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

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In developing recommended practices for burning, we have used rules of thumb and simple classifications and descriptions
of world climates. The descriptions include those weather elements most important in predicting probability of good
burning weather and in selecting dates for desiccation treatments.
Probability of Incendiary Success
When considering forest incendiary operations for any area of the world, the first climatic determination should be
whether the locality will support fires in nearby forests at any time of year. This can be judged quite quickly from the
climatic records. Find the driest month and the warmest month. Then calculate whether either month meets both of the
following conditions:
I. The mean monthly precipitation (inches) is less than the mean daily temperature for the month (OF.) times 0.03.
2. The number of rainy days (0.1 inches or more) is less than the mean daily temperature divided by 10.
If both conditions are not met for either the warmest or the driest month, then the location should not be seriously
considered as a suitable target for forest incendiary operations. This does not mean that suitable weather for burning may
not occur occasionally, and such occasions should be taken advantage of on an opportunistic basis. But the opportunities
for successful burning will be too infrequent for inclusion in planned operations. For example: The driest month in Washington.
D.C. is November, with a mean temperature of 47 degrees and 2.6 inches of precipitation occurring on 5.9 rainy
days. The warmest month is August, with a mean temperature of 75 and 4.3 inches of rain on 7.5 days. Neither month
meets our criteria (November would have to have less than 1.4 inches and fewer than 4.7 days; August has few enough
rainy days, but must have less than 2.25 inches of rain). Consequently, any guerrilla forces in the woods surrounding the
Pentagon will have to be routed by some other means than fire.
On the other hand, Missoula, Montana, has its driest month in February with a mean temperature of 27, and 0.9
inches of precipitation on 4.4 days. Its warmest month is July: mean temperature 68, precipitation 1.0 inches on 3.7
days. February does not fit the criteria, but July does; so if military operations were being conducted in the vicinity of
Missoula, the use of fire should be explored more carefully.
Once it is decided that an area can support planned incendiary operations over forests, the next step is to determine
the frequency and extent of periods when successful burning is likely. Figure 26 is a plot of mean monthly temperature
versus the PD value (mean monthly precipitation times the number of days with precipitation Z 0.1 inch). The diagonal
line represents the 50/50 probability that a day chosen at random during the month will have weather suitable for forest
burning. A point above the line means the chances are less than 50 percent-the greater the plus (+) value between the
point and the line the poorer the odds. Likewise, the greater the negative (-) value below the line the better the odds for
occurrence of a suitable burning day. When all 12 months of climatic data are calculated for a location, the result is a good
picture of the forest fire possibilities during the year.
The climatic data for Missoula are in Table 4 and Figure 27. The plotted points show a better than even chance of
weather favorable for incendiary operations from early July through early September-and little chance the rest of the
year. The data show (-) humidity values of 4.0 and -3.9 for the two months of good burning weather-July and August.
They also show that weather most favorable for growth of green vegetation occurs during May through June-when (+)
values are highest. The decision as to whether 2 months of summer burning weather will justify the planned use of forest
incendiaries will depend on the projected military situation, and on the posibilities for drying the green vegetation ahead
of burning.
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