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Reading the Fire: Smoke and Air Track

In general terms, lighter colored smoke frequently contains a substantial concentration of unburned (and highly flammable) pyrolysis products.

As discussed in the previous article, fire behavior indicators can be grouped into five general categories: building, smoke, air track, heat, and flame. A simple mnemonic for remembering the categories is B-SAHF ("be safe"). Click here for a poster illustrating B-SAHF.

This article focuses on two related sets of indicators; smoke and air track. Smoke, as a product of combustion is a fairly obvious indicator of fire behavior. Air track indicators are often included in the "smoke" category, but this article will make the case for thinking of them as different, but highly interrelated.

Smoke Indicators

Smoke indicators include: location, color, density, and volume. In many cases these indicators can be observed from the exterior of the structure, providing an early indication of what is going on inside. However, these indicators can also be observed (although some times not as easily) while working inside a burning building. The interrelationship of these factors is graphically illustrated in Figure 2 (if you would rather look at a list of smoke factors, click here for Figure 1a).

Volume and Location

In many cases visible smoke may be the only indicator that there is a fire in the building. The volume and location of smoke discharge provide some indication of fire location and extent, but alone may prove to be unreliable. Ventilation controlled fires tend to produce a greater volume of smoke than those that are fuel controlled, however it is important to consider volume and location in conjunction with the other fire behavior indicators to obtain a clearer picture of fire conditions.

Smoke volume is often referred to as either "heavy" or "light". However, this could be confused with density (does heavy smoke sink and light smoke rise?). If you use heavy and light consistently to refer to volume, it is likely that the firefighters and officers you work with will understand, but it is something to think about when looking to describe fire behavior clearly.

Smoke visible from the exterior of the building may provide a useful clue as to the location of the fire. However, it is important to place this observation in the overall context provided by the fire behavior indicators (looking at smoke alone may be misleading).

What do you make of the smoke indicators visible in Figure 2? A moderate amount of smoke is visible from several openings (less from the upper window and doorway). Smoke is optically dense and buoyant. It is difficult to tell the level of the neutral plane (at the doorway) due to the effect of the stream being directed through the door. Back to this photo in a bit with a look at the related air track indicators.

Location continues to be important when working inside. Consider the extent of smoke filling each compartment as you work your way through the building. The term "smoke logged" as used in Figure 1 refers to a compartment that is filled (or largely filled) with smoke.


Smoke color can vary considerably depending on the nature of the fuel that is burning. Petroleum products, rubber, and many plastics will produce black smoke, while wood and other ordinary combustibles will commonly produce smoke ranging from light gray to yellowish, dark brown or even black when the fire is under ventilated. It is essential to remember that smoke color is only one of a number of indicators used to predict fire behavior and it must be considered in context.

In general terms, lighter colored smoke frequently contains a substantial concentration of unburned (and highly flammable) pyrolysis products. Under these conditions, smoke can ignite (given adequate oxygen and a source of ignition) and present a significant threat to firefighter safety. Dark smoke generally results from an under ventilated fire and/or combustion of petroleum products. In examining smoke color, it is also important to consider changes over time (e.g., smoke becoming darker or lighter). It may not be clear what is the major influence on smoke color (fuel or ventilation limitations), as with the other indicators it is important to consider color in context.


As with the earlier discussion of "heavy" and "light smoke", density can be a bit confusing as it is used in two different contexts. Most commonly, dense smoke is so "thick" that you can't easily see through it. Optical density refers to how difficult it is to see through the smoke. Thick or optically dense smoke contains a high concentration of particulates and is difficult to see through. High particulate concentration can also give the smoke the appearance of having texture (like velvet). Like color, optical density is related to fuel type and ventilation. Hydrocarbons and many synthetic fuels as well as under ventilated conditions will result in increased optical density.

Physical density refers to the buoyancy of the smoke. Smoke that is buoyant will rise quickly and smoke that is not will hang low to the ground. Generally buoyancy is related to the temperature of the smoke, the higher the temperature, the less (physically) dense the smoke and the greater the buoyancy. Early in fire development smoke may not be that buoyant due to limited heat release. Later in the development of the fire, buoyancy may be affected by the operation of automatic sprinklers (or application of water from hoselines) or it may simply cool as it moves away from the fire.

