Where the Wind Doesn't Blow: A Climatology of Air Stagnation Events in the United States

Air stagnation events can greatly impact fire planning. Stagnant air can cause major complications for fires primarily because of diminished smoke ventilation, which in turn limits visibility and may negatively affect safety and public health. Warnings for air stagnation can halt a prescribed burn for an extended period of time, even if stagnation does not occur. Currently, there is no widely accepted definition of stagnation, as the frequency and duration of stagnation events varies greatly around the country. We therefore wanted to build a climatology of air stagnation events that would be consistent everywhere, and could help fire managers when making burn decisions.

To build the climatology, we decided to start with a long-term dataset at the surface, where the effects of stagnation would affect fires most greatly. We collected hourly surface observations from stations around the United States from 1981 to 2010, and extracted the winds from each station for the entire period. After a perusal of the data, we selected the thresholds for a stagnation event as a maximum wind speed of 6 kt for at least 24 hours. Results from the surface analysis, including distributions of the duration of the event, the hour of the end of the event, and more will be shown. Further, to expand the spatial coverage of the surface observations, we will also present similar distributions and calculations from gridded surface datasets. Comparisons between surface observations and gridded datasets should help us determine confidence for stagnation occurrences in locations where surface stations are sparse.