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”.
The gases are also carrying:
The rising column of hot gases, called the convection column, affects the behaviour of the fire by:
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:
|All times||Daytime sunshine||Nighttime|
|Surface wind speed (km/hr)||Strong||Moderate||Slight||Thin overcast, or 4 octas of cloud or more||Less than 4 octas of cloud|
|<8||A||A – B||B||E||F|
|8 – 12||A – B||B||C||D||E|
|12 – 18||B||B – C||C||D||D|
|18 – 23||C||C – D||D||D||D|
and note that:
Interpretation of the classess can be done, based on work for planning around Albury-Wodonga, produced this Table:
|Stability Class||Amount of wind swing over 2 hours||Vertical diffusion produced||Lateral diffusion produced|
|A||Over 135o||Very large||Very large|
|B||105o – 135o||Large||Large|
|C||75o – 105o||Moderate to large||Moderate to large|
|D||45o – 75o||Moderate to small||Moderate to small|
|E||15o – 45o||Small||Small|
|F||Under 15o or very little rapid variation and slow variation under 30o||Very small||Very small|
|G||Under 15o or very little rapid variation and slow variation over 30o||Very small||Large|
“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.
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).
STABILITY TERMT850 – T700 (oC)
|5 or less||1|
|6 to 10||2|
|11 or more||3|
MOISTURE TERMT850 – DP850 (oC)
|5 or less||1|
|6 to 12||2|
|13 or more||3|
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.