Geoscience Reference
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speeds. Winds blowing from the interior of the conti-
nent warm adiabatically as they drop in elevation from
the highland areas of Victoria to the coast, producing
what is commonly known as a föhn wind. This adiabatic
warming can push temperatures to 40°C or above at
the coast. The process also reduces relative humidity to
levels below 20 per cent. Similar winds exist elsewhere,
especially in mountainous Mediterranean climates
dominated by Hadley cells. Local variants of föhn
winds are termed 'mono' in central California, 'Diablo'
around San Francisco, 'Santa Ana' in southern Califor-
nia, 'sirocco' in southern Europe, and 'bora' in the
Balkans. In Australia, strengthening of föhn winds is
aided by convergence of isobars, and reinforced by a
fast-moving subtropical jet stream in the upper tropo-
sphere on the western side of the high-pressure cell.
Because this sequence of winds moves eastward across
the continent over several days, there is a gradual shift
in the bushfire threat from South Australia to Victoria,
and then to New South Wales. In Western Australia,
fire weather is characterized by positioning of high-
pressure cells with north-easterly winds out of the
interior.
Fire behavior and local topography ultimately
determine the severity of the fire hazard. Often fires
accelerate as the day progresses, because fuel moisture
content drops and wind speed and atmospheric
instability increase owing to diurnal heating. Addition-
ally, as the fire intensifies during the day, convection is
generated by the extra heat supplied to the atmos-
phere. Radiation from the fire dries out fuel in front of
the fire, and spot fires can prepare forward areas for
the passage of the main fire. Spotting in front of fires is
quite common in Australia because of the presence of
stringy-barks. Spotting ensures that convection occurs
in front of the main fire, thus drawing the main fire-
front towards the spot fires.
As wind speed increases, the rate of fire movement
increases exponentially. A wind speed of 10 km hr -1
will move a fire - in Australian bushland of average fuel
content - at the rate of 0.5 km hr -1 . At 20 km hr -1 , this
rate increases to 0.8 km hr -1 and, at 40 km hr -1 , to
1.8 km hr -1 . If the fire actually reaches treetops and
burns through the crowns of trees (Figure 7.4), it can
accelerate and travel at speeds of 20 km hr -1 . With
spotting, fires can move forward or 'propagate' faster
than this, creating a firestorm that can travel at
60 km hr -1 or more. The rate at which a bushfire moves
does not taper off with increased wind velocities.
Fig. 7.4 Crown fire in A) Eucalyptus forest, Australia (photograph by
N.P. Cheney, National Bushfire Research Unit, CSIRO,
Canberra)
Fig. 7.4 and B) a boreal forest in northern Ontario, Canada
(photograph by Jim Bryant, Ontario Department of the
Environment). Note the similarities between the fires.
Eucalyptus is perceived as more flammable, but boreal
forests produce three to four times more energy per unit
time because of their greater biomass.
 
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