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

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