The Hot Gas Layer and Neutral Plane

As the temperature of a gas is increased it will expand, becoming less dense and more buoyant (Charles Law). If gases are confined within a compartment and heated, pressure will increase (Gay-Lussac's Law).

Charles Law: Gases expand in direct proportion to the absolute temperature (temperature in degrees Kelvin, Ko = Co + 273) applied to them. If the absolute temperature of a given quantity of gas is doubled its volume will double.

  • Key Points for Firefighters:
    1. Gases expand when heated.
    2. Gases become less dense and will rise when heated.

    Gay-Lussac's Law: When the volume of a gas remains the same and temperature is increased, pressure increases in proportion to the absolute temperature of the gas.

  • Key Points for Firefighters:
    1. When gases are confined and heated, pressure increases.
    2. Increased pressure indicates higher temperatures

    When a fire develops in compartment, a plume of hot smoke rises to the ceiling and spreads horizontally through the compartment in the form of a ceiling jet. Increased temperature reduces gas density. Less dense gases will rise. The difference in density between hot smoke and cooler air below causes them to separate into two distinct layers. The boundary between these two layers is called the neutral plane. This is because the hot gas layer is trying to expand (Charles Law), but if it cannot, pressure will rise (Gay-Lussac's Law). Fluid pressure is exerted in all directions in an attempt to reach equilibrium. This is easy to observe when there is an opening in the compartment such as a doorway, hot smoke exits from the upper level due to higher pressure in the compartment while cooler air enters at the lower level due to lower pressure. The point at which the pressure inside and outside the compartment is the neutral plane. This is the height of the bottom of the hot gas layer at the opening. However, inside the compartment the level of the hot gas layer is dependent on the difference in density between the hot smoke and cooler air below (this may be at a considerably different level than observed at openings such as doors and windows.

    While level or thickness of the hot gas layer is categorized as a smoke indicator, the neutral plane relates to movement of smoke and air and would be categorized as an air track indicator. This illustrates the close interrelationship between smoke and air track indicators and criticality of looking at the big picture when reading fire conditions.

    Remember that smoke indicators continue to be important after making entry. In addition to the indicators discussed to this point, firefighters should consider the thickness of the hot gas layer. While often thinking about the space between the floor an hot gases (our working area), it is even more important to think about the space between the bottom of the hot gas layer and the ceiling (the volume of hot smoke over your head). Consider the difference between a compartment with a ceiling height of 8 feet and one with a ceiling height of 24' (a considerable difference in the potential volume of smoke (think fuel) over your head).

    Early in fire development the hot gas layer is likely to be poorly defined with warm smoke defusing into the slightly cooler air in the compartment. As the fire develops, increase temperature differential between the smoke and cooler air below will sharply define the hot gas layer and it will become lower. If the fire continues to burn in a ventilation controlled state, the smoke and hot gases can lower completely to the floor.

    Air Track Factors

    Air track includes factors related to the movement of smoke out of the compartment or building and the movement of air into the fire. Air track is caused by pressure differentials inside and outside the compartment and by gravity current (differences in density between the hot smoke and cooler air). Air track indicators include velocity, turbulence, direction, and movement of the hot gas layer. The interrelationship of these factors is graphically illustrated in Figure 4 (if you would rather look at a list of smoke factors, click here for Figure 4a).

    Generally the movement of air is not directly visible, but can be inferred from watching movement of smoke. Air track indicators are closely related to and to some extent intertwined with those related to smoke and are always considered together.

    Velocity and Flow

    Velocity and flow are two interrelated air track factors. Velocity refers to the speed with which smoke is traveling (either out of an opening in the compartment or building or within a compartment). Flow may be either smooth (laminar) or turbulent. This is dependent to a large extent on velocity. High velocity generally results in turbulent flow through a compartment (such as a hallway) or out an opening (e.g., doorway or window). High velocity smoke discharge and turbulent flow is generally indicative of high temperature within the compartment (another connection, in this case between air track and heat). However, it is important to consider the size and nature of the opening from which the smoke is being discharged (for a given volume, velocity and turbulence will be higher through smaller openings).


