By Lorie Woodward Cantu
Editor's Note: This is the fifth installment in a 6-part series on prescribed burning, which was formerly called controlled burning. Although fire is an important tool in a range manager's tool box, its role is sometimes misunderstood by the general public, who see only its potential for destruction. In dry times, particularly when burn bans are in effect, county leaders find themselves in the difficult position of balancing the conservation needs of ranching constituents with public safety. To help facilitate informed discussions and sound decision-making, this series will provide an introduction to the topic and then explore: the effect of prescribed burning on the environment; equipment of a prescribed burn; public relations of a prescribed burn; managing smoke during a prescribed burn; and safety practices of a prescribed burn.
Managing the smoke from a prescribed burn is as important as managing the fire.
"As a burn manager, concerns about smoke keep me awake at night," says Rich Gray, who, as state fuels program coordinator, oversees prescribed burns for the Texas Forest Service throughout the state. "Some of the larger accidents and most significant injuries have resulted from smoke, not escaped fire as one might expect."
Planning is the key to mitigating any potential negative impacts from smoke, Gray says. The first step is to consider weather conditions, because they impact smoke behavior as surely as they do fire behavior.
According to Jason Runyen, fire weather program leader at the National Weather Service in Corpus Christi, the 2 primary meteorological considerations when it comes to smoke management are mixing height and wind.
Mixing height is the distance that smoke will rise before mixing with the atmosphere and dispersing.
"Visualize the mixing height as a lid within the atmosphere," Runyen says. "Smoke will rise until it hits that lid, which is a layer of warm air, and then it will stop. If there are adequate transport winds, the smoke will begin to disperse. If there are not, the smoke will linger."
Gray says when it comes to mixing heights, the higher the better. The mixing height should be at least 1,500 feet above ground level, he says. While this is a good theoretical minimum, it cannot be considered a standard because there are several local factors that enter into consideration, he adds.
"The goal is to have the smoke rise as far as possible above the surface of the earth to minimize impacts," Gray says. "If the mixing height is too low, the smoke can collect near ground level, creating visibility problems and safety hazards for health and travel."
The mixing height changes on a daily and seasonal basis, Runyen says. For instance, the mixing height is lowest early in the morning and begins to rise as the day progresses. When night falls, the mixing height decreases, and can create what is known as an inversion, causing smoke to sink back down toward the ground.
According to Runyen, mixing heights are higher in the summer and lower in the winter. They can also take significant dips immediately following Arctic cold fronts.
Local topography can also affect mixing height, Runyen says. Valleys have lower mixing heights than hilltops. Large water bodies, such as bays, can lower the mixing height as well, he says.
"We had a situation in Corpus Christi where a large prescribed burn was conducted on an inland ranch where the mixing height was 3,000 feet to 4,000 feet, which is very good," he says. "But the prevailing winds brought the smoke over Corpus Christi Bay, where, on that particular winter's day, the mixing height was 1,000 feet. The smoke blanketed downtown and gave prescribed burning a black eye."
As this example illustrates, wind direction and wind speed are also critical factors to consider when planning a burn.
Gray says, "Obviously, wind direction tells you which way the smoke will travel, and wind speed tells you how far it will go."
Runyen says a good transport wind falls in the range of 8 to 20 mph. Slower than that and the smoke won't move out of the area quickly enough; higher than that and the smoke will stay closer to the ground.
From a burn manager's perspective, it's important to consider the near-term and long-term impacts of the smoke, Gray says. By law, a burn manager is required to notify in person and in writing any "sensitive receptors" such as schools, hospitals, nursing homes and residences within 300 feet of the burn site, he says. From a good neighbor standpoint, it is wise to distribute a public notice through a local newspaper or radio station about the impending burn, he says. It is also neighborly to directly contact people who may be in the predicted path of the smoke, even if they're outside the 300-foot limit.
Another consideration for burn managers who are planning a fire within proximity to a major metropolitan area is the city's air quality restraints, Gray says. If the city is facing an ozone action day or is facing a non-attainment classification, it may be wise to postpone a burn that could affect the city's air quality ratings.
"In some cases there are legal considerations, but primarily it's a public relations issue for prescribed burning," Gray says. "If we want to continue using our tool to manage habitat and rangeland, we can't afford to cause non-point source pollution problems for our urban neighbors."
Runyen says an additional meteorological consideration is the stability of atmosphere. When the atmosphere is stable, there is no vertical air movement. When the atmosphere is slightly unstable, there is some vertical air movement. When the atmosphere is unstable, air moves upward very rapidly.
To measure stability and help predict fire behavior, burn managers rely on the Haines Index, which uses indices ranging from 2 to 6. The lower the number, the more stable the atmosphere.
"A burn manager is looking for a Haines Index of 3 or 4," Runyen says. "If the number is lower, the smoke won't rise adequately. If the number is higher, fire behavior becomes so erratic that is dangerous and unwise to burn."
Smoke is produced when there is an incomplete combustion of fuel due to lack of oxygen; therefore, the fuel or type of vegetation that is being burned affects the amount of smoke produced and how long the smoke lingers.
Gray says fine fuels, such as grass, tend to produce a high volume of smoke for a short duration, while heavy fuels, such as cedar or timber, produce an even higher volume of smoke that can linger for days.
The amount of moisture in the fuel also affects the amount of smoke it produces, he says. The higher the moisture content, the more smoke a fuel produces.
"When it comes to fuel, a prescribed burn manager is once again performing a balancing act," Gray says. "The fuel needs to have enough moisture to make the fire manageable, but not so much moisture that the smoke is unmanageable."
Active smoke management begins when the prescribed fire is set and does not end until the smoke completely dissipates and no longer presents a threat, Gray says.
Because smoke travels, at least 1 member of the burn team should be mobile during the burn, checking nearby roads and towns in the path of the smoke for impacts. Is the smell of smoke barely noticeable in the town, or is the smoke visible and the stench strong? Is the smoke blowing across the highway barely visible, or is it so thick that flaggers need to be posted to help warn motorists? These impacts should be reported back to the burn manager.
"As a burn boss, I need to know how the smoke is acting, because it will be up to me to react appropriately," Gray says. "If the smoke is presenting a hazard, I may need to change the burn plan and burn much less than I originally planned. If I light the fire, I own the smoke — and the responsibility for ensuring public health and safety."
For useful forecasting tools, including mixing height, prescribed burners can go to: weather.gov/fire. This will access the Fire Weather page, which includes a national map. Typing in a specific location will give you access to a wide range of information.
"Prescribed Burn: Smoke Management" is from the October 2011 issue of The Cattlemen magazine.