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

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SHumid,
long bum season (HULB)-5, or more, "'B" months, or yearly (-) value of 26, or more; (for yearly (+) values
see Table 5). Most locations have 3 to 9 "H" months and I to 7 "D" months.
Nonhumid, long dry season (NHLB)-5, or more, "B" months, or yearly (-) values of 26, or more; yearly (+) value
at least I but too low for humid (see Table 5).
JDry yearlong (DRYL)-Most months designated as "B'; no yearly (+) value (see Table 5). No months are "H"; all,
or most, are "D".
The climate at any location can be readily classified by use of the identification key in Table 5. For example, the
coldest 3 months at Missoula have an average mean temperature of 25°F and an en-rage monthly lowest of -2°F. This
puts the location in the long cold (LC) winter. From Table 4, 2 months have (-) values with a yearly total of-7.9, and
yearly (+) values total +24.2-the climate is LC-NHSB (Long cold winter; nonhumid, short burn season).
The climate at any location will fall into one of the 42 possible climatic situations shown in Table 5. A number of
these situations seldom occur at locations for which climatic records are available, or they will occupy only limited geo.
graphic area. Appendix B lists for the most important climates-or combinations of climates-the typical vegetative cover,
litter weights, desiccation requirements, burning conditions, and representative locations.
Fuel Characteristics
Fu.-l and air are the two factors that determine whether and how a forest fire will burn. In the two preceding sections,
we have discussed forest fire behavior in terms of vegetation and climatic types. But these are'merely convenient
descriptors for generalizing a large number of more basic fuel and atmospheric factors. In order to have a real understanding
of forest fire behavior, it .s necessary to examine forest combustion, not in terms of biology or geography, but asa
physical-chemical process.
To oversimplify only slightly, combustion of wood is a chemical chain raction that is the exact reverse of the pro.
cess of photosynthesis which originally produced the wood. In the combustion of wood, cellulose (C 6 H 0 0 5 ) and oxygen
combine to produce carbon dioxide, water and excess energy in the form of heat. This process takes place in three
distinct phases: charring, flaming, and glowing. To examine the process more closely, lets look at what happens when we
put a lighted candle underneath the Christmas tree, too close to one of the branches.
First, heat from the burning candle is absorbed at the surface of nearby needles and twigs. This absorbed heat is
transfered by conduction into the interior q•f the needle or twig, and heats the woody material beneath the surface. The
rate of heat conduction varies inversely with the density of this material and directly with its moisture content. The temnperature
of the wood rises as heat moves from the surface inwards. Once a layer begins heating, the moisture in that layer
-starts to vaporize and is driven off into the air as steam; When the temperature has risen further, the wood itself begins to
d-.ompose into carbon charcoal and a complex mixture of flammable gaseous hydrocarbons which are also driven off, to.
gether with the steam coming from the deeper, cooler layers of the needle or twig. Soon the surface layer consists of nearly
pure charcoal. This is the charring phase of combustion.
Once the charring process has reached deeply enough into the twig so that the vapors driven off contain more hydro.
3 carbon than steam, then the vapors themselves will burn as they mix with oxygen in the air around the twi& This flaming,
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