    Consider the following observations. You arrive at a fire in a commercial building and observe smoke showing from a door on floor 1 (Figure 5). The smoke discharge fills the upper half of the window while it appears that air is moving in the bottom half of the window. What can you infer from this? What would you infer if the smoke discharge completely filled the window?

    The direction of the air track can also provide valuable cues to fire behavior. When air moves in an opening (inlet) without any smoke discharge, it is likely that smoke is exiting from another opening (exhaust). When this condition is reversed, and smoke comes out with no inward movement of air, it is likely that another opening is serving as an inlet. When the air track is bi-directional and air moves in at the bottom and smoke moves out at the top, this may be the only opening in the compartment or ventilation from other exhaust openings may be inadequate. Smoke discharge without inward movement of air or bi-directional air track both indicate that the fire is likely to be moving toward the opening.

    Mixing of smoke and air occurs at the interface between the hot gas layer and cooler air below. This is a critical factor in creating the conditions required for backdraft and many types of fire gas ignitions. Pulsing air track, outward movement of smoke followed by an inward movement of air is indicative of an under ventilated fire and potential backdraft conditions (consider other indicators in determining if backdraft conditions are likely to exist). It is critical to remember that these pulsations can vary in duration and that backdraft does not generally occur immediately upon making an opening. The time between making an opening and occurrence of a backdraft is dependent on many factors including distance of the compartment with backdraft conditions from the opening. Air track is an extremely useful indicator, but it must be integrated with a big picture evaluation of fire behavior indicators.

    Movement of the Hot Gas Layer

    Even more important than the height of the hot gas layer, are changes in height. A sudden rise could indicate that ventilation has occurred (either performed by firefighters or caused by the fire). Gradual lowering of the hot gas layer could indicate worsening conditions and increased potential for flashover. However, inappropriate or excessive application of water can also cause lower the hot gas layer (this will be addressed in more detail in the fire streams and fire control chapters of this text). Sudden lowering could indicate worsening conditions caused by flashover in an adjacent compartment. While not commonly known as a backdraft indicator, raising and lowering of the hot gas layer is similar to a pulsing air track observed at an opening (however in this case the compartment is not fully smoke logged, so the expanding and contracting gases cause the bottom of the hot gas layer to move up and down).

    Height and vertical movement of the hot gas layer provide only part of the air track picture. It is also important to observe the horizontal direction of smoke and air movement while inside the building (this may not always be visible, but when it is horizontal movement can provide a critical clue to the direction of fire spread.

    Study and Discussion Questions


    1. What are the five categories of fire behavior indicators?
    2. What is indicated by the volume and location of smoke discharge observed from the exterior of a structure?
    3. What is meant by the term smoke logged?
    4. What factors influence the smoke color?
    5. What may be indicated by light colored, but optically dense smoke?
    6. Why is it important to observe changes in smoke color over time?
    7. What is indicated by optically dense smoke? What factors may cause optical density to increase?
    8. What influences the buoyancy of smoke?
    9. Why might smoke become less buoyant?
    10. What is meant by the term neutral plane? How is this different from the depth of the hot gas layer below the ceiling?
    11. While the height of the hot gas layer above the floor is important, it can be misleading when you consider potential volume of fuel (smoke) overhead? Why? How can we overcome this limitation (think about building factors)?
    12. What causes air track (note that there is more than one causal factor)?
    13. What are the major categories of air track indicators?
    14. Where should firefighters and fire officers watch for air track indicators?
    15. What factors influence the velocity of smoke discharge from an opening?
    16. What does a bi-directional air track indicate (smoke out the top and air in the bottom)?
    17. You approach a door to make entry into a burning building and observe a pulsing air track at the door (smoke pushing out and then air moving inward). What does this indicate?
    18. What might be indicated by if the bottom of the hot gas layer rises and falls? How is this similar or different from a pulsing air track?



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