Reading the Fire: Heat and Flame

Dec. 8, 2007
While all of us have a commonsense understanding of heat and temperature, these concepts are frequently misunderstood.As discussed in previous articles, fire behavior indicators can be grouped into five general categories: Building, Smoke, Air Track, Heat, and Flame (Figure 1). A simple mnemonic for remembering the categories is B-SAHF ("be safe"). This article focuses on heat and flame indicators.

While all of us have a commonsense understanding of heat and temperature, these concepts are frequently misunderstood.

As discussed in previous articles, fire behavior indicators can be grouped into five general categories: Building, Smoke, Air Track, Heat, and Flame (Figure 1). A simple mnemonic for remembering the categories is B-SAHF ("be safe"). This article focuses on heat and flame indicators.

Flames and heat get quite a bit of attention. Flames showing from a couple of windows are sure to increase a firefighters pulse and we often talk about how "hot" a fire was. It is important to remember that while heat and flame are important fire behavior indicators, they provide only part of the picture. There is also a reason why they are last on the B-SAHF list. Flame and heat indicators must be integrated with Building, Smoke, and Air Track indicators to gain a more complete picture of incident conditions.

Heat and Temperature
While all of us have a commonsense understanding of heat and temperature, these concepts are frequently misunderstood. Heat is a form of energy and temperature is a measure of the average amount of thermal energy in a substance or object. Click here for more information on heat and temperature!

Heat Indicators
Neither heat nor temperature can be observed directly. However, there are indicators of heat and temperature that can provide critical information about current and impending fire behavior. Heat indicators can be divided into two categories, those that can be seen (observed) and those that are felt (tactile) as illustrated in Figure 2.

Observed Indicators
Air track often provides an early heat indicator. Observation of turbulent smoke pushing from the building at high velocity is a reliable indicator of a tremendous amount of heat energy and high temperatures inside the structure.

A thermal imaging camera (TIC) provides a highly effective means for examining the building for heat (Figure 3). Use of a TIC should begin on the exterior and continue during interior operations. Despite the tremendous advantage provided by use of a TIC, it is essential to not become completely dependent on technological information. Integrate thermal image data with visual observations to obtain a more complete picture of temperature conditions.

Other visual indicators include bubbling paint, melting roofing, crazing glass, and condensation of pyrolyzate on windows. Fire stream effects such as evaporation of water from a hot surface (such as a door) or lack of return from a temperature check (brief application of water fog into the hot gas layer to check overhead temperature) also provide an indication of temperature (Figure 4)

Tactile Indicators
Tactile effects include sensing temperature or temperature change. Firefighters may sense temperature and changes in temperature, but this is limited by the extent of thermal protection provided by their protective clothing and focus on the task at hand. Firefighters' protective clothing effectively insulates them from the thermal hazards typically encountered in firefighting. The multiple layers of insulation in the protective ensemble slows (but does not stop) heat transfer. This time lag makes it difficult for the firefighter to appreciate the amount of heat flux that they are being exposed to (Bryner, Madrzykowski, & Stroup, 2005).

Personal alert safety system (PASS) devices may be equipped with a temperature sensing function that provides warning at a specified exposure value when the specified temperature is exceeded for a specified time period (Figure 5). However, National Fire Protection Association 1982 Standard on Personal Alert Safety Systems (PASS) (NFPA, 2007) does not address thermal sensing and there is not standardized test protocol for these types of devices (Bryner, Madrzykowski, & Stroup, 2005). Thermal sensing devices use a temperature response curve to provide warning for long duration exposure to lower temperature and short duration exposure to higher temperature. However, during rapid increases in temperature such as those encountered in flashover or other forms of rapid fire development, adequate early warning to permit egress is unlikely due to limited sensitivity of the sensors (Bryner, Madrzykowski, & Stroup, 2005). While firefighters must be attentive to heat level and temperature change, it is often difficult to perceive these changes quickly enough to react to rapidly developing fire conditions. This reinforces the importance of integrating all the fire behavior indicators in your ongoing size-up and dynamic risk assessment.

Flame Indicators
Firefighters' attention is often drawn to flames like a moth to a candle. However, this is only one of many fire behavior indicators. Visible flames may provide an indication of the size of the fire (i.e., fire showing from one window vs. fire showing from all windows on the floor). The size or extent of the fire may also be indicated by the effect (or lack of effect) of fire streams on flaming combustion (Figure 6).

Size and Location
Location of the flames may provide important information. If flames are visible from outside the structure, where are they coming from? It is important to connect this information with building factors such as compartmentation. Is fire showing from a single window due to compartmentation or simply because that is the only window that has failed? Are the flames pushing from inside a compartment or is smoke igniting and burning outside?

While working inside the building, what is the flame height? Are the flames impinging on the ceiling and bending to travel horizontally? Do you observe flames in the hot gas layer (i.e., ghosting, rollover)? Fire development speeds considerably after flames in the plume of hot gases reach the ceiling and begin to travel horizontally in the ceiling jet. Flames in the hot gas layer or development of rollover are an important indicator of imminent flashover.

With flame indicators, it is not just what you see that is important. What you do not see is equally important. Remember that the low oxygen concentration in backdraft conditions may preclude flaming combustion (at least in that compartment). However, conditions can vary widely from compartment to compartment (void spaces are compartments too!) and you may have visible flames from the exterior, but quite different conditions inside the building.

As with other fire behavior indicators, change over time is an important indication of fire development or progress towards control. This is particularly true with flaming combustion. Once fire control operations have started, firefighters and fire officers must evaluate the effect of fire streams. Failure of water application to reduce the size of the fire indicates that either the flow rate is inadequate, the application point is ineffective, or both.

