Agriculture Reference
In-Depth Information
Fire Weather
Most large fires owe their origin to severe fire weather that includes high tempera-
tures, low humidity and wind. These conditions contribute to drying of live fuels,
thus reducing the heating required to drive off moisture and effect combustion.
Wind accelerates fire spread rate by both increasing oxygen supply and carrying
heated air to adjacent fuels on the downwind side, thus raising the fuel tempera-
ture and driving off water vapor. Most MTC regions have extraordinarily severe
wind events during either summer or autumn (see
Box 1.3
), which greatly exacer-
bates the fire danger.
Wind is particularly critical because it is capable of carrying firebrands, which
often occur when gusts are greater than 16 km hr
1
(Green
1981
). Such firebrands
may be carried more than a kilometer from the fire front and ignite new fires.
Firebrands are lifted by the convective buoyancy of the flaming front and thus
fast-moving crown fires are particularly likely to create spot fires far ahead of the
front (Pyne
et al.
1996
). The extent to which firebrands effectively spread fire is a
function of the fuel type and relative humidity. Fine fuels such as grass do not
produce firebrands as effectively as woody fuels. Spot fires are a function of not
just the proper fuels for creating flying embers but the state of fuels where they
land. Landing in dead fuels is generally required for ignition of spot fires and such
ignitions are greatly affected by relative humidity.
In the absence of substantial winds, flames are nearly vertical and fuels ahead of
the fire front are not preheated by hot gases, making such plume-driven fires far
more dependent on fuel structure (Morvan & Dupuy
2004
). Other factors that
affect plume-driven fires are the fact that such plumes may rise to more than
10 000 m (see
Fig. 1.2
) and contribute to long-distance firebrand transport and
spot fires. In addition, on some landscapes they can create hazardous internal
winds. This commonly occurs when substantial fuel loads contribute to a massive
plume that rises faster than the ambient winds and increases airflow into the fire.
Also, this plume may collapse as the fire moves into lower-volume fuels, causing a
downward rushing of air in every direction and spreading fire rapidly outward
(Mutch
2003
). A similar phenomenon may be driven by rapid cooling as convec-
tional air masses form ice at higher altitudes, generating microbursts of wind
blowing outward at ground level (Pyne
et al.
1996
).
Topography
Steep terrain may act to spread fire much like winds do because the incline
increases the extent to which the flaming front parallels the ground and heats
fuels ahead of the front. Indeed, fire spread roughly doubles for each 13-degree rise
in slope (Green
1981
). In addition, topographic features may cause unstable
changes in velocity and direction as winds adapt to the topography. On coastal-
facing slopes onshore winds may be channeled up-canyon and produce eddies at
