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

•Sunshine:
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Solar radiation is an important factor in determining the moisture content of forest fuels. The effect is
primarily indirect. Forest litter lying in the sunshine is heated to temperatures as much as 70 degrees higher than that of the
surrounding air. Consequently, the equilibrium moisture content is much lower than onewould calculate from the temper.
ature and humidity of the air aone. The difference between the actual and the calculated fuel moisture depends on the duration
of exposure to sunshine, and on the intensity of solar radiation. A good rule of thumb for middle latitudes is that fuel
moistures of fully exposed materials at midafternoon on a clear day will be one-half those calculated from atmospheric
temperature and relative humidity. The pine needles of the example on page 80 would have a moisture content below
3-1/2 percent if they were out in the open on a calm, sunny afternoon. They would be even drier at the end of a long
summer day at high latitudes, or exposed to the intense sunlight of the tropics. Whenever possible, incendiary operations
should be limited to days when cloud cover is 3/8 or less.
Except in forest openings, litter is seldom exposed to full sunlight for prolonged periods. The very overstory trees
and shrubs that provided the litter act to shade it. One of the primary reasons for partially defoliating shrub and hardwood
stands prior to burning is to remove the shade provided by the foliage and let solar radiation down to the ground litter.
Solar radiation also has a slight direct effect on fire behavior. The intensity of solar radiation on a sunny afternoon
in the tropics adds about 20 percent to the radiation received 2 feet from a ground fire in pine needles This effect, although
too slight to proxduce a statistically significant difference in rates of spread, is sufficient to make a visually noticeable difference
in fire intensity if a cloud passes in front of the sun when a small ground fire is burning
Wind: Wind also affects forest fuel moisture, although its greatest influence on forest fire behavior is a direct one.
Wind serves to mix the air in the layer immediately around the fuel, removing moisture when fuels are drying and bringing
in fresh moisture when fuels are absorbing water vapor. Wind also reduces the difference between the reported air temperature
and the temperature at the fuel surface when fuels are exposed to sunlight. Fuels in the shade dry faster on windy
days. Fuels in full sunlight dry more slowly when the wind is blowing.
In addition to its effect on fuel moisture, wind has a major effect on heat transfer within the fuel bed. Wind increases
flame length and tilts the flames forward into unburned fuel thus greatly increasing the rate of radiation transfer
between flame and fuel. At the same time, wind also moves hot gases into the fuel downwind of the fire. By increasing
the amount of energy transferred directly to the fuel, wind effectively increases combustion efficiency. Rate of spread
varies exponentially with the velocity of the wind at flame height. When the wind at flame height is 7 knots, a fire moves
about 5 times as fast as it does when the wind is 2 knots. Sloping ground has the same effect as wind; the flames are tilted
up the slope. For a fire spreading up a slope, the rate of spread will approximately double for each 15 degree increase in
slope.
. Slopes and other topographic features also affect surface wind patterns. This subject is well covered by Buck, C. C.,
Winds over wildiands - A guide over forest management. USDA-Forest Service. Agric. Handbook No. 272,33 pp.,
lius. November 1964.
1& The entire preceding discunaon of weathervariables is primarily applicable to surface fires in their initial stages. Weather
effects on large ;ntense fires are much more complicated. An excellent reference is Countryman, C. M. Project Flambeau...
An investigation of Mass Fire (1964-1967). USFS Pacific Southwest Forest and Range Experiment Station
118 pp., •L June 1969.
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