Fire Behavior - 1st Article in a Series

Posted by Jeff Saley (photo credit: Ray Ford) -

How fire "behaves" - or misbehaves - is due to some very basic physical and chemical processes. We will try not to bore you with this short explanation of these basics, but it's important to you as a property owner in planning what to do with your property to make it more fire resistant and defensible.

Combustion and Heat Transfer

Fire releases heat through combustion. Combustion is a physical and chemical process that unleashes solar energy stored in fuels as heat. Fuels can be anything flammable, however in a wildland fire it is commonly the surrounding vegetation. In a wildland fire in an urban interface environment like Montecito, the fuels can also include your home.

Combustion is supported when fuel, heat and oxygen (air), also referred to as the "fire triangle", combine in just the right amounts. For a fire to grow and spread, heat must be transferred to surrounding fuel through convection, radiation or conduction. Conduction is the transfer of heat by contact; radiation is the transfer of heat by contact and through the air; and convection is the transfer of heat by (electromagnetic waves through the air);  contact, through the air and by direction (rising).

As wildland fires spread, radiation and convection are the primary methods of heat transfer from one fuel to another.

Have I still got your attention? There will be a quiz at the end of this article....

How Fire Spreads

The four general ways of propagation by which a fire can spread from one area to another are:

• Ground fires - restricted to the layer of duff, roots, and buried or partially buried dead and decaying logs - very much like most of us have in our yards adjacent to our houses.


• Surface fires - propagate through fuels less than six feet high, which are commonly small trees, shrubs (including decorative), herbaceous vegetation and litter.


• Crown fires - advance through the aerial strata of the fire environment more or less independently of surface fires. In Montecito this phenomenon can occur in Oak and Eucalyptus groves, especially when the canopies of trees are close to one another.


• Spotting - occurs when firebrands or embers are transported by wind beyond the main perimeter of the fire. Embers can be carried 1-3 miles in a wind driven fire. Examples of this type of spread are all too vivid in our memories of the recent Tea and Jesusita fires.

Wildland fires that occur in the Montecito area usually spread as a result of a combination of all of these.

How Fire Grows

Measuring or estimating flame height and length can help understand fire intensity and predict fire effects.
Under real life conditions the fire's shape and rate of spread will continually change. Fuel, weather and topography combine to influence a fire's rate of spread and overall behavior.

Flame Characteristics

Flame height is the vertical distance from the flame base or ground to the yellow flame tip.
Flame length is measured from ground level from the mid-point of the base of the flame to the tip of the flame, whether the flame is vertical or slanted.

In Montecito, flame lengths may vary from less than 1 foot in a slow moving "backing fire" in leaves and short grass to over 100 feet in extreme wildfire conditions in dense shrubs and Oaks. (A "backing fire" is one that is spreading into - or against - the wind or down slope.)

Please be patient - we're almost there!

Fire Danger Rating

Fire danger describes how easy it is to ignite vegetation, how difficult a fire may be to control and how much damage a fire may do. Rating systems produce qualitative and or numeric indices of fire potential based on fuels, topography and weather. These rating systems allow fire managers to estimate present and future fire danger for a given area.

The four fire danger rating signs in Montecito are located at Mountain Drive and Coyote Rd, Coldsprings and Sycamore Canyon Rd, San Ysidro Rd and E. Mountain Drive, and on Romero Canyon Rd. They are maintained daily and present the current fire danger rating for the Montecito area.

Watch for our next article in the Montecito Journal - Defensible Space!

Now...time to see how much of this you remember.

Short Quiz (fill in the blanks)
1. The three legs of the fire triangle are _______, _________ and _________.
2. In Montecito, crown fires can occur in ________ and __________ groves, especially when the canopies of these trees are close to one another.
3. When firebrands and embers are transported naturally by wind beyond the main perimeter of the fire, the resulting method of spread is called ________.
4. In the Montecito wildland fire environment, flame lengths vary from less than _______ft. to over ________ft. depending on fuel, weather and topography.
5. Public display signs measuring the current fire danger rating are located at what four locations in Montecito?

‹‹ Return To Index