Wednesday, February 12, 2014

A CFA Leave and Live television advertisement aimed at residents of rural towns 
to warn them of the danger of 'staying' spells out in large letters  
People are taking this to mean the air temperature can reach 800 degrees in our town, around our home, during a bushfire. And so there's nothing they can do but Leave and Live. 

This is NOT air temperature, not even near the fire front.
 This is the temperature inside a flame.  
Deep within the core of fire front flames. 
To experience 800C during a bushfire, 
you would have to walk into the fire front.

Temperatures have been accurately taken within the flames of fire fronts burning in dry eucalypt forests by an amazing system of fire proof thermometers set up by a research group of eminent bushfire scientists.  

They registered a maximum of 1100C near the flame base, 
and minimum 300C  at the flame tip.

If it was true that during a bushfire air temperature can reach 800 degrees where you are during a bushfire, no one who stayed at Kinglake or Marysville during that worst of worst bushfires, Black Saturday 2009, would have been left alive.  All those who 'sheltered' on the sports ground would been cremated. No building would have remained.

What purpose is served by this terrifying, distorted, advertisement?

 It is not difficult to vastly lower the amount of heat that can impinge on your property during a bushfire threat, by reducing the amount of flammable vegetation on it. Thin out thinned, unclutter and tidy your garden plantings; include fire resistant species; use no-flammable mulch.

The intensity of heat emitted from any flame front is directly related to the density of vegetation in which it is burning. There is even a formula for this: ‘the intensity of a bushfire increases as the square of the dry fuel weight’ (I = W2).

No matter how fierce is the flame front burning in the bush, it HAS TO immediately die down when it reaches a less dense area of vegetation. Five metre flames burning in 2 metre high grass HAVE TO drop or stop when they reach dirt, gravel, concrete, or 2 cm high grass.

Google: "Flame temperature and residence time of fires in dry eucalypt forest", by B. Mike Wotton, James S. Gould, W. Lachlan McCaw, N. Phillip Cheney and Stephen W. Taylor 2011  (International Journal of Wildland Fire 21(3) 270 – 281. Full text