The singular role of the atmospheric stability in forest fires

Abstract

Low stability favors the upward transport of the hot air generated by combustion. On the contrary, strong stability diminishes the potential for this vertical movement. Therefore, it could be concluded that low stability favors fire development. But this study shows that low stability increases fire activity only in dry atmospheric conditions. Statistical analysis of more than 70,000 fires in the period 1993-2005 (July- September) for Galicia (NW Spain) indicates that the daily number of forest fires is greater on dry days with low stability. This dependence on stability, however, reverses for moist days. The lowest values of the daily number of forest fires occur on those days with low stability coupled with high humidity. This bimodal and seemingly contradictory dependence of fire occurrence with respect to atmospheric stability causes misleading results in simple statistical correlations between stability and the number of fires. Furthermore, this contradictory role of the atmospheric stability implies ambiguous values in risk indices that apply simple numerical addition to stability and humidity.