Geoscience Reference
In-Depth Information
However, in grassland, fire movement decelerates at
wind speeds above 50 km hr
-1
because of fragmenta-
tion of the fire head. This is not significant when one
considers that, under the same wind speed, grass fires
can move about eight times faster than a bushfire. If
fires move upslope, their rate of advance will increase
because rising heat from combustion dries out fuel.
Fires will also move faster upslope because the wind
velocity profile steepens without the influence of
ground friction. In rugged terrain, spot fires are more
likely because embers can be caught by winds blowing
at higher speeds at the tops of hills or ridges.
Atmospheric stability also controls fire behavior. If
the atmosphere is unstable, then convective instability
can be established by the fire's heating of the
atmosphere. Once initiated, convective instability will
continue even after the heat source is removed. Under
conditions of extreme instability, fire whirlwinds are
generated and can reach speeds exceeding 250 km hr
-1
.
These whirlwinds are of three types. The first is related
to high combustion rates and generates a rotational
updraft, which can move embers a couple of hundred
meters beyond the fire-front. The second type repre-
sents a thermally induced tornado. This type originates
in the atmosphere on the downwind side of the
convective column or the lee side of topographic
relief. The winds are capable of lifting large logs and
producing gaseous explosions in the atmosphere.
These vortices are particularly dangerous because
wind is sucked towards the fire tornado from all
directions. The third type of whirlwind is the post-burn
vortex (Figure 7.5). It forms because of the heat
emanating from a burnt area up to a day after the fire.
These vortices are usually less than 20 m in diameter,
but they can carry embers from burnt to unburnt areas.
Whirlwind associated with a clearing after burning (photograph
by A.G. Edward, National Bushfire Research Unit, CSIRO,
Canberra).
Fig. 7.5
The latter group includes burning that gets out of
control, smoking, children playing, fires due to trains,
and camp fires. In Canada, lightning is responsible for
32 per cent of all forest fires, whereas in the United
States it accounts for fewer than 8 per cent of fires. The
latter figure is misleading because lightning has played
a major role in igniting fires in isolated areas of the
United States. Between 1940 and 1975, over 200 000
fires were started by lightning in the western United
States alone. This represents an average of 15.6 fires
per day. Single storm events or pressure patterns can
also produce a large number of lightning fires in a short
space of time. Between 1960 and 1971, the western
United States experienced six events that produced
500-800 lightning fires each. The largest such event, in
June 1940, started 1488 fires.
Humans are the greatest cause of fires. Arson by
mentally disturbed people accounts for 32 per cent of
all fires in the United States and 7 per cent in Canada.
CAUSES OF FIRES
(Luke & McArthur, 1978; Pyne, 1982; van Nao, 1982)
No matter how dry a forest or grassland becomes,
there can be no fire unless it is ignited. The recorded
cause of a fire depends upon how rigorously fire statis-
tics are collected in individual countries. For instance,
in European countries, 45 per cent of all fires are of
unknown cause; however, in Canada, only 5 per cent of
fires have unknown causes. In Europe, only 2 per cent
of the known causes of bushfires can be attributed to
natural causes such as lightning. Over half are the
result of arson and 40 per cent, of human carelessness.
