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
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
SECRET<br />
Relative Humidity: Once the excess water added by rain or dew h<strong>as</strong> evaporated, a different mechanism begins to<br />
control fuel moisture. <strong>Forest</strong> fuels are hygroscopic; at moisture contents below 25-30 percent they gain or lose moisture<br />
directly to the surrounding air without any free moisture such <strong>as</strong> rain or dew being present. The water exchanged between<br />
air and wood at moisture contents below 25 percent is known <strong>as</strong> bound water because it is chemically bonded to the cell<br />
walls of the fuel.. The amount of bound water in forest fuels is controlled primarily by the relative humidity of the surrounding<br />
air, with air temperature having a strong influence. Because of chemical and physical differences in the structure<br />
of various forest fuels, each fuel type h<strong>as</strong> a different relationship between its moisture content and the air's relative humidity.<br />
Figure 32 shows the relationships between fuel moisture content and relative humidity for several common forest<br />
fuels. The curves in Figure 32 represent the moisture content that the fuel would reach if exposed for an indefinite<br />
period of 70 0 F. and the specified relative humidity. The curves can be corrected for temperature by multiplying<br />
the fuel moisture by I - (<br />
7<br />
0 ). For example, the curve shows pine needles to have a fuel moisture of 9 percent<br />
when the relative humidity is 60 percent and the temperature is 70 degrees Fahrenheit. The fuel moisture at 60 percent<br />
humidity and 100 degrees would be 9 X I - 30/100 or 9 X 0.7 - 6.3 percent.<br />
Because bound water is chemically bonded to the fuel, energy is required to remove it from the fuel surface and<br />
energy is rele<strong>as</strong>ed when water is absorbed from the air into the fuel. For this re<strong>as</strong>on, the relationship between fuel moisture<br />
and relative humidity for a particular fuel is different when the humidity is rising (late afternoon, for instance) than<br />
it is when it is falling (in the morning). Figure 33 shows a typical set of curves.<br />
<strong>Forest</strong> fuels do not react instantaneously to changes in temperature or humidity. Their rate of change in moisture<br />
content depends on the chemical and physical properties of the fuel particle, particularly its thickness, on the magnitude<br />
of the change in humidity, and on whether the fuel is adsorbing or desorbing moisture. Table 9 shows the time required<br />
for common forest fuels to reach approximate equilibrium when the relative humidity is varied between 20 percent and<br />
80 percent.<br />
Table 9<br />
Timelags of Common <strong>Forest</strong> Fuels<br />
Timelag (hours)<br />
Adsorption Desorption<br />
Fuel (20%- 80% RH) (80%- 20% RH)<br />
Gr<strong>as</strong>s 3/4 1/2<br />
Shrub or Hardwood leaves 5 4<br />
Conifer Needles 7 5<br />
1/2-inch Twigs 30 25<br />
Because of the important, but complex, relationship between fuel moisture and changes in atmospheric humidity,<br />
accurate prediction of forest flammability requires knowledge of the daily humidity pattern, not just a single mean or<br />
instantaneous value. If a single value must be used for planning purposes, the daily minimum humidity is the most useful;<br />
it gives a good indication of whether or not fuel moisture will drop low enough to permit fires to spread. Incendiary success<br />
is virtually impossible unless the minimum humidity is below 70 percent, and conditions are marginal unless minimum<br />
humidity drops below 50 percent. The minimum humidity value alone given no information on how long fires will continue<br />
to spread, once ignited.<br />
SECRET 46