Haines Index (EN)

The Haines Index (or Lower Atmosphere Severity Index) is a meteorological index used to estimate the potential for intensification and erratic behavior of wildfires in the presence of a dry and unstable atmosphere.Proposed in 1988 by American meteorologist Donald A. Haines, it aims to measure the atmosphere's propensity to favor rapid developments of already existing fires.
Unlike integrative indices such as the Weather Forest Index, the Haines Index focuses exclusively on the conditions of the lower atmosphere without taking into account the characteristics of fuels or terrain.

What is the HI?

The HI is a purely meteorological index, consisting of two components observed in a layer of the atmosphere depending on the altitude of the terrain:
  • Atmospheric Stability (∆T): temperature difference between two levels (for example, 950–850 hPa for low altitudes). The higher ∆T is, the more unstable the air is, favoring potentially intense vertical movements.
  • Humidity (dew point vs temperature): dew point deficit at a chosen level (for example, 850 hPa). The drier the air, the more quickly the fuels can dry out.
Each component is coded from 1 to 3, then summed to obtain a total index ranging from 2 to 6.

What does the HI take into account?

The index relies on two fundamental meteorological components:
    Atmospheric stability, which indicates the air's ability to develop vertical movements.
    Air dryness, an estimate of the potential for vegetation desiccation and enhanced combustion.
These parameters are measured from atmospheric soundings (radiosondes) or derived from numerical weather prediction models.

Formula

The index is calculated according to the formula:
HI = A+B
where:
  • A represents atmospheric stability (thermal gradient),
  • B represents air dryness (dew point depression).
Each component is coded from 1 to 3. The final index therefore varies from 2 to 6.

Index Scale

Value
Interprétation
2 – 3
Low potential
4
Moderate potential
5
High potential
6
Very high potential (risk of explosive development)


Advantages
Limits
  • Simple calculation;
  • Easily accessible data;
  • Proven ability to identify extreme fire conditions;
  • Good environmental indicator for operational monitoring.
  • Does not take into account the wind, a critical factor in the spread of fires.
  • Ignores the terrain and fuel characteristics (type of vegetation, soil moisture…).
  • Formulation empirical and sometimes too simple for complex weather contexts.
A study showed that the maximum value (6) appears only about 6% of the time, but it is associated with more than 70% of the burned areas.
Comparison with the HDWI
  • The HDWI is based on a clear physical formulation (VPD × wind), integrating temperature, humidity, and wind, and is calculated near the ground (~500 m).
  • The HI, which is simpler and empirical, does not take wind into account, which limits its relevance for assessing extreme conditions.

Bibliographic References

  • Haines, D. A. (1988). A lower atmospheric severity index for wildland fire. National Weather Digest, 13(2), 23–27.   FRAMES.gov : A lower atmospheric severity index for wildland fires     A lower atmospheric severity index for wildland fires | Fire Research and Management Exchange System 
  • 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. DOI:  https://doi.org/10.1071/WF18015    
  • Potter, B. E. (2018). Quantitative Evaluation of the Haines Index’s Ability to Predict Fire Growth Events. Atmosphere, 9(5), 177. DOI:  https://doi.org/10.3390/atmos9050177 
  • Mills, G. A., & McCaw, L. (2010). Atmospheric stability environments and fire weather in Australia – extending the Haines Index. CAWCR Technical Report, 20.  https://www.publish.csiro.au/wf/pdf/WF18015