Fire Behavior

Overview

In this lesson, we will learn about fire itself.  What is fire and what factors influence how it burns?  Building on the lesson on fire ecology, students will perform an experiment to compare how different types of fuels influence fire behavior.

Learning Outcomes

Students will know the following:

• The three elements of the fire triangle
• The distinction between fire intensity and severity

Students will understand the following:

• How environmental conditions influence wildfire behavior
• How different fuels burn in wildfires

Students will be able to do the following:

• Develop a hypothesis regarding an experiment based on past results

Essential Questions

• What is fire?
• What are the factors that impacts how it burns through a landscape?

NextGen Science Standards

• MS-PS1-2 Matter and its Interactions
  • Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• MS-PS1-4 Matter and its Interactions
  • Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

Materials

• Grill (at least 12” x 12”)
• 2 handfuls of small twigs (<1/4 inch in diameter)
• 2 handfuls of large twigs (>1/4 inch in diameter)
• 1 handful of green needles
• 2 handfuls of dry needles
• 4 paper bags
• 6 matches
• Glass jar/cup
• Stopwatch
• Scale
• Fire extinguisher

Glossary

• Aerial fuel: Fuels located above ground level
• Combustion: The chemical process of burning
• Fine fuels: Fuels less than a ¼ inch across
• Fire intensity: How much energy is released during combustion
• Fire severity: The environmental impact of fire on an ecosystem
• Fire triangle: The three components a fire needs to burn: pilot heat, oxygen, and fuel
• Flaming: The stage of combustion that releases a visible flame rising above the fuel
• Fuel: Material that can burn
• Ground fuel: Fuels located in the soil
• Heavy fuels: Fuels more than ¼ inch across
• Herbaceous plants: Plants, like grasses, that have flexible green stems and tend to die back in the winter
• Hypothesis: An educated guess of how a phenomenon or pattern happens, which is tested with an experiment
• Ignition: The process of starting combustion
• Pilot heat source: A heat source necessary for ignition of fuels
• Smoldering: The stage of combustion when the combustion is localized to the fuel surface
• Surface fuel: Fuels located on the forest floor
• Topography: The physical layout of the land, including slope, elevation, direction, and shape
• Woody plants: Plants with a hard stem and survive above ground in the winter, including trees and shrubs

Activating Strategy

Time: 5 minutes

Pair up students and have the pairs brainstorm factors that influence fire for two minutes.  Then, have the students share what they brainstormed with the whole class, writing the unique factors on the board.

Learning Approaches

Time: 45 minutes

In the last lesson, we learned about the influence fires have on ecosystems.  But what is fire and how does the environment influence it?  Fire is a rapid and persistent chemical reaction, which is to say, it starts fast and keeps going.  It combines fuel and oxygen to produce heat and light.  An external heat source is usually needed to start the reaction.  These three components (fuel, oxygen, and pilot heat source) make up what is called the fire triangle (Figure 3).  Which of the factors you all brainstormed during the activating strategy fall under these categories?

Oxygen is present in the atmosphere.  Our atmosphere is approximately 20% oxygen, so a wildfire should have enough unless it starts burning underground, where the oxygen levels are lower and can extinguish the fire.

Many pilot heat sources can ignite a wildfire.  Some of these sources can be natural, like lightning or volcanic eruptions.  They can also be caused by human activities, like campfires left unattended, sparks from a fallen electrical line, or a car left idling on some grass.  It can be surprising how long smoldering embers can retain enough heat to cause ignition.  Fires can last for months underground and then start burning again when conditions are right!

Fuels in wildfires tend to be different kinds of biomass.  Scientists who study wildfire classify wildfire fuels differently.  We can classify the fuels based on where they are located:

• Surface fuels include all the burnable materials on the ground, like sticks, fallen logs, and needles.
• Aerial fuels are the living and dead vegetation that stands above surface fuels.
• Ground fuel are all the burnable materials below the surface fuels, like roots and organic matter in the soil.

We can also classify fuels based on the types of plants that are burning and their size.  Herbaceous and fine woody fuels become dry and burn quickly, but do not release as much heat when they do.  By contrast, heavy woody fuels are harder to light, but will burn hot and for a long time.

