We examined the influence of average climatic conditions and nightly weather variations using bat detectors on the summer activity of bats. Average summer precipitation was the principal climate variable correlated with differences in bat activity along a latitudinal array of sites, with the highest activity levels occurring at sites located in montane rain shadows. On a nightly basis, the occurrence of rain and low temperatures had strong negative correlations with flight activity. However, the variation in nightly activity at a site that was explained by weather was relatively small. Our results suggest that the use of long-term climatic data offers potential to predict variations in bat activity among sites. Such information may be useful in recognizing conservation priorities for the management of bats in the Pacific Northwest where topography is complex and climate conditions are variable.