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The local-scale precipitation (LSP) is mainly driven by thermal convection. Here we reveal a decreasing trend in the summertime LSP frequency over both China and the United States by utilizing the hourly rain gauge data from 1981 to 2012. The contrasting aerosol trend likely contributes to this same declining trend of LSP in both countries. As aerosol optical depth (AOD) goes beyond the turning zone of 0.25-0.30, the impact of aerosol on precipitation changes from invigoration to suppression. The mean AOD is generally less and larger than this range and of opposite trends in China and United States, respectively, which likely accounts for the same declining trend of LSP hours in the two countries. The observed boomerang shape points to the importance of aerosol loading, which matters as much as, if not more than the AOD trend, thereby potentially serving as a constraint for climate model evaluation.

Plain Language Summary Local-scale precipitation (LSP) is an integral part of the freshwater cycle. Here, we show that summer LSP hours have significantly declined in the United States and China over the past three decades, a phenomenon that cannot be well explained by global warming. The relationship between LSP hours and aerosol loading is a boomerang shape; a turning zone exists for the shifting effect of aerosols from enhancing to suppressing rainfall as aerosol loading increases. China is above this zone with an increasing aerosol trend, and the United States is below it with a decreasing trend, but they have similar reductions in LSP hours. This disparate role of aerosols in the rainfall process requires holistic thinking about air pollution and climate change.