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Mother Nature is no sweet old lady. She's one tough mother, red in tooth and claw and with a thousand ways of disposing of mortals. Make that a thousand and one – an article in New Scientist(subscriber only) reveals that she has fuel-air weapons as well.
One witness describes an explosive encounter during a forest fire near Canberra:
"The hairs on the back of my neck stood up and I thought, 'There's something not right here.' Then this furnace just hit us. The trees in front of us snapped off and the mirrors of the truck smashed. The flames were 2 metres off the ground to about 8 metres in the air and were weird blues and oranges. It was like the air was on fire."
It seems that during forest fires, volatile plant material can be vaporized in a process called pyrolysis, forming a cloud of water vapor and flammable gases including hydrogen and carbon monoxide. When these gases mix with the air, and when the temperature hits 350 degrees centigrade, the cloud ignites, giving the impression that the air itself is on fire. The question is how burning air, fireballs and explosions can happen some distance from the fire itself. One researcher may have the answer.
*John Dold, a combustion researcher at the University of Manchester, UK, has a new theory to explain the strange events observed during the Canberra fire. He says gases produced by pyrolysis must have somehow escaped from the fire front and accumulated away from the flames, forming an invisible and highly flammable mixture with the air. Flames can propagate so quickly through such a mixture that the changes in pressure create shock waves, leading to an explosion. For Dold, the snapped trees, blue flames and strange shimmering fire all add up to one thing - the advancing flames must have caused just such a violent combustion. "The evidence seems quite strong that it did happen," he says. *
Dubbed 'the finger of death', nature's own fuel-air weapon can reach out and kill fire-fighters some distance from a blaze. 14 fire-fighters died a fire at South Canyon near Glenwood Springs, Colorado in 1994 and the survivors' description is of just this sort of 'explosive event.'
Now researchers hope to be able to determine what sort of conditions giver rise to the 'finger of death' in particular the lay of the land. This should help fire-fighters avoid this kind of lethal hazard in future. But, as usual with rare and unexplained phenomena, other scientists don't necessarily agree.
Footnote: if the fuel and air are mixed poorly, the result is a spectacular fireball without a major shockwave. Better mixing gives 'deflagration', a flame front which moves at less than the speed of sound and produces some blast. Only under certain circumstances will there be an actual detonation with a supersonic shockwave and massive blast effects. So you might say that Mother Nature mainly just uses a flamethrower, with fuel-air for special occasions.
