The Haines index, also called the lower atmospheric stability index, condenses the atmosphere's contribution to erratic fire behaviour into a single number from \( 2 \) to \( 6 \). It is built from two terms scored \( 1 \) to \( 3 \) each: a stability term, the temperature difference between two pressure levels, and a moisture term, the dewpoint depression at a level, with three variants of the level pair chosen for low, mid and high elevation terrain. High values, \( 5 \) or \( 6 \), mark a dry unstable lower atmosphere in which plumes grow and downdrafts reach the ground; low values mark a moist or stable one. Its most important limitation is honest and well documented: it says nothing about wind, ignition or fuels, so a high Haines is a permission, not a prediction, and it is a poor discriminator on its own. The confusion this resolves is reading it as a fire danger rating. After this Concept you can interpret a Haines value as a statement about atmospheric potential and name what it deliberately omits.
The Haines Index (formally the Lower Atmosphere Severity Index, LASI) is an additive index combining an atmospheric stability term (environmental lapse rate, categorized 1–3) and a moisture term (dew…