In this study, we examine the climatology of cold air pools in western United States using the gridded data from the North American Regional Reanalysis for the period of 1979 through 2011. The results show that while diurnal cold air pools that develop at night and breakup the next morning can form throughout the western US, persistent cold air pools occur primarily in valleys or basins and the longer the cold air pools, the more limited they are to major valleys and basins. The preferred locations for long-lived cold air pools include the Columbia Basin, Snake River Valley, Lake Lohanton Basin, Bonneville Basin, Uinta Basin, Colorado Plateaus Basin and the Central Valley. Averaging over the entire western US, the frequency of persistent cold air pools exhibits a decreasing trend within the past three decades. This appears to be connected to a negative trend in the 500-hPa geopotential height over the region for the same time period. The frequency of occurrence and the length of the cold pools also show strong interannual variability. Results from EOF analyses reveled that the first mode (accounts for about 30% of the total variance) for both the frequency and the length of cold pools is in-phase across the region and it is related to Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO). The second mode (accounts for about 10% of the total variance) mainly show an out-of-phase variability between Columbia Basin and the other basins and valleys and it is related to anomalous 700-hPa and surface temperature fields.