The contributions of water vapor and temperature to the interannual variability of the precipitation of different seasons and locations, which can be controlled by different factors and interactions in the climate system, are evaluated by using a unified approach and the North American Regional Reanalysis. The central question we address is whether the higher precipitation amounts in wet years are due to more atmospheric water vapor or lower temperatures (i.e., enhanced prospects for saturation). For each season at each grid point, the contrast between the composites of the 9 wettest and the 9 driest years of 1979-2005 is used to characterize the interannual variability of precipitation. We found that the extent of water vapor saturation at levels below 350hPa can account for the majority of interannual variability of precipitation. By contrasting water vapor and temperature from the composite dry year to the composite wet year, we define three patterns of water vapor and temperature that contribute to the interannual variability of precipitation: a) moistening, b) cooling, and c) moistening-cooling. This means that the higher precipitation of wet years is due to more water vapor, lower temperature, or both. We find that high winter precipitation of wet years is contributed by more water vapor in high-latitudes, lower temperature in the mid-latitude around 45„aN, and both in low-latitudes. For summer precipitation, the moistening pattern contributes mainly in low-latitudes, the cooling pattern is responsible mainly in mid-high latitudes, and the moistening-cooling pattern is present in all latitudes. The distributions of contributing patterns for the precipitation in spring, fall, and the annual total all show a transition between the distributions for the winter and summer precipitation. For the precipitation of each season, there are places where the same contributing pattern controls the entire atmospheric column below 350hPa. Atmospheric circulation may play an important role in the formation of the different contributing patterns of water vapor and temperature to the interannual variability of precipitation.