The mean valley heat deficit for the 39-year period of record (POR) is quite variable from year to year, driven mainly by synoptic-scale weather systems and the attendant frequency of persistent multi-day cold-air pools. The variability is such that no long-term interannual trend in wintertime heat deficit (a measure of atmospheric stability) can be determined statistically. A heat deficit threshold of 5 MJ m-2 is able to separate normal atmospheric stabilities from the persistent multi-day events. Multi-day cold-air pools typically last about 3 days, but the longest persistent cold-air pool in the POR lasted 16 days. Particulate air pollution builds up gradually from day to day in the persistent events, so that the worst air quality is in the latter stages of the events. The events are accompanied by weak winds in the valley. Like for air pollution, moisture often builds up within the pool over time, sometimes resulting in the development of fog and low-level stratus. While the heat deficit often builds up gradually over a period of days, the cold pool breakdown often occurs rather abruptly. Because the breakdown of the pool occurs with a declining heat deficit and an increase of wind speeds aloft, a Froude number approach is tested to see if it is useful in discriminating cold-pool breakup.