Fire Danger

[Footy]

Can anyone post the formula used to calculate degree of fire danger.
Is it a 0-100 scale? or does it go higher?

Tah

[Pipster]

From the BOM site :

The CSIRO meter as described in "Grassfires: fuel, weather and fire behaviour" by Phil Cheney and Andrew Sullivan (CSIRO Publishing, 1997) is used as the basis for the calculation of the fire danger ratings. As a formula that is,
FDI = 10^(0.009254 - 0.004096*(100-C)^1.536 + 0.01201*T + 0.2789*V^0.5 - 0.09577*RH^0.5 )

where

C = curing expressed as a percentage
T = air temp in degrees Celcius
V = wind speed in km/h (10m, 10 minute mean)
RH = relative humidity expressed as a percentage
FDI = Grassland Fire Danger Index

Pip

[Footy]

thanks for that pip,
what does the curing mean?

Compton 24 is now fully governmental with official blue plates...

[littlejohn]

what does the curing mean?

Curing = how dry the grass/vegetation is.

At a guess they'd use the Dry Matter percentage (I stress it's a guess, because I can't be bothered looking up the reference document).

Green grass at this time of the year has a DM = ~ 20%. Dry grass in late summer DM = ~ 90-100%.

[Pipster]

Perhaps a more user friendly version of the formula can be found on the CSIRO wesbite at:

http://www.ffp.csiro.au/nfm/fbm/Software/grasscalc.html

Pip

[allan]

In respect of "is the scale 0 to 100, or does it go higher?" (see first post).

When McArthur developed the scale (back in the 1950s) he considered that 100 would be the worst case, however recent studies (and live burn tests) has shown that it can go well over 100, so the scale is now generally considered to be 'open ended". But remembering that anything over 50 is 'extreme', the difference between say 100, and 125 is getting to be academic. It's frightening!

Also recollect that McArthur's original ratings (ie. low, medium, high, extreme) are designed to describe to firefighters how difficult it will be to control a fire. It is more often used (in the public mind at least)to describe the risk of a fire starting and spreading, which is probably true enough.

However, as firefighters I believe we should always think back to the original concept - FDI above 50, rating extreme, it is going to be extremely difficult to control a fire under these conditions!

[Pipster]

And as I understand it, MacArthur's scale is now considered to be inaccurate when the wind speed reaches a particular speed...(I think it was 15 km/hr, but that may not be correct).

His protegee, King Phil, has undertaken a huge amount of research into bushfire behaviour, and as a result of the work undertaken, I believe a more accurate scale has been prodcued.

Perhaps to put a  scale into perspective, the AWS (Automatic Weather Station) based at North Shields (near Pt Lincoln) which was active on Black Tuesday 2005, the FDI based on those weather conditions was 336 (as per the official data).   There was speculation that it went higher than that, but the fire went through and cut all the power, and the data wasn't being collected, and hence, couldn't be confirmed....


Pip