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Wild Fires - The Immediate Face of Climate Change? (Weather)

by dulan drift, Wednesday, November 14, 2018, 13:53 (32 days ago) @ dan

I was a bit surprised when you posted that the rate of spread of the bushfire was only one mile per hour - would have thought it would be much faster. Found this on the Black Saturday fires in Australia, which was similar in terms of devastation to the Camp fire. Also includes some other frightening details such as how 'fire-balls' occur:

The rates of spread of the fires in forest equalled the maximum previously recorded on
“Ash Wednesday”, averaging 12 km/h for an hour in the case of the Kilmore East fire.
There was an exceptional run of fire in the Canberra fires of 2003 which spread at a rate
of about 26 km/h for an hour due to the “pull” of a major convection column on either
side of it. However, it was the extent and prolific spotfires that made the behaviour of
these fires unique. The very hot and dry conditions were conducive to keeping the
firebrands alight for longer than normal, and upper winds of around 100 km/h carried
firebrands and started spotfires up to 35 km ahead of the fire front. The previous
authenticated spotting distance was 30 km (Luke and McArthur, 1978).

The rate of spread of the fire for shorter periods of time would have been much faster
than the hourly average. Prolific short-distance spotting was occurring about 10
minutes before the arrival of the main fire, i.e. about one kilometre ahead of the main
front. Black smoke indicated that the burning efficiency of the fire was very poor due
to the air-fuel mixture being too rich to burn. This resulted in fire flares and what have
been described as “fireballs” where parcels of flammable gases released from the forest
fuels have been blown ahead or above the fire and when they have mixed with sufficient
air (oxygen), they ignite. This phenomenon could occur several hundred metres ahead
of the fire front.

Modelling of these fires, using PHOENIX RapidFire, indicated that
fires could have spread as fast as 60 km/h in grassland and 40 km/h in forest for one
hundredth of an hour – about half a minute. The fire then effectively moves in pulses
where several hundreds of meters of forest can be set alight and then it takes a few
minutes for the fuel to be fully incorporated into the fire and then it pulses again. The
rate at which these pulses move could be as fast as the prevailing winds, which in the
case of “Black Saturday” was 50 to 60 km/h.

Flames were observed to leap 100 m or more into the air. Radiation from the flame
would make survival within a distance of three to four times this flame height difficult
except for very short exposures. Flame temperatures would have been 900 to 1200o
C, depending on the amount of moisture in the fuels and the depth of the flames.


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