (lat. 7.43° N, long. 3.90° E), Nigeria, have been analysed using the Fourier series method. The pertinent amplitudes, phase angles, and harmonic angles have been obtained for each year of the series and for each variable. The study reveals that only the annual harmonic contributes in any significant way to explain the variance of the average temperature and the mean relative humitidy series, with percentages ranging between 57% and 91% for temperature and between 75% and 93% for the relative humidity. In the case of global solar irradiation, both the first and the second harmonics contribute almost equally to the total variance, with about 40% each. Typical annual time function parameters are provided for each of the three climatological variables.