A typical example of the air temperature and relative humidity histories monitored inside the solar dryer throughout three consecutive days is presented in Fig. 4. The observed cyclic pattern reveals a sinusoidal variation, reflecting day and night increase/decrease in temperature and corresponding decrease/increase in relative humidity. This sinusoidal pattern is also present in the data of daily variation of radiation flux density (Fig. 5), which was generated with Charles-Edwards and Acock model (Eq. (2)).