Whither the Wind? Developing an Air Stagnation Climatology for the United States

Air stagnation events can greatly impact fire behavior and 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 air stagnation, as the criteria for the frequency and duration of dangerous stagnation events varies greatly around the country. We wanted to build a climatology of air stagnation events that would be consistent everywhere, and could thus help fire managers when making burn decisions.

To build the climatology, we decided to start with a long-term surface weather dataset, since air stagnation at the surface would affect smoke dispersion most greatly. We collected 30 years of hourly surface observations from weather stations around the United States, and extracted the winds from each station for the entire period. After perusing the data, we selected thresholds for a stagnation event as a maximum wind speed of 6 kt for at least 24 hours. For individual stations, we will show frequency distributions based on each event's duration, month of occurrence, and first and last hour below threshold speed. Then, to expand spatial coverage beyond disparate (individual?) surface observation sites, we will present distributions calculated from higher-resolution gridded surface analyses. Comparisons between surface observations and gridded datasets will help us determine confidence for stagnation occurrences in regions where surface stations are sparse.