Low
Fires are not easily started. Fuels do not ignite readily from small
firebrands, although a more intense ignition source, such as lightning,
may start many fires in duff or punky wood. Fires in open cured
grassland may burn freely a few hours after rain, but fires burning in
forested areas spread slowly by creeping or smoldering, and burn in
irregular fingers. There is little danger of spotting.
Moderate
Fires start easily and spread at a moderate rate. Fires can start from
most accidental causes, but with the exception of lightning fires in
some areas, the number of starts is generally low. Fires in open-cured
grassland will burn briskly and spread rapidly on windy days. Woods
fires spread slowly to moderately fast. The average fire is of moderate
intensity, although heavy concentrations of fuel may burn hot.
Short-distance spotting may occur, but is not persistent. Fires are not
likely to become serious, and control is relatively easy.
High
Fires start easily and spread at a fast rate. All fine dead fuels
ignite readily and fires start easily from most causes. Unattended
campfires are likely to escape. Fires spread rapidly and short-distance
spotting is common. High- intensity burning may develop on slopes, or in
concentrations of fine fuel. Fire may become serious and difficult to
control unless they are hit hard and fast while small.
Very High
Fires start very easily and spread at a very fast rate. Fires start
easily from all causes, spread rapidly and intensify quickly. Spot fires
are a constant danger. Fires burning in heavy fuels may quickly develop
high-intensity characteristics, such as long-distance spotting and fire
whirlwinds. Direct attack at the head of such fires is rarely possible
after they have been burning more than a few minutes.
Extreme
The fire situation is explosive and can result in extensive property
damage. Fires under extreme conditions start quickly, spread furiously,
and burn intensely. All fires are potentially serious. Development into
high-intensity burning will usually be faster and occur from smaller
fires than in the very high danger class. Direct attack is rarely
possible, and may be dangerous, except immediately after ignition. Fires
burning in heavy slash or in conifer stands may be unmanageable while
the extreme burning condition lasts. Under these conditions, the only
effective and safe control action is on the flanks until the weather
changes or the fuel supply lessens.
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