Useful definitions

15 Useful Definitions

Illustration of aspect.
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Aspect: The direction of the slope. Generally, a south or southwesterly aspect presents the greatest hazards from wildfire. The aspect determines the intensity of solar heating and rate at which wildland fire fuels might be expected to dry out during the fire day. Aspect also affects what grows naturally on a slope. 

Canopy: Woody upper layer of a forest formed by mature tree crowns.

Conifers: Woody plants. Typical conifers include cedars, Douglas-fir, cypresses, junipers, redwood and pines. There are over 600 living species within the conifer family.

Ground Fuels.
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Crown Fuels: Crown fuels are what are above the stem of the plant – the branches, twigs, needles, and leaves, often called the overstory. Separating crowns from each other gives each plant its own piece of the sky. Cleaning the crown of dead and dying vegetation is important in keeping potential flame height and ignitability low.

Deciduous: Shedding or losing leaves annually – the opposite of evergreen.

Dominant Vegetation (Eucalyptus)
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Dominant Vegetation: Vegetation type determines fire behavior – the flame length, intensity, and rate of spread – and can be predicted through measurement of fuel type.

Ground Fuels: The low ground fuels are grasses, dead branches on the ground, shrubs and brush. This is often referred to as the understory. Removing ground fuels cools a fire and disallows its spread.

Hardscaping is the use of rock and other non-combustible hard surfaces such as concrete sidewalks, brick patios and asphalt driveways.

Home Ignition: There are three forms of ignition:

Direct flame impingement.
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1) Direct flame impingement (convection), in which flames overwhelm a structure. This results from having no defensible space.

Radiant Heat.
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2) Radiant heat, when objects nearby are so hot that they provide sufficient heat for the home to burst into flames. This possibility is greatly reduced as space between fuels is increased.

3) Embers, which can come as a blizzard and ignite any available light fuels such as needles or dry vegetation.

Vents with minimal opening help reduce structual flammability.
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Non-combustible roofing materials help reduce structual flammability.
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Ignition-Resistant Construction (fire hardening):  These construction methods or components increase exterior building ignition resistance to a wildfire. Non-combustible roofing materials, double window glazing, vents with minimum openings, fire-resistant siding, and other materials will help to reduce structural flammability. Building or remodeling with these materials adds to overall survivability when done in combination with vegetation management measures for defensible space.

Ladder fuels leading
to crowns.
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Ladder Fuels: Fuels that provide vertical continuity between layers of vegetation, thereby allowing fire to travel from surface fuels into the crowns of shrubs and thence into trees. Ladder fuels initiate and assure the continuation of crowning (crown fire) during wildland fires.

Overstory: The upper crowns or canopy of a forest.

Prevailing Wind: Many wildland fires are wind-driven. Knowing the direction of prevailing winds and how they might behave is crucial for wildland fire protection planning. Typically, in the late summer and fall, hot, dry northeasterly winds occur. Northeast winds have been a major factor in virtually every California firestorm.

Pyrophytic, “fire loving” fuels.
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Pyrophytic Species: Literally, “fire loving” vegetation which is adapted to or which contributes to rapid burning, high heat output, and ember creation.

Many brush species – such broom, manzanita, coyote bush, and juniper – are highly flammable and burn with an oily heat. Anything that smells when crushed has oils which volatize and burn readily, including bay, fir and eucalyptus trees.

Shrubs: Woody plants. Distinguished from a tree by its multiple stems or lower height, usually less than 15-20 feet tall.


Steep slope.
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Slope: The steeper the slope, the greater the rate of fire spread. The critical slopes are those downhill of the structure because these pose the greatest threat. Slope is measured as rise over run. Thus a 20 foot vertical drop in 100 feet of horizontal distance is a 20% slope.

Eucalyptus understory.
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Understory: The area of a forest that grows in the shade of the forest canopy.

Alta Heights, where wildland meets homes.
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Wildlife Urban Interface (WUI): The wildland/urban interface is any location where a fire can spread from vegetation (wildland fuels) to buildings (urban fuels), resulting in multiple house fires that overwhelm fire protection efforts.

Napa Firewise

  To empower the citizens of Napa County with the information, knowledge, and support they need to survive a wildfire.