Abstract. The Haines Index is an operational tool for evaluating the potential contribution of dry, unstable air to the development of large or erratic plume-dom:inated wildfires. The index has three variants related to surface elevation, and is calculated from temperature and hmnidity measurements at atmospheric pressure levels. To effectively use the Haines Index, fire forecasters and managers must be aware ofthe climatological and statistical characteristics ofthe index for their location. However, a detailed, long-term, and spatially extensive analysis of the index does not currently exist. To meet this need, a 40-year (1961-2000) climatology of the Haines Index was developed for NOlihAmerica. The climatology is based on gridded (2.5" latitude x 2.5 0 longitude) temperature and humidity fields from the NCEP/NCAR reanalysis. The climatology illustrates the large spatial variability in the Haines Index both within and between regions using the different index vmiants. These spatial variations point to the limitations of the index and must be taken into account when using the Haines Index operationally.

The Haines index is used in wildfire forecasting and monitoring to evaluate the potential contributions of atmospheric stability and humidity to the behavior of plume-dominated wildfires. The index has three variants ("low," "mid," and "high") that accommodate differences in surface elevation. As originally formulated, the low variant is calculated from temperature observations at the 950-and 850-hPa levels and humidity observations at 850 hPa. In the early 1990s the National Weather Service implemented a new mandatory level for radiosonde observations at 925 hPa. Following this change, measurements at 950 hPa became less frequent. An informal survey of several forecast offices found no formalized adjustment to the calculation of the low Haines index to take into account the nonavailability of 950-hPa measurements. Some sources continue to use 950-hPa temperature, usually interpolated from 925-hPa and surface temperatures, to calculate the low Haines index. Others directly substitute the 925-hPa temperature for the originally specified 950-hPa value. This study employs soundings from the central United States when both 950-and 925-hPa levels were available to investigate the impact of different calculation approaches on the resulting values of the low variant of the Haines index. Results show that direct substitution of 925-hPa temperature for the 950-hPa temperature can dramatically underestimate the potential wildfire severity compared with the original formulation of the Haines index. On the other hand, a low-elevation variant of the Haines index calculated from the interpolated 950-hPa temperature is usually in close agreement with the original formulation of the index.

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