In a 1988 paper, Mr. Donald Haines of the USDA Forest Service’s North Central Research Station proposed what he called the “Lower Atmosphere Stability Index.” Renamed the Haines Index in his honor since then, it has a value of 2, 3, 4, 5, or 6 and is a simple measure of how strongly atmospheric conditions near the earth’s surface might contribute to an existing fire becoming a dangerous, erratic fire with a strong, well defined updraft. It reflects atmospheric stability and dryness for a layer of the atmosphere roughly 1 to 5 km above the surface.
Just as important as recognizing what the Haines Index does indicate, is knowing what it does not indicate. The Haines Index says nothing about fuel conditions – whether fuels are abundant or sparse, wet or dry. It does not reflect surface temperature, humidity or wind conditions. And, because it does not include any measure of winds at the surface or aloft, it does not necessarily indicate anything about danger, behavior or growth for fires when strong winds are present.
The Haines Index depends on temperature and dew point values measured above the ground, across a layer of the atmosphere identified by the pressures at the top and bottom of the layer. Depending on the surface elevation of the region of interest, there are three possible layers used for computing the Haines Index, and these three layers provide what are commonly known as the low, mid and high elevation Haines Index variants. The pressure layers used for the variants are 950 millibars (mb) to 850 mb for the low variant; 850 mb to 700 mb for the mid variant, and 700 to 500 mb for the high variant.
For all three variants of the Haines Index, there are two components. The A component reflects the temperature difference between the top and bottom of the appropriate layer. The B component reflects the difference between air temperature and dew point (this difference is often called the dew point depression) at the top of the layer (in the case of the low variant) or the bottom of the layer (for the mid and high variants.) For each component, the differences are converted to values of 1, 2, or 3 based on thresholds specified by Haines. The final Haines Index is the sum of these two component values, a number between 2 and 6, inclusive. Values of 2 or 3 are considered “Very Low,” a 4 is “Low,” a 5 is “Moderate” and a 6 is “High.” The use of “low” and “high” to describe both the value and the variant of the Haines Index can lead to some confusion, and users must be clear on which aspect they mean.
Originally, Haines computed the index using measurements taken at 00 UTC, which corresponds to local mid-afternoon to late evening, depending on local time zone. However since its creation, researchers and practitioners have computed it for other times of day also, using observations or forecast information.