A transient CFD model is developed to investigate the parameters affecting the productivity of a solar still. Instantaneous weather data is accounted as boundary conditions while the absorption of solar irradiance, as well as radiative heat transfer is considered by DO equation. Evaporation and condensation rates are evaluated by solving species transport equation for water vapor and application of Fick's law. Latent heats are added to the energy equation as sink and source terms by considering a coefficient that accounts for the latent heat shared between neighboring bodies. Effects of wind speed, glass cover thickness, water depth, water to cover distance and cover inclination on performance of a passive solar still are investigated. Stepped solar stills are also modeled and effects of number of steps and water to cover distance are discussed. Results show that a 14.4% increase in the productivity is observed when the wind speed increases from 1 m/s to 6 m/s, and a 3.5% improvement occurs by decreasing the glass thickness from 4 mm to 2 mm. The optimum values of water depth and water to cover distance are found to be 2 and 8 cm respectively, with the cover tilt equal to the local latitude angle. Moreover, by converting the basin solar still to the appropriate stepped solar still, the productivity increases about 17.4%.