In summer, weaker winds move the clouds more slowly. Heat absorbed by the city and pollution's interference with raindrop formation interact to cause the clouds to intensify before producing precipitation. The onset of rainfall from a cloud leads eventually to its demise by cooling off the air near the ground. the air pollution delays the onset of precipitation, so that the intense storm clouds can build higher and larger before they start precipitating and subsequently dissipating. Therefore, these larger and more intense thunderstorm clouds produce eventually heavier rainfall on the city and the downwind areas. First is the unpolluted, then the polluted case.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 220.127.116.11.0