Chapter 2: Cause and Effect

What Does What to What?

I have given this simple test to many firefighters and always get similar results. Take a minute to answer the questions below to test your perceptions of fire behavior.

  1. What best defines fire behavior? (Circle one or more)
    1. The rate of spread
    2. The intensity
    3. The variables of A and B
  2. How are changes in fire danger expressed? (Circle one)
    1. By weather measurement
    2. By calculation of rate of spread or intensity
    3. By a number, plan or key phrase, such as “low,” “moderate,” “extreme”
  3. How do you predict changes in a fire’s intensity or rate of spread? (Circle one)
    1. I go by my instincts.
    2. I use my experience of like situations
    3. I use the weather readings I take on site.
  4. How does hot and dry air affect fuels? (Circle one or more)
    1. It heats fuels
    2. It dries fuels
    3. It cools fuels
  5. Which is trying to reach equilibrium on a hot day? (Circle one)
    1. Fuel
    2. Air

See answers[1] below.

Some Common Perceptions

Fire behavior training includes a lot of information about the weather’s effects upon fire behavior. Somehow, most trainees conclude that the air affects fuels and that the fuels dry causing a change in the fire behavior. We deduce that rising air temperature causes forest fuels to dry, increasing the intensity of a fire. We form the opinion that the forest fuels are reacting to the condition of the air so that they might reach equilibrium with the atmosphere.

We are taught to use atmospheric measurements to made fire predictions. Indeed, we are instructed to correlate the relative humidity to the degree of fire intensity, and, sometimes, to predict extinction from forecasts of dew on the fire area.

These perceptions are WRONG! Measurements and forecasts of air temperature and humidity are indicators and predictors of fire danger, not fire behavior. Fire behavior is the manner in which a fire reacts to the influences of fuel, weather, and topography. It’s important to understand that differences in fire behavior can exist under all ranges of air temperature and humidity.

Think of a point some distance away from a fire’s front. How will you use air temperature and humidity to predict how the fire will behave when it gets there? Forecasts of changing air temperature or relative humidity are too generalized to be useful for tactics. We need to use other information to predict change: when or where fire intensity may vary.

Let’s Get It Right This Time

First, let’s address fire danger. Fire danger is expressed using values such as extreme, high, moderate or low. This denotes the relative danger of fire starts and fire intensity. These terms are derived from a scale of atmospheric readings including air temperature, relative humidity, fuel stick moisture content, wind velocity, etc.

Fire behavior, on the other hand is the manner in which a fire reacts to the influences of fuel, weather, and topography.

The degree of fire danger does not explain the variations in fire intensity that you can observe across the fireground. These variations—or fire behavior changes—occur under all fire danger designations.

As the fire danger gets higher, the potential for variations in fire behavior becomes more extreme. Air temperature, relative humidity or fuel moisture readings and forecasts are fire danger factors that can exacerbate fire behavior, but do not account for the variations exhibited in fire behavior on the fireground.

What information is necessary for us to determine the potential variation in fire intensity? Picture in your mind a one-foot patch of fuel bed consisting of pine needle litter. At morning’s first light, the air is cool and the humidity is highest; there may be some dew on the fuel. Assume that this patch of needles is the same temperature as the air with which it comes in contact. The air and fuels are in equilibrium.

Next imagine the sun rising, radiating its heat toward the earth. The sunlit fuels give off steam as they heat.

Regarding sunlit fuels, think about the order in which changes are occurring. Does the sun warm the air, raise the air temperature, decrease the humidity and dry out the fuel? Yes, all that does happen, but not in that order.

What, then, is the order of action and reaction? The sun heats the fuel by radiation. The solar heating raises the fuel temperature and drives off the fuel moisture. The air is relatively unaffected by the sunlight so it is not heated by radiation. Instead, the air contacting the solar heated fuel is heated by conduction; its temperature begins to rise but lags behind the fuel’s temperature. The fuel and air are not in equilibrium at this stage. The air is trying to catch up and is, therefore, the element that is out of equilibrium. The air is the element that is out of equilibrium.

In our example, the sunlit portion of the pine needle patch heats up to 30, 40, maybe 50 degrees Fahrenheit higher than the air temperature above the fuel. Place your hand on sunlit litter and you will quickly feel that the fuel is hotter than the air.

So, our general perception that the atmosphere is the element that heats and dries the forest fuels is not correct. In fact, the air cools the fuel. The air is receiving heat from the fuel by conduction. Therefore, in the heat exchange, the air cools the fuel. The air is out of equilibrium.

Knowing how to use the weather and fire danger information is important. Using the information correctly is necessary for us to predict changes in fire behavior based upon our observations.

