The cool air gravity current is the path of air/oxygen feeding the fire as it enters from a ventilation point and travels toward the fire in the lower atmosphere near the floor. We are all aware of ventilation as it relates to getting the heat, smoke, and toxic gases out of the building in the upper atmosphere. However, let’s take a look at how fresh air/oxygen comes into the fire area in the lower atmosphere and feeds the fire to sustain the burning process, creating a cool air gravity current, and how we can use this to locate the seat of a fire.
Fresh air/oxygen enters the structure from ventilation points, even if it is your point of entry. This flow of air/oxygen lays out a path directly to the seat of the fire as it feeds the fire.
Often, we can locate the fire fairly rapidly and make a quick knock on it. However, in some instances, it can be more challenging. In those situations, following the cool air gravity current may be helpful in locating the fire more rapidly. It may not always be visible in extremely heavy smoke and high-heat conditions—in these situations, it may be necessary to evacuate to a safer location and ventilate the area to allow the cool air gravity current to resume and then proceed to locate the seat of the fire. Use caution when ventilating so as not to create a fire flow path toward your crew and place them in a more hazardous position. Well-coordinated ventilation is always essential!
Throughout our basic and advanced training, we have been taught that a fire starts in the smoldering stage, it progresses to the growth (flaming) stage, flashover occurs, and then the fire transitions into the decay stage. Smoke rises to the ceiling, spreading across to the walls, and then mushrooms downward until it finds the path of least resistance and moves to the next room. We’ve seen the benefits of reading smoke from the exterior of the building to determine what is going on inside the building. We have all seen the fire flow path and the benefits of learning this. However, little emphasis has been placed on air and smoke movement near the floor, in the lower atmosphere or the cool air gravity currents. Lower atmosphere air movement is pulling in the required air/oxygen to sustain the burning process; thus, the cool air gravity current can be defined as air/oxygen that enters in the lower atmosphere; is drawn toward the fire; becomes consumed by the fire; is converted to heat, smoke, and toxic gases from the burning process; and then is expelled into the upper atmosphere and exits the structure. Look at cool air gravity currents more closely to help you find the fire faster and more efficiently when other indicators have not worked.
Because of more synthetic products, today’s fires burn much hotter and with more intensity than in the past. The newer buildings of lightweight construction give us little time to work. Double-/triple-pane insulated windows make the building more airtight, allowing the fire to grow more rapidly and with more intensity. Increased British thermal units from the synthetic products allow for a more rapid production of thermal radiation feedback into the room, resulting in flashover occurring faster than ever. The upper atmosphere contains the heat, smoke, and toxic gases released by the fire. Contained and without ventilation, the smoke and heat build and drop to the floor, making a room untenable. Thermal radiation feedback causes everything to heat up in the room and flashover occurs.
Ventilation allows smoke and heat to exit the structure, sometimes with force. Additionally, ventilation provides a source for fresh air/oxygen to enter. The heat and smoke exit the ventilation point in the upper atmosphere, while the fresh air/oxygen enters in the lower atmosphere, creating the cool air gravity current. The cool air gravity current brings in the cooler, denser fresh air as it flows under the less dense hot and buoyant air. The simple task of forcing the door for entry creates a ventilation point and creates a point of entry for fresh air/oxygen to feed the fire. Even with vertical ventilation, fresh air/oxygen is pulled into the fire area from someplace—possibly from the point of entry you made.
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Too often, we focus on or react to what occurs in the upper atmosphere while giving little regard to what may be occurring closer to the floor (lower atmosphere). Pay close attention to the cool air gravity current when you enter a structure and begin searching for the seat of the fire. Cooler, denser air and smoke that drop down from the upper atmosphere and into the cool air gravity current are now moving toward the fire mixed with the fresh air/oxygen. Following this smoke on the floor can lead you directly to the seat of the fire. The lower atmosphere is where you also will find a cooler environment in which to operate and where most victims with a better chance of survival are found because it is cooler and provides more oxygen than the upper atmosphere.
There are many ways of locating the seat of the fire and a variety of techniques used. A good size-up and general knowledge of building construction help; thermal imaging makes it easy. Unfortunately, not everyone is carrying a thermal imaging camera (TIC), and sometimes the smoke is so thick you can’t see the TIC screen. This is when you have to search to find those deep-seated fires with heavy interior smoke conditions. For your safety, constantly check the floor under you as you advance, but have you ever really noticed the smoke down there? This smoke is moving toward the fire, sometimes very rapidly, carrying the needed fresh air to feed the fire.
(1) Notice the lower section of the doorway as it allows the fresh air/oxygen to enter. This creates the cool air gravity current leading directly to the seat of the fire. (Photo by Trevor James.)
When you’re at an intersection while advancing a handline, how do you determine which way to turn? Turning in the wrong direction will cost you valuable time, time that you do not have, when it’s your responsibility to get water on the fire. It is your responsibility to protect the primary search crews. It is your responsibility to protect an interior stairwell, and the best way to do it is to get water on the fire.
