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