Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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Flammability of shrub types depends on the amount of litter plus dead stems, twigs and leaves, as well as on flam.
inability of the green leaves and twigs. In a mature stand of shrubs, the amount of dead material is largely a function of the
climate. Shrub types are most flammable in climates where periodic droughts or extreme cold kills many branches, or
entire plants, and where a layer of litter has developed under the shrubs. In this situation evergreen shrub types will burn
naturally after 3-5 consecutive weeks of low rainfall averaging less than 1/4 inch per week. In most situations, however.
burning of shrub types is not successful unless the amount of dead material has been increased by killing and desiccating
the plants, through treatments such as mechanical crushing of the shrubs or spraying them with herbicides.
Simple systems relating fire behavior characteristics to fuel weight and weather cannot be prepared for shrub types
as a whole. The types vary too widely in their physical characteristics, and in their yearly moisture trends as related to
soil moisture and physical processes.
The manner in which air temperature, relative humidity, and windspeed affect the approximate relative rate of fire
intensity is shown in Tables 1, 2, and 3 for a single situation. This is an evergreen shrub type-with sufficient litter
and other dead material to be flammable without desiccation treatnment-2 months into the dry season. The numbers are
relative index values; that is, a fire will burn twice as intensely at an index of 40 as it will at an index of 20.
Table I
Shrub Burning Index
Wind 0 - 10 miles per hour
Relative
Humidity
Temperature (OF)
(%) 40 50 60 70 80 90 100 110
10 9 I1 12 14 15 17 18 20
20 3 5 7 9 10 12. 14 15
30 2 3 5 6 7 9 11 12
40 1 2 3 4 6 7 9 10
50 0 1 2 2 3 5 7 8
60 0 0 0 1 1 2 3 6
70 0 0 0 0 1 1 2 5
80 0 0 0 0 0 1 2 4
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