Examining the influence of mid-tropospheric conditions and surface wind changes on extremely large fires and fire growth days

Background. Previous work by the author and others has examined weather associated with growth of exceptionally large fires (‘Fires of Unusual Size’, or FOUS), looking at three of four factors associated with critical fire weather patterns: antecedent drying, high wind and low humidity. However, the authors did not examine atmospheric stability, the fourth factor. Aims. This study examined the relationships of mid-tropospheric stability and dryness used in the Haines Index, and changes in surface wind speed or direction, to growth of FOUS. Methods. Weather measures were paired with daily growth measures for FOUS, and for merely ‘large’ fires paired with each FOUS. Distributions of weather and growth were compared between the two fire sets graphically and statistically to determine which, if any, weather properties correspond to greater growth on FOUS than on large fires. Key results. None of the factors showed a robust difference in fire growth response between FOUS and large fires. Conclusions. The examined measures, chosen for their anecdotal or assumed association with increased fire growth, showed no indication of that association. Implications. Focus on wind changes and mid-tropospheric properties may be counter- productive or distracting when one is concerned about major growth events on very large fires.