This short paper presents an investigation on how human activities may or may not affect precipitation based on numerical simulations of precipitation in a benchmark case with modified lower boundary conditions,representing different stages of urban development in the model.The results indicate that certain degrees of urbanization affect the likelihood of heavy precipitation significantly,while less urbanized or smaller cities are much less prone to these effects.Such a result can be explained based on our previous work where the sensitivity of precipitation statistics to surface anthropogenic heat sources lies in the generation of buoyancy and turbulence in the planetary boundary layer and dissipation through triggering of convection.Thus only mega cities of sufficient size,and hence human-activity-related anthropogenic heat emission,can expect to experience such effects.In other words,as cities grow,their effects upon precipitation appear to grow as well.