Flame Color
Flame color is largely dependent on the type of fuel involved and the extent to which the fuel and oxygen are mixed. For example, in a candle flame, the air and fuel mix as combustion occurs resulting in incomplete combustion and a bright yellow flame (diffusion flame). With a propane torch, the fuel and air mix prior to combustion, resulting in more complete combustion and a blue flame (premixed flame). Because there are several influences on flame color, it is important to interpret this information in context with other fire behavior indicators. Organic materials (natural or synthetic) will tend to burn with light yellow to reddish orange color depending on oxygen concentration as illustrated in Figure 7. If organic fuel gas or vapor is premixed with air, flame color will be bluish. A bright white flame is usually indicative of high temperature such as that generated by burning metal (i.e., magnesium). One other influence on color will be flame contact with other materials. For example, flame impinging on copper will have a blue green color.

Study and Discussion Questions
Use the information presented in this article to answer the following questions:

  1. What is the difference between heat and temperature?
  2. What are the two categories of heat indicators?
  3. How do your observations of air track provide an indication of the temperature inside a compartment or building?
  4. A thermal imaging camera (TIC) can be a valuable tool on the fireground. How should you use the TIC as part of size-up?
  5. What other heat indicators might you observe from the exterior of the structure?
  6. How can your hoseline be used to evaluate temperature conditions prior to and following entry?
  7. Why might your perception of temperature and temperature change while working inside a poor indicator of the thermal hazards encountered in the fire environment?
  8. What are the limitations of thermal sensors provided on personal alert safety system (PASS) devices?
  9. What are the basic categories of fire behavior indicators associated with flames?
  10. What can observation of flames showing from the exterior of a structure tell us about conditions on the interior?
  11. What flame indicators observed on the interior warn of imminent flashover?
  12. What factors influence flame color and how might this information be useful as a fire behavior indicator?

Review of the FBI
Effective size-up and dynamic risk assessment requires that firefighters and fire officers recognize key fire behavior indicators and have the ability to translate that recognition into a prediction of what is happening now, how the fire will develop, and how fire control and ventilation tactics will influence fire behavior.

B-SAHF, building, smoke, air track, heat, and flame, provides an effective framework for reading the fire. Each situation will be different with each category of indicator having varied importance. However, it is critical to look at conditions holistically and not focus in on one or two indicators.

The building is the starting point. Information about the building is best obtained before the fire starts, but this process must continue during the incident as well. Fire conditions can change both ventilation profile and structural stability. Anticipate these changes!

Smoke and air track are closely related and provide critical information about fire behavior. More important than what you see on arrival or at a specific point in time during the incident are the changes that happen over time. Monitor changing conditions on an ongoing basis.

Heat and flame are important indicators, but must be taken in context with the building, smoke, and air track. Remember that your perception of temperature conditions is modified by the personal protective equipment that you are wearing. Conditions can change faster than you will perceive them!

Putting it All Together
The purpose of making a detailed study of fire behavior indicators is to develop and continuously improve skill in reading the fire as part of ongoing size-up and dynamic risk assessment on the fireground.

Developing a high level of knowledge and skill requires ongoing and deliberate practice. Every time you see a photograph of a structure fire, examine it using B-SAHF and determine what fire conditions are and what you think will happen next! Discuss your observations with the members of your crew. Do they have a different perception? Why? Draw on one another's experience to improve your understanding. Use Figure 8 to practice your skill in reading the fire.

The next article in this series will focus on the interrelationship between the stages of fire (incipient, growth, fully developed, and decay) and the fire behavior indicators.


  • Grimwood, P., Hartin, E., McDonough, J., & Raffel, S. (2005). 3D firefighting: Techniques, tips, and tactics. Stillwater, OK: Fire Protection Publications.
  • Bryner, N., Madrzykowski, D., Stroup, D. (2005). Performance of thermal exposure sensors in personal alert safety system (PASS) devices, NISTR 7294. Retrieved September 1, 2007 from
  • National Fire Protection Association (NFPA). (2007) 1982 Standard on personal alert safety systems (PASS). Quincy, MA: Author.

Related Training Articles:

Do you have a good photograph or video clip illustrating building, smoke, air track, heat, or flame indicators? If you do and would be willing to share it, please send a copy to [email protected]. I will be working to include photographs and video clips in future articles to provide you with an opportunity to apply your knowledge of fire behavior and skill in reading the fire.

Ed Hartin, M.S., EFO, MIFireE, CFO is a Battalion Chief with Gresham Fire and Emergency Services in Gresham, Oregon. Ed has a longstanding interest in fire behavior and has traveled internationally, studying fire behavior and firefighting best practices in Sweden, the UK, and Australia. Along with Paul Grimwood (UK), Shan Raffel and John McDonough (Australia), Ed co-authored 3D Firefighting: Techniques, Tips, and Tactics a text on compartment fire behavior and firefighting operations published by Fire Protection Publications. Ed has delivered compartment fire behavior training (CFBT) and tactical ventilation training in the US, Australia, and Malaysia. Ed has also authored articles in a number of fire service publications in the US and UK, and presented at the British Fire Service College's annual research conference in 2006. The International Association of Fire Chiefs (IAFC) at its 2006 Annual Conference recognized Gresham Fire and Emergency Services compartment fire behavior training (CFBT) program as a finalist for an Award of Excellence. At the same conference, the Commission on Fire Accreditation International awarded Ed Chief Fire Officer (CFO) designation.

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