Atmospheric Stability

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 growth is 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.

It is important that you understand stability. This is covered in the “Fire Weather” training module. It is also important that you understand how stability is referred to in the Fire Weather Broadcasts. This is by means of the “Stability Class” and the “Haines index”.

Stability Class

Much of the heat released by a fire is removed by the convection of heated gases. These gases want to do two things:

• rise, because they are warmer than the surrounding air (and thus lighter); and
• travel sideways with the wind.

The gases are also carrying:

• smoke, which reduces visibility and makes operations difficult; and
• burning embers, which may start spot fires downwind.

The rising column of hot gases, called the convection column, affects the behaviour of the fire by:

• altering the angle at which flames meet unburnt fuel;
• making the entire air mass more energetic (as measured by the Haines Index);
• producing spot fires which develop their own convection columns; and
• spawning off smaller turbulent features (fire devils) that may carry burning embers outside the fire perimeter.

Stability is not just based on the heat released by a fire – it reflects the structure of the atmosphere above the fire. Clearly inversions are a serious issue for smoke dispersal. “Inversion” refers to a reversal of the temperature trend with increasing altitude. Rising buoyant air, and the smoke that it is carrying, may become trapped by a layer of warmer air above it.

But also the rate of change of temperature with altitude varies from time to time and place to place. It is measured by weather balloon flights. The differences may permit more, or less, vigorous convection column formation, with all of the resulting implications for fire management.

So, the Stability Class is clearly important to firefighters. So how is it derived?

We use the “Pasquill Index”, which prescibes one of a series of classes, which may be named by a single letter or a descriptive phrase:

Calculation of the class differs between daytime and nighttime. Surface wind speed is always relevant. During the day the amount of sunshine is needed, while at night the level of overcast is used. The table below shows the workings:

and note that:

• heavy overcast at day or night is always D
• G is only used for clear, calm, cold nights

Interpretation of the classess can be done, based on work for planning around Albury-Wodonga, produced this Table:

“Wind swing” is of key importance to fire controllers, as it determines the spread of a headfire, and thus the difficulty and safety of containment. “Vertical diffusion” is a good measure for the development of convection columns. Clearly Classes A & B, and possibly C require the attention of fire controllers. Also fire tower operators could experience difficulty in pinpointing the base of a smoke plume when moderate or lower values are met. “Lateral diffusion” refers to the sideways spread of the smoke plume as it moves. This impacts on air pollution control, aerial operations and also the ease with which fire tower operators can locate fires.

REFERENCE

Pasquill, F. (1961). The estimation of the dispersion of windborne material, The Meteorological Magazine, vol 90, No. 1063, pp 33-49. [Note that a lot of the paper suggests that instead of talking about smoke plumes and smoke concentrations we could as easily talk about radioactive clouds and dosages – funding for such research has not always come from morally sound sources!]

The Haines Index

By observing the vertical structure of the atmosphere when "plume driven" fires occurred in the US, 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 – as shown below - and the two are added together (thus the range of 2 to 6 for the Haines Index).

The findings in Tasmania suggest that using Haines as well as McArthur gives a better readiness indicator. In fact 84% of area burnt occured on days with a Haines Index of 5 or 6.

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.

REFERENCE

"The Evaluation of Idaho Wildfire Growth using the Haines Index" by Paul Werth & Richard Ochoa 1993.
From Weather and Forecasting, Vol. 8 pages 233-234.

Rick McRae