Generation of diurnal solar radiation, temperature, and humidity patterns

Diurnal patterns of solar radiation, temperature and humidity are needed for energy and water use evaluations of greenhouses and other agricultural structures, as well as for plant growth simulation studies. Moreover, these diurnal weather pattern data are not readily available for most locations around the world. Somewhat more available are mean monthly values of total daily solar radiation, of maximum and minimum air temperature, and of vapor pressure. Some simple models were taken from the literature and modified to generate diurnal patterns from such data. Model values were then compared to actual diurnal data from a total of eleven locations in New Zealand, Denmark and the United States. For solar radiation a simple half-cosine model had an overall root-mean-square (rms) difference from observations of 16.3 W m⁻², but the daylength was adjusted slightly to attain this low rms value. For air temperature, the model of Parton and Logan was modified to force the nighttime decrease in temperature to the minimum near sunrise. This modified equation had an overall rms difference from the observations of 0.64°C. The model for vapor pressure was a simple constant equal to the daily average, and it had an overall rms difference from observations of 0.042 kPa. Diurnal relative humidity values could then be calculated from the constant vapor pressure and diurnal temperature values. Thus, these simple models should be adequate for generating diurnal solar radiation, temperature, and humidity patterns for many applications.