Haines Index

The Haines Index (also called the Lower Atmosphere Severity Index) estimates the potential for the atmosphere to amplify fire development based on stability and dryness. Proposed by meteorologist Donald A. Haines in 1988, it identifies conditions where atmospheric structure can cause fires to develop rapidly and behave erratically.

Unlike the FWI, the Haines Index focuses exclusively on the lower atmosphere — it does not consider fuels, terrain, or wind.

Why it matters for fire weather

Most fire weather indices focus on fuel moisture and wind. The Haines Index adds a different dimension: atmospheric structure. An unstable, dry lower atmosphere allows fires to develop tall convection columns, which in turn:

• Draw in surrounding air, creating stronger surface inflow winds near the fire
• Loft embers high into the atmosphere, enabling long-distance spotting
• Can trigger pyroconvection — fire-generated thunderstorms that create erratic and extreme conditions

Research by Haines found that the maximum index value (6) occurs approximately 6% of the time but is associated with over 70% of total burned area — illustrating how atmospheric structure amplifies fire impact (Haines, 1988).

How it works

The index combines two components measured at atmospheric pressure levels (the specific levels depend on terrain elevation):

1. Stability (A) — temperature difference between two atmospheric levels. A larger difference means more unstable air, favouring intense vertical development (convection columns, fire whirls).
2. Dryness (B) — dew point depression at a reference level. Drier air means faster fuel desiccation and enhanced combustion.

Each component is scored 1 to 3, and the total is:

HI = A + B (range: 2–6)

Key thresholds

Value	Atmospheric contribution
2–3	Stable, moist atmosphere. The atmosphere is not amplifying fire development.
4	Moderate instability or dryness. Some atmospheric support for fire growth.
5	Significant instability and/or dryness. The atmosphere favours rapid fire development and convective plume growth.
6	Very unstable and dry. Conditions associated with the most extreme fire events — strong convection columns, long-distance spotting, and potential pyroconvection.

Strengths and limitations

Strengths	Limitations
Simple calculation from standard atmospheric data	Does not include wind — a critical fire spread factor
Effective at identifying atmosphere-driven extreme fire events	Ignores fuel characteristics and topography
Proven association with large burned areas	Empirical design; coarse 2–6 scale limits nuance

Comparison with HDWI

The HDWI complements the Haines Index by including wind in its formula (VPD × wind speed). The Haines Index captures atmospheric instability and dryness aloft, while the HDWI captures the direct drying and mechanical effects of surface weather. Together, they provide a more complete picture of atmospheric fire conditions than either alone.

Potter (2018) has argued that the Haines Index should be revised or replaced with more physically-based measures, but it remains widely used due to its simplicity and proven track record.

How to read it in Wildflyer

The Haines Index appears in the expert view alongside other fire weather indices. Because it captures atmospheric structure rather than surface weather, it is most useful for identifying days when the atmosphere itself could amplify an existing fire beyond what surface conditions alone would suggest.

Sources

• Haines, D.A. (1988). A lower atmospheric severity index for wildland fire. National Weather Digest, 13(2): 23–27.
• Potter, B.E. (2018). The Haines Index — it’s time to revise it or replace it. International Journal of Wildland Fire, 27(7): 437–440.