A study of solar radiation climate at Cairo urban area, Egypt and its environs

The measured values of the hourly global solar radiation, G , and its diffuse component, D , on a horizontal surface for a 12‐year period (1992–2003) have been collected and used to examine the solar radiation characteristics in the urban area of Cairo, Egypt. The corresponding values of the diffuse fraction of global radiation, K , have been calculated. Diurnal variations of the results have been investigated. Also, the daily total values and their monthly and seasonal means, as well as their frequency distributions, have been calculated and investigated. The results reveal that Cairo has annual mean values of G , D , and K equal to 18.57, 6.10 MJ m −2 and 0.35, respectively. The seasonal and climatic effects on the fluctuation of the results are discussed. These effects were particularly large during the winter and spring months owing to the high fluctuation of the atmospheric conditions with respect to the amount of water content, clouds, and concentration of aerosol and dust particles. It was found that the reduction of G due to the effect of clouds is small (2.8%), which in turn reflects the low degree of cloudiness over Cairo due to the impacts of urbanization. A comparative study has also been done to investigate the effect of urbanization processes on the global radiation values received at urban Cairo. It was found that G values received during the non‐urbanized period (1969–1973) highly exceeded the radiation values received during the recent urbanized period (1999–2003) for all months of the year, and the annual mean of the relative reduction was found to be 17.63%. Copyright © 2006 Royal Meteorological Society