214
R. Il-4IJBENMIRE
I n the early stages of grass fires surface wind velocity, temperature
and relative huniidity are doininant factors determining the direction
and r:it.e of sprwtd. A lit,tle IiLter R strong convection column typically
becomes domixiant with wind and relative humidit,y diminishing to a
minor role. Then RS the t)urning fruiit widens, the direction and velocity
of wind again becomes important, (MrArthur, 1960). Owing to the
effects of gust.y winds, expressions of fire behavior uwally represent
averages over a period of time.
The amount and rate of heat released as vegetation burns depends
on three classes of factors.
(1) Weather conditions leading up to and those prevailing a t the time
of burning determine the dryness of t,he fuel, moistness of soil (as this
provides a licat sink arid a t the same time facilitates heat conduction),
current temperature of t,lie combustible ma.teria1 (as this determines
the addit.iona1 heat required for expelling and igniting gases), and wind
velocity. Thoroughly dry grass in Venezuela may ignit,e when its temperature has been raised to only 1 2 V C (Vareschi, 1962). Wind tends to
have a cooling effect on an open fire (Masson, 1949), but this may be
more than offset by its bringing in fresh supplies of oxygen which speeds
up the rate of combustion and so increases temperatures (Byram, 1948;
Morton, 1964). Owing t o increased oxygen supply, backfires, those
spreading against the wind, move somewhat faster into a strong wind
than into a light wind, but the effect of wind incretlsing the rate of spread
is much greater in a head fire, i.e., one spreading with the wind (Byram,
1958). The higher the moisture content of grass fuel the greater the
wind velocity required to sustain a fire or to carry it at a given rate
(McArthur, 1!#63). Grass fuels commonlv dry sufficiently to carry fire
within a few hours after (I shower, although the intensity of burning
under such circurnRtanceu i s minimal. In Australia it was found that as
relative humidity brings t,he moisture content of dry grass up to 9%
the incidcrico of fire in the region diminishes, and when it rises to
13-14%, few grass fires get started (McArthur, 1966). The moisture
content of dry grass fluctuates daily with a minimum about 15.00 h
and a maximum between 05.00 and 06.00 h. Fires start more readily
st the time of maximum intensit.y of solar radiation (approximately
11 .OO h) rather than at the time of minimum moisture content of the
grass.
(2) Topography affects the character of a grass f i e since fies move
faster upslopo than on level ground, advancing still more slowly when
moving downslope. Fire spreads up a 10" slope twice aa fast aa on the
level, and progresses four times as fast up a 20" slope. Where the slope
is very steep, the rate of spread may exceed 65 km/h (McArthur, 1963).
The direction the slope faces is important through its control over
R. Il-4IJBENMIRE
I n the early stages of grass fires surface wind velocity, temperature
and relative huniidity are doininant factors determining the direction
and r:it.e of sprwtd. A lit,tle IiLter R strong convection column typically
becomes domixiant with wind and relative humidit,y diminishing to a
minor role. Then RS the t)urning fruiit widens, the direction and velocity
of wind again becomes important, (MrArthur, 1960). Owing to the
effects of gust.y winds, expressions of fire behavior uwally represent
averages over a period of time.
The amount and rate of heat released as vegetation burns depends
on three classes of factors.
(1) Weather conditions leading up to and those prevailing a t the time
of burning determine the dryness of t,he fuel, moistness of soil (as this
provides a licat sink arid a t the same time facilitates heat conduction),
current temperature of t,lie combustible ma.teria1 (as this determines
the addit.iona1 heat required for expelling and igniting gases), and wind
velocity. Thoroughly dry grass in Venezuela may ignit,e when its temperature has been raised to only 1 2 V C (Vareschi, 1962). Wind tends to
have a cooling effect on an open fire (Masson, 1949), but this may be
more than offset by its bringing in fresh supplies of oxygen which speeds
up the rate of combustion and so increases temperatures (Byram, 1948;
Morton, 1964). Owing t o increased oxygen supply, backfires, those
spreading against the wind, move somewhat faster into a strong wind
than into a light wind, but the effect of wind incretlsing the rate of spread
is much greater in a head fire, i.e., one spreading with the wind (Byram,
1958). The higher the moisture content of grass fuel the greater the
wind velocity required to sustain a fire or to carry it at a given rate
(McArthur, 1!#63). Grass fuels commonlv dry sufficiently to carry fire
within a few hours after (I shower, although the intensity of burning
under such circurnRtanceu i s minimal. In Australia it was found that as
relative humidity brings t,he moisture content of dry grass up to 9%
the incidcrico of fire in the region diminishes, and when it rises to
13-14%, few grass fires get started (McArthur, 1966). The moisture
content of dry grass fluctuates daily with a minimum about 15.00 h
and a maximum between 05.00 and 06.00 h. Fires start more readily
st the time of maximum intensit.y of solar radiation (approximately
11 .OO h) rather than at the time of minimum moisture content of the
grass.
(2) Topography affects the character of a grass f i e since fies move
faster upslopo than on level ground, advancing still more slowly when
moving downslope. Fire spreads up a 10" slope twice aa fast aa on the
level, and progresses four times as fast up a 20" slope. Where the slope
is very steep, the rate of spread may exceed 65 km/h (McArthur, 1963).
The direction the slope faces is important through its control over