Sulfate aerosol is formed in the atmosphere from SO₂ by gas-phase oxidation (GPO) involving the hydroxyl radical and from the reaction of SO₂ in aqueous phase with ozone or peroxides. The latter mechanism is referred to as “heterogeneous” chemical oxidation (HCO). SO₂-to-sulfate HCO occurs in hydrated aerosols, cloud droplets and rain drops. Modeling aerosol formation requires simulating the HCO process and, hence, the presence of clouds. Little has been done, however, to determine how well physical-chemical atmospheric modeling systems simulate clouds. Simulations of three summer episodes, using the Urban-to-Regional Multiscale (URM) model, have been studied to determine model performance with regard to the spatial and temporal distribution of clouds. The presence of non-convective, non-precipitating clouds was parameterized using a threshold of relative humidity output from a meteorological model. Precipitating clouds were defined using simulated values of cloud top heights and the fractional coverage of convective and stratiform cloud systems. Fractional cloud cover and cloud ceiling height simulated by the URM model are compared to ground observations. Data on the observed vertical structure of convective clouds are used to judge the realism of model simulations of such clouds. The influence of clouds on simulated concentrations of sulfate aerosol is summarized.