Investigations in recent decades have shown an unexpected increase in aerosol concentrations in metropolitan and industrialized regions. It was sometimes followed by heat islands and probably decreases in precipitation due to an inadvertent cloud overseeding process.
Particle pollutants act like small cloud condensation nuclei in which they form large collections of small droplets in cloud. Collision coalescence efficiency of tiny droplets is decreased and the following formation of rain drops elapsed. In a similar way in mixed clouds the formation of ice crystal precipitation due to a reduction in the accretion aggregation the process of small crystals was dilating.
The relationship between precipitation and particle pollutants in Tehran as a metropolitan area has been investigated. This study plays an important role for meteorologists and environmental researchers.
The influence of particle pollutanst on the precipitation process in various regions of Tehran including northeast (Aghdasie), northwest (Geophysics), east (Sorkhe hesar), west (Mehr Abad) and the center (Bazaar) has been studied .These investigations have been carried out in two ways: desirable days (particle pollutants less than 100 ) and undesirable days (particle pollutants more than 200 ) in warm and cold seasons over a period of 5 years (1999-2003).
The analysis of isohypse/particle pollutant isograms in undesirable conditions for each precipitation event shows that in both warm and cold seasons the amount of precipitation during the day increases from downtown toward the north of the city, due to the decrease of particle pollutant concentrations.
The average of precipitation in the northern stations is higher than the central, western and eastern stations due to their higher elevations. This average on undesirable days decreases from West to east and increases on desirable days. The precipitation trend increases for all stations in desirable conditions in warm and cold seasons and decreases in undesirable conditions.
A study of all data on desirable and undesirable days in cold and warm seasons shows that, in desirable conditions the precipitation trends increase, probably due to inadvertent cloud seeding. In undesirable conditions due to an increase in particle pollutants as cloud condensation nuclei (CCN), the precipitation trends decrease probably due to cloud over seeding.
A majority of stations in the warm season shows that the maximum value of precipitation on desirable and undesirable days is slightly more than its value in the cold season, mainly due to the greater thickness of clouds and more precipitation intensity than in warm the season.