The number and size of large wildfires have increased dramatically in the western United States during the past three decades. Prior understanding was that the increase in fires was mainly attributable to warming temperatures and earlier snowmelt. In this study, a research team contrasted the three, main hypothesized climatic drivers of recent increases in western US wildfire activity: decreased snowpack, increased temperature, and decreased precipitation. Data was pulled from satellite-derived maps of forest wildfire area burned from eight western US ecoregions, and daily gridded temperature, humidity, and snow water equivalent datasets, to examine the sensitivity of wildfire area burned to standardized hydrologic and precipitation indices. Scientists then quantified trends from1979 to 2016.They found summer precipitation totals and the duration of dry spells were the strongest controls on forest area burned by wildfire. Declining summer rain rather than declining winter snow or increasing wildfire season length has been a primary driver of increasing wildfire in the western U.S. Also, less summer rain exacerbates increasing temperatures as well because less water is available for evapotranspiration. Researchers and scientists can use an understanding of where future summer rain declines are likely to occur to guide adaptation planning.