You sometimes can feel a heat difference to determine the direction in which to head. That, however, it is not always possible. Turnout gear has offered some of the best thermal protection we have ever seen. However, it provides so much protection that temperature changes around us can sometimes go unnoticed. Following the smoke traveling at floor level, when you’re able to see it, will almost always lead you directly to the fire.
As a young firefighter, full of energy and pumped up with adrenalin, you may not feel the heat and the direction it is coming from as you move forward toward a fire. As you gain experience and more knowledge of the environment in which you operate, you become more aware of your surroundings and pay closer attention to the temperature changes. Now pay closer attention to the lower atmosphere and the cool air gravity current. You can see smoke travel; even if you have to put your face piece to the floor and shine your light, you may be able to see the direction the smoke is moving.
This technique may also be of value to the search and rescue crews who move ahead of the handline to perform a primary search, locate the fire quickly, search the fire area, and then search back to the point of entry. Also, it can be used during vent-enter-search when you enter through a window to do a rapid search under deteriorating conditions. Making your entry through a window, it is critical to quickly close the door to the room to buy time to perform a search of the room. The window you opened to enter has just created a ventilation point as well as a cool air gravity current. This ventilation point has now created a point of entry for fresh air/oxygen to feed the fire. The fresh air immediately drops to the floor (and hopefully you did too!); now the flow of air across the floor leads to that door you need to close.
Follow a search pattern! Do not wander off to the middle of the room following the smoke without a hoseline or search rope. Stay safe; stay on the walls, keep your orientation, but also watch that air flow in the lower atmosphere. It may get you to that door faster, maybe allowing you to pass over an open closet door that serves as a false sense of security when you closed it. That cool air gravity current and smoke on the floor are moving toward that doorway, and you are still not safe to conduct your search until that door is closed.
Advancing in on a basement fire with no exterior entrance, we’ve always been told it’s much cooler when we get to the bottom and to stay low. Why is it cooler? It is cooler because the fresh air feeding the fire is being drawn from a ventilation point in the lower atmosphere or a cool air gravity current. As you advance down, ensure the stairs are intact as you proceed. When you arrive on the basement floor, again, pay close attention to that lower atmosphere. The heat, smoke, and toxic gases will flow over your head, upward, and out of the basement entrance; the cooler gravity current is moving toward the seat of the fire. Basement fire tactics can vary from jurisdiction to jurisdiction, but rapidly getting water on the fire makes all things better.
Used at a Fire
I was a new lieutenant on an engine company. We arrived on the scene of a large two-story single-family residence of lightweight construction. After determining we had a working basement fire with extremely heavy smoke conditions and no exterior basement entrance, we advanced our handline down the interior stairs and reached the bottom. There was absolutely no evidence of where this fire was located. Smoke was banked to the floor, heat was increasing throughout, and we were just not sure in which direction to head to locate this fire in a very large basement. Feeling pretty uncomfortable at this point, I requested an exterior basement window to be ventilated. Once the window was vented, the cool air gravity current changed and the smoke lifted slightly. The fire grew quickly and started drawing more fresh air/oxygen in from the interior stairwell that we entered. This allowed us to follow the cool air gravity current and make a great stop on a rapidly developing fire that was very close to collapsing a section of floor above us.
As the smoke lifted, we saw the basement had exposed lightweight wooden I-beams above us. The room above us was now showing signs of a sagging floor. Quick extinguishment of this fire prevented a lightweight floor collapse that may have seriously injured my crew or worse. The newer buildings of lightweight construction today do not allow us much time, considering the structure can fail six to 12 minutes after the fire attacks the structural components. The inherent nature of lightweight construction allows for larger void spaces in the structure. Once the fire enters the voids and attacks exposed structural components, the fire spreads rapidly and the time clock starts for structural failure, thus making it even more important than ever for quick water on the fire.
Paying more attention to the lower atmosphere where this cool air gravity current occurs may allow us the opportunity to get water on the fire more rapidly and improve the overall conditions of the building much faster. Nothing makes a chief happier than seeing black, brownish/yellow smoke turn to steam. It may also allow you, when searching ahead of the nozzle, to find the fire more rapidly and start a search in the fire area. Add this tool to your toolbox for the future. Consider this on the next fire you go to or even the next training fire you attend. Pay closer attention to the cool air gravity current and use this to expand your knowledge in finding the fire.
Robert L. Daley is a captain (ret.) and a 26-year veteran of the Montgomery County (MD) Fire/Rescue Service. He has 32 years of volunteer service with the Maugansville (MD) Goodwill Volunteer Fire Company Inc. He studied fire protection at Catonsville Community College, Montgomery College, and attended numerous fire rescue classes through Maryland Fire and Rescue Institute, University of Maryland.