(upbeat music)
- In this video you will learn about fire behavior
and why an understanding of fire behavior is critical
for fire managers.
You will learn:
One, how is fire behavior related to the fire environment?
Two, what are the basic measures of fire behavior?
Three, what are the parts of a fire?
Four, what are the types of fire
and their associated fire behavior?
Five, what are the three types of crown fires?
One, how is fire behavior related to the fire environment?
The three principle environmental elements
affecting wildland fire behavior,
which are found in the fire environment triangle,
are fuels, weather, and topography.
The changing states of each element
and how they interact with each other,
determine the behavior of a fire.
For example, wind speed influences
how fast fire will spread,
and the amount of fuel available to burn
influences how hot a fire will get.
Two, what are the basic measures of fire behavior?
To understand the relationship between the fire environment
and fire behavior,
we first need to understand some basic measures
of fire behavior.
I will introduce four basic concepts in this video
that are used by fire managers to describe fire behavior.
Rate of spread, fire line intensity, flame length,
and flame height.
Let's start with rate of spread.
Rate of spread is basically how fast the flaming portion
of the fire is moving.
This is expressed as an average distance over time,
such as feet per minute or chains per hour.
A chain is a popular and convenient unit of measurement
used to describe distance,
with one chain equal to 66 feet,
80 chains equal to one mile,
and 10 square chains equal to one acre.
The rate of spread is different for different parts
of a fire, which is explained further later in this video.
All parts of the fire environment triangle
can influence the rate of spread.
For example, high wind speeds, steep slopes,
and low fuel moistures can lead to rapid rate of spread.
More details on fire environment are introduced
in other videos.
There are several ways to measure the intensity of a fire,
but we will only introduce fire line intensity here.
It describes the rate of heat transfer per unit length
of the fire line,
which is expressed as kilowatts per meter,
or BTUs per foot per second.
A BTU, or British Thermal Unit,
and how it is used in intensity equations,
is further discussed in later videos.
Characterizing the intensity of both prescribed
and wildland fires is important information
for fire managers to prepare for expected fire effects
and suppression planning.
Again, all legs of the fire environment triangle
can influence fire intensity.
For example, low relative humidity and heavy fuels
can lead to high fire intensity.
The last two concepts I will introduce are flame length
and flame height, which are commonly used as surrogates
to describe fire line intensity,
because they are easier to observe and measure in the field.
The flame length is the distance between the flame tip
and the base of the midpoint of the flame at ground level.
Flame height is only measured at the leading edge
of the front of the fire,
and is the average maximum vertical extension of the flame.
All legs of the fire environment triangle
can influence the flame height.
For example, one would expect high flame lengths
on steep slopes with tall vegetation,
high wind and low relative humidity.
Three, what are the parts of a fire
and how do they behave?
Most wildfires start at a single point called the origin,
and spread most rapidly in the direction of the wind
or uphill, generally forming the shape of an ellipse.
The fire perimeter is the edge, or boundary,
around the fire.
The fastest spreading part of the fire is called the front,
or head, and a fire spreading with the wind is called
a head fire.
The slowest spreading part of the fire, opposite the head,
is called the back, or rear.
A backing, or heel fire is normally moving into the wind
and/or down slope.
The size of the fire, or flanks, will spread
at intermediate rates and burn outward at right angles
to the wind or slope, and this is called a flanking fire.
It is important to note, that the behavior of backing
or flanking fires can change quickly with the wind shift,
changing the slow-rate-of-spread flanking fire
into a fast-rate-of-spread head fire.
Fires can take other shapes depending on fuels,
topography, and weather.
Fingers are the long narrow extensions of a fire
projecting from the main body.
The unburnt indentations in the fire perimeter formed
by fingers are called pockets.
An island is an area of unburned fuel
inside the fire perimeter.
And a spot fire is a new fire ignited
outside the main fire perimeter.
Four, what are the types of fire
and their associated behaviors?
There are three types of fire,
ground, surface, and crown.
Ground fires spread primarily by smoldering combustion
through subsurface fuels, including deep duff, peat,
roots, rotten buried logs, and other organic material.
Most fires have a ground component to them, when duff burns.
Ground fires have a slow rate of spread.
An ecosystem that supports ground fires is swamp land
in Florida.
Surface fires spread by flaming combustion
through fuels at, or near, the surface,
including needles or leaves, grass, woody debris,
small plants, and shrubs.
Surface fires have a higher rate of spread
than ground fires.
The Ponderosa pine forest type supports surface fires
under normal conditions.
Crown fires burns through shrub and tree crowns
and canopies.
They are fast spreading and release an enormous amount
of heat and energy over a short period of time.
Ecosystems that support crown fires include chapparral,
lodgepole pine, and spruce fir forests.
It is important to note that these fires
generally do not occur in isolation.
For example, a surface fire will often burn
through surface and ground fuels, and in a crown fire,
all three fuel strata will often burn simultaneously.
Five, what are the three types of crown fires?
There are three types of crown fires,
passive, active, and independent.
A passive crown fire involves the burning
of individual trees or small groups of trees.
An active crown fire moves consistently from crown to crown
and is sustained by the intensity and speed
of the surface fire below it.
An independent crown fire spreads
through the crown fuel layer without the aid
of a supporting surface fire.
However, it is thought that independent crown fires
rarely occur.
No matter the type of fire, the rate of spread,
intensity, and flame height will be determined
by the three legs of the fire environment triangle,
fuels, weather, and topography.
And now, let's review.
One, the three principle environmental elements
affecting wildland fire behavior are fuels, weather,
and topography.
Knowledge of these elements is critical
to understanding how a fire will behave on the landscape.
Two, the four basic measures that describe fire behavior
are rate of spread, fire line intensity,
flame length, and flame height.
Three, the front, flank, and rear
of a fire behave differently,
and fire behavior can change quickly
with a change in wind direction or slope.
Four, the three types of fire are ground, surface,
and crown.
Five, there are three types of crown fires,
passive, active, and independent.
To conclude, the behavior of all types of fire is controlled
by all elements of the fire environment triangle.
(upbeat music)