Dry, unstable air increases the probability that wildland fires will become large and/or erratic. This paper describes an atmospheric index for these fires, based on the environmental lapse rate of a layer of air coupled with its moisture content. In low-elevation regions of the United States, the index is derived from a lapse rate value using 950- to 850-mb temperature differences and a temperature-dew-point spread at 850 mb. At mid-elevations, the index uses 850- to 700-mb temperature differences and the 850-mb temperature-dew-point spread. In the high-elevation western regions, a similar calculation uses a 700- to 500-mb lapse rate value and the temperature-dew-point spread at 700 mb. A preliminary comparison of the low-elevation variant of the index with climatology showed that only 5% of all fire-season days fell into the high-index class. A similar comparison for the high-elevation region showed 6% of all fire-season days in the high-index class , but 45% of days with large and/or erratic wildfire in that class.
[This publication is referenced in the "Synthesis of knowledge of extreme fire behavior: volume I for fire managers" (Werth et al 2011).]