Practical Application of Countryman’s Research

In 1966, C. M. Countryman, Research Forester, published a paper titled, The Concept of Fire Environment [4], through the Pacific Southwest Forest and Range Experiment Station. His research provided information on how the fire environment is related to the fire’s behavior. The information presented by Countryman provided great insight for firefighters but still did not tell the entire story. Following are excerpts from Countryman’s paper and my explanations and amplifications of this important work.

Interrelationships of Components

“Fire environment is not static, but varies widely in horizontal and vertical space, and in time. The fire environment components and many of their factors are closely interrelated. Thus, the current state of one factor depends on the state of the other factors. Also, a change in one factor can start a chain of reactions that can affect the other factors.

“For example, consider the simple topographic factor of slope aspect. The amount of heating of fuel by the sun on a slope depends partly on aspect. A slope facing east begins to warm first, and its maximum temperature occurs early in the day. A slope facing south reaches its maximum temperature about two hours later, and it has potential for higher fuel temperatures than the maximum of the other aspects. A slope facing west reaches its maximum temperature still later. The north slope and flats also have their distinctive diurnal trends.”

C.M. Countryman

Solar preheating by time of day and aspect. Countryman 1966

Solar heating: fuel temperatures (F) by time of day and aspect. Countryman 1966

How Does This Affect Fire Behavior?

Fire intensities vary because of differences in the flammability of the fuels. Fuel that is heated (regardless of the source) is more flammable than fuel that is not heated.

The sun is a powerful heat source that heats fuel as much as 80° F above the air temperature. As each aspect heats and cools, the fuel flammability on that aspect also increases and decreases. Each aspect has its own peak (time period) of flammability. The potential for extreme fire behavior often coincides with the shortest shadow on the aspect. I call this event of the shortest shadow the peak of the flammability curve.

Fire and the Fire Environment

“Where does fire fit into the picture? In an environment without fire, radiant energy from the sun is almost the only source of heat. This energy heats the earth’s surface and to a minor extent the air above. Most of the energy that directly and indirectly modifies the air mass and fuel components of the fire environment comes from the heated earth surface.

Because of differences in slope, aspect, and ground cover, heating by the sun is not uniform; some areas become much warmer than others. This variation in the local heat sources creates the variability in local weather and fuel conditions.”

“When the surface of a slope is heated, it transmits this heat to the air above it by conduction, convection, and radiation. The resulting increase in air temperature changes the relative humidity. In addition, local winds also are often strongly affected by the differences in air temperature resulting from the differential heating of slopes of different aspects.”

M. Countryman

Let’s Get This Straight

The sequence of events that lead to changes in atmospheric conditions like wind, air temperature and relative humidity, start with the sun’s radiant energy heating the earth. The sun heats the earth and the fuel upon it, and the earth and fuel, in turn, heat the air causing a reduction of humidity.

By now you should be starting to see that observing variations in fuel temperature—or fuel flammability—to predict changes in fire behavior is vastly superior to any attempt to use relative humidity to make short term predictions. The fuel flammability varies over the terrain while the humidity remains considerably constant.

The sun heats the fuel, which becomes hotter and drier due to the solar radiation. Air in contact with the fuel then reacts, becoming warmer and dryer. The heated fuel is the primary factor in the change within the microclimate. In fact, fuel temperature is the causative agent in charge of the air’s condition. The air temperature and moisture content are reactive to the fuel temperature changes.

The Fire Environment in Fire Behavior Predictions

Fire Behavior or Fire Danger? Making a Distinction

First of all, a fire behavior prediction should not be made solely upon air temperature and relative humidity readings from a representative location. Such a prediction would be flawed because it assumes the fuel flammability is stable. As we’ve demonstrated, this is definitely not the case: fuel flammability varies by time and place.

Fire behavior is the manner in which a fire reacts to the combined influences of fuel, weather, and topography, whereas fire danger is a value representing the factors affecting inception, spread, and resistance to control, and subsequent fire damage. Fire danger is often expressed an index with values like extreme, high, moderate or low. The general weather conditions over the fireground are an indication of the level of the fire danger. The fire danger level should be used as an indicator of the range of intensities that a fire may exhibit, but not when or where this fire behavior will occur.

You may want to review some pertinent excerpts from the Fire Weather Handbook [3] contained in Appendix 1. The handbook is issued by the U.S. Department of Agriculture and it contains basic information to verify and substantiate the conclusions made by the author regarding the relation of fuel temperature and air temperature. If you are unfamiliar with this source, skip over to it now and you’ll have a good foundation of the basic fire physics that fit a firefighter’s needs.


Air temperature and relative humidity readings and forecasts are general conditions that can be an indication of fire danger across the entire fireground.

Variations in the fuel bed’s temperature and moisture are causative factors in microclimate’s air temperature and relative humidity, not the results.

[1] Answers: (1) c, (2) c, (3) a,b,c, (4) b,c, (5) b

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