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Fire Weather Topics: What 'Fuels'
A Wildfire?
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| Fuels are one of three main components which drive wildfires.
The other two are terrain and weather. Fuels can be split up into two classes
or components: live and dead. An example of live fuels is shown in photo
below and to the left with green shrubs and grasses evident in the landscape
near San Ysidro. The dead component is illustrated in the photo below and
to the right showing an example of a "jungle" of dead sticks
that rats pile up and live in. |
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| The live component is generally related to the growing season
or green-up period. During the course of the year across New Mexico there
are typically one or two green-up periods (spring and summer) with a dormant
period during winter. |
| Typically, during the month of June, fuels across the lower elevations
(see photo to the right) are either cured or curing, that is, drying. This
includes annual grasses, weeds and forbs. Woody shrubs will also observe
lowering moisture levels as intense heating and drying during this period
begins the curing process. |
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A transition occurs in higher elevations where grasses
can still be fairly green although there is some slight drying. The woody
shrubs across the higher elevations tend to be
fairly green (see photo to the left) and typically contain higher moisture
levels. During significant drought years, less moisture is found within the
growing
or live
fuels. This promotes fire spread after lightning or human ignition.
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| The dead fuel component also contributes to fire ignition
and spread. This includes the "duff layer",
which is comprised of several years worth of accumulating needlecast
and
leaf
litter. The photo below on the left shows a combination of leaf litter
and needlecast left, while the photo on the right illustrates a full layer
of only needlecast. |
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The drier and deeper the duff layer, as well as the drier
the soil, the more susceptible this fuel bed is to lightning
strikes.
Fire will smolder for days and even weeks within a dry and deep duff layer.
The photos below illustrate a contrast in that both depict a dry and deep
duff layer, but soil is exceptionally dry in the left image and has some
moisture evident in the right photo. Crackling duff and powdery soil as
indicated by the video below represent a very efficient lightning ignition
source (click on image to start video).
Dry Fuels and Duff Layer, Jemez Mountains
Watch for the blowing dust and dirt.
duration: 18 seconds
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| The dead fuel component is also comprised of wood logs and
twigs. These photos illustrate a "Punky Log", or a dead rotting
log on the forest floor. Look carefully at the close up on the right to
see that
while the outside of the log is dry fuel, there is some moisture in the
middle of the log. When dry, these types of logs are efficient lightning
ignition sources. |
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The dead fuel component does not need to be on the surface
but can also include dead limbs on trees (photo below and left) as well
as "snags", or dead fuel still upright (photo below and right).
Snags,
especially
tall
ones,are
susceptible
to
lightning strikes as they contain ample amounts of dead wood that
lightning can easily ignite. Typically, during the month of June, the dead
fuel component
is
at its driest levels for the year. The drier the dead fuel component, the
more
susceptible this fuel regime is to lightning ignitions. |
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| The moist portion of the monsoon will act to raise moisture
levels within the dead fuels and generally promotes green-up in various
live species such as perennial grasses. The overall effect of the moist
monsoon is a reduction in lightning fire ignitions and a reduction in spread
after ignition. |
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Weather Service Weather Forecast Office