Let’s do an experiment to see how different fuels burn.  First, we need to develop a hypothesis, or an educated guess, about how the fuels burn.  Come up with a hypothesis based on what we’ve discussed in this module.  Which of the following fuel mixes do you think will burn with the lowest intensity?  The highest?  The slowest?  The fastest? 

Once we have our hypothesis, we need to test it with an experiment.  We will collect data about the intensity and speed of different burning fuel mixes.

1. Assemble the following fire “recipes” in paper bags
   1. Handful of small twigs <1/4” in diameter and handful of larger twigs >1/4” in diameter
   2. Handful of larger twigs >1/4” in diameter and handful of green needles
   3. Handful of larger twigs >1/4” in diameter and handful of dry/dead needles
   4. Handful of smaller twigs <1/4” in diameter and handful of dry/dead needles
2. Weigh the bags (labels correspond with the recipes above) and record them in Table 3 below.

3. Place the bags in a fire-safe location like a grill and light them one by one
4. Record the time each of the “recipes” burn
5. After the bags are fully extinguished, weigh the bags again
6. Calculate the percent change (loss) in weight using the following formula:

% change =

Now that we have our experimental data, we can compare our previous hypothesis to the results.  Was your hypothesis verified?  Or did something surprising happen?  If the latter, what might have caused this different result?  Can you come up with a new hypothesis that reflects this new information?  The beauty of science is there are no wrong answers; it’s a process by which we better understand how the world works.  We can learn as much from an experiment that supports our hypothesis as we do one that does not!

Now, let’s do an experiment to better understand the process of combustion.  Light a match and start a stopwatch.  Assign one or two students to be in charge of the stopwatch. Have students watch how and how long the match burns.  After the match is fully extinguished, have students list the stages of burning they observed and record the time it burned.  Repeat the experiment, lighting another match, and place it under a glass jar or cup.  What element of the fuel triangle have we changed, and how does that influence how long the match burns?

A fire can be divided into four steps:

1. Pre-ignition: A pilot heat source will dry the fuel and create a cloud of flammable gases.
2. Ignition: The pilot heat source will light the gas cloud around the fuels; combustion is now self-sustaining and doesn’t require the pilot heat source anymore
3. Flaming: Flames rise, dry the fuel, convert them to flammable gasses, and then burn them; the flames will expand to consume more intact fuels
4. Glowing/smoldering: As the fuel is converted to gas and consumed, resulting in less heat and fuel, the flames will gradually die back, the zone of combustion will shrink back to the surface of the fuel itself, and more smoke will rise as we enter the final stage of combustion before the fire is extinguished (however, the process could begin again if conditions change allowing for ignition)

As we can see, the type and amount of fuel influences how long and hot a fire burns, also known as fire intensity.  Environmental conditions also influence fire intensity.  Hotter, drier, and windier conditions will typically lead to more intense wildfires.  Weather and topography can also influence fire behavior.  Fire will generally move in the same direction as the wind, and it will move more quickly uphill.  This is called “making a run.”

It is important to note that fire intensity is related to, but distinct from, fire severity.  Fire intensity is tied to how much energy is released through combustion.  Fire severity is an assessment of how much of an environmental impact the fire has; the more vegetation is killed and the more soil is scorched, the higher the fire severity.  In general, higher fire intensity results in higher fire severity, but as we discussed in the Fire Ecology lesson, some ecosystems are more adapted to fire and can handle more intense fires without much disturbance.

Let’s consider a few wildfire scenarios.  What would happen in a fire burned in the ecosystems below?  What are the elements of the fire triangle in each scenario?  How will it influence fire intensity? 

Summarizing Strategy

Time: 10 minutes

With each word worth 10 cents, ask students to write a $2 summary of what they learned from the lesson.  Have them write for 5 minutes, and share their work with the class for another 5 minutes.

Assessing Strategy

• Students develop testable hypothesis for the fuel recipe activity
• Students are able to evaluate ecological conditions to determine wildfire behavior in different forests
• Students are able to identify each of the three elements of the fire triangle in the matchstick activity