OBJECTIVE
Students will describe the factors that affect fire intensity.
TEACHING STRATEGY
Students use cards to create a fire intensity line.
MATERIALS
Tundra and Forest Fire Component Cards (pdf)
Tundra Fire Fact Sheet (pdf)
Forest Fire Fact Sheet (pdf)
Small pieces of paper and tape
TEACHER BACKGROUND
Fire intensity is a term used to describe
the amount of heat a fire produces - it will be a hot, cool, or moderate
fire
Most people have heard about the fire that burned out of control near
Tok, Alaska, during the summer of 1990. There have been many other fires
in Alaska that firefighters were not able to stop. These fires are often
controlled only after a change in weather helps to cool the fire. What
makes these fires burn so hot? Topography, ground slope, humidity, temperature,
and the amount of fuel all work together to determine fire intensity.
Topography is important when discussing fire intensity. Slopes that
face south, southwest, and west tend to be warmer and drier because
they receive more sun. Fires on these slopes will burn more readily
than fires on north-facing slopes. Fire will burn up a steep slope more
rapidly than on level ground because the fire and heat move up more
quickly and dry out the vegetation.
Moisture in the air and air temperature also affect how fuels burn.
Fires that occur in the spring burn less intensely than fires during
the dry summer months because of lower temperatures and increased moisture
in the soil and air. While rain can cause fires to cool down and lessen
in intensity wind can fan a fire and cause the intensity to increase.
A low intensity fire means that the fire
is burning slowing and is not very hot. These usually occur in moist
areas, in wetter months, low winds, and minimal fuels.
A moderate intensity fire is faster burning
and very hot. They usually occur in dryer months and in moderately dry
conditions. There are adequate fuels to continue the fire.
A high intensity fire is one that burns very
fast and extremely hot. These usually occur in dry months with dry soils
and a large amount of fuels. These fires are very hard to contain and
ignite other areas quickly often traveling great distances.
The amount and condition of available fuels will also influence fire
intensity. There are three basic types of fires: surface, ground, and
canopy (crown) fires. Each burns differently depending on the kind of
fuel present.
A surface fire burns fuels that are on the
ground as well as shrubs and trees. Fuels small in size and very dry
(e.g. branches, bark, broken and downed trees, dead shrubs, etc.) will
cause a fast moving fire. Grass fires generally produce lower temperatures
and burn quickly. A fire through brush such as alder or willow burns
quickly with high temperatures because of the woody fuel. Some shrubs,
such as Labrador tea, have an oily sap that is very flammable. If a
fire burn fast, but without much intensity, the soil and trees are often
not damaged. Surface fires can help keep surface fuels from building
up and will stimulate herb and shrub re-growth.
A ground fire can occur when the duff layer
becomes very dry. Duff is the organic layer of the soil consisting of
decaying leaves and other plant parts, dead branches, and wood. It can
be from a few inches to several feet thick. A ground fire can creep
slowly through the duff, similar to the way charcoal burns. It not only
burns the dead leaves and wood, but will also burn the roots of living
trees and plants. Generally ground fires are of moderate intensity but,
like the charcoal on a grill, can smolder and burn much longer than
a surface fire.
A canopy (crown) fire burns the higher leaves
and branches of trees and shrubs, moving from tree to tree through the
treetops. The worst canopy fires occur in dense forests.
The fire will usually begin in the shrub layer, spreading into the
dry, lichen-covered lower branches and into the canopy. If there is
a strong wind the fire can build very quickly. Large fires can create
their own winds as they use up large amounts of oxygen and as heat from
the fire rises. The winds from these huge fires can flatten stands of
trees and scatter them like toothpicks! These fires are so powerful
they can create their own weather by heating and drying out vegetation.
The moisture rises above the trees and forms cumulus clouds, which can
then cause lightning that starts other fires.
PROCEDURE
- This activity is done after the "Fire Triangle" activity.
Students use the fire triangle they created in that activity.
- Discuss with students the fuels that create surface, ground, and
canopy fires using the "Tundra and Boreal Forest Fire Fact Sheet"
for a reference. Talk about fire intensity and the factors that make
some fires more intense than others.
- Draw a fire intensity line on the blackboard or bulletin board.
Label the left hand side "low intensity fire." Label the
right hand side "high intensity fire."
- Students or groups of students will write the three components of
their fire triangle on a small piece of paper. They then use the "Tundra
and Boreal Forest Fire Fact Sheet" to categorize their fires
as surface, ground, or canopy fires. Finally, they will use the heat,
oxygen, and fuel information given them on the "Tundra and Boreal
Students or groups of students will write the three components of
their fire triangle on a small piece of paper. They then use the "Tundra
and Boreal Forest Fire Fact Sheet" to categorize their fires
as surface, ground, or canopy fires. Finally, they will use the heat,
oxygen, and fuel information given them on the "Tundra and Boreal
Forest Fire Fact Sheet" to determine their fire's intensity.
It may be difficult to distinguish between the intensities of some
fire triangles; each fire has its own individual characteristics and
is subject to many, often rapidly changing, influences.
- Individual students are to read to the class the components of their
fire triangle and the type (surface, ground, or canopy) of fire. They
then tape their triangle (slips of paper) onto the fire intensity
line. Class discussion may occur if there are questions about where
each fire triangle is placed along the line, fires of similar intensity
can be clustered together. Students can add magazine pictures of fire
to make the intensity line more interesting
