The actual solar radiation recoded inside the greenhouse model was lower than that outside, due to the reflectance, absorptance, and transmittance factors of the greenhouse covering material. The hourly averages solar radiation recorded outside and inside the greenhouse model were 555.7 and 298.7 W/m2, consequently, the effective transmittance of the covering material was on the average 53.75% as revealed in Fig. (3). To determine the solar radiation flux incident on a horizontal surface inside the greenhouse model (Ri), all the data recorded