Cities tend to be 1-10 degrees Fahrenheit warmer than surrounding areas. The added heat destabilizes and changes air circulation around cities. During the warmer months, the added heat creates wind circulations and rising air that produces new clouds enhances existing ones. Under the right conditions, these clouds evolve into rain-producers or storms. Scientists suspect that converging air due to city surfaces of varying heights, like buildings, also promotes rising air needed to produce clouds and rainfall.
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 188.8.131.52.0