The Haines Index Explained

For some time now the A.C.T. Bushfire Service has had the Haines Index read out over the radio during weather forecasts. We have been experimenting on the use of this index with the Bureau of Meteorology, as have other places such as Tasmania. (It is more widely used in the US)

Some severe fires, such as the Peirces Creek Fire of April 1991, are driven by surface winds, which push the fire across the ground. In fact at that fire the tops of some pines were scorched and frozen in a horizontal position, like wind vanes. This indicates a very strong wind and a very brief peak heat pulse. Other severe fires are driven by vertical motion of air - i.e. the air's instability.

To quote from a research paper...(see Note 1)

Holding fuel type, topography, and all other meteorological factors constant, atmospheric instability promotes the spread and intensity of fires by increasing the following:

  1. The height and strength of smoke columns. Smoke columns contain large amounts of moisture since water is a significant by-product of fuel combustion. These columns are convective in nature and can be thought of as "chimneys". Smoke columns, like clouds, grow taller in unstable air and thus develop stronger indrafts. Large smoke columns are similar in size to well-developed thunderstorms. Stronger indrafts increase fire activity by increasing wind speeds and also by entraining more oxygen from the surrounding environment.
  2. The chance of firebrands, or burning ember, being lofted by the smoke column outside the fire perimeter. This method of fire growthis called spotting.
  3. The chance of dust devils or fire whirls, which may move outside the fire line, resulting in new fires (spotting).
  4. Other convective winds at the surface." By observing the vertical structure of the atmosphere when the "plume driven" fires occurred, Haines was able to note certain conditions that lead to explosive growth of fires, extreme spotting and frequent crowning. These were combined into an index that ranges from 2 to 6, which show the potential for large plume-driven fire growth...
    • Index 2 or 3 - very low potential
    • Index 4 - low potential
    • Index 5 - moderate potential
    • Index 6 - high potential

For the technically minded, the index reflects the temperature difference between heights were the air pressure is 850 and 700 hectopascals; and also the dew point depression at the 850 hectopascal height. Each of these are scored from 1 to 3, and the two are added together (thus the range of 2 to 6 for the Haines Index).

The finding in Tasmania suggest that using Haines as well as McArthur gives a better readiness indicator. In the ACT we have had no chances to test whether the same holds true - in the years that we have had Haines Indices forecast we have had no suitable fires.

This is good.

So whenever the Haines Index is a six, or maybe a five, Fire Controllers should be aware of the potential for the fire to grow well beyond what would be expected from the McArthur Indices. A large, vigorous convection column could be dragging down very dry air from aloft to replace the air moving upwards. If a large cumulus cloud forms in the convection column of a large fire, watch out, as evaporation of moisture from the cloud can generate a sudden downwash of air that can make a fire explode.

Rick McRae

Note 1: "The Evaluation of Idaho Wildfire Growth using the Haines Index" by Paul Werth & Richard Ochoa 1993.
From the 'Weather and Forcasting' Journal Vol. 8 pages 233-234.