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Environmental Considerations When Burning

Posted Feb. 1, 2010

When planning a prescribed burn, there are many environmental factors to consider. This article addresses topography, fuel loads and weather. Combined, these factors influence the progress and success of a burn.

The lay of the land influences fire behavior. If a burn unit is relatively level, topography is not a major factor. If a burn unit has hills, gullies, creeks or bottomland, fire will behave differently across the different types of areas. The most important thing to remember about topography's effect on fire is that fire increases in intensity when burning uphill. When igniting along fireguards, remember to burn downhill. Many accidents occur when people or equipment are positioned ahead of a fire running uphill.

Vegetation can change with topography, such as a creek bottom dominated by tall fescue or a sparsely vegetated rocky hilltop. A fire might not carry through these areas and may need to be reignited on the other side. Topographic features such as a rock bluff, running creek or other bodies of water can serve as fireguards.

Fuel loads
Fuel loads can be defined as the amounts of dead and growing plant material that carry a fire across a burn unit. Fuel loads can be determined by taking uniformly sized clippings (usually one-quarter of a square meter) from several random locations across a burn unit. Once collected, the samples are dried in an oven and then a dry fuel load weight can be calculated. Fuel loads are commonly expressed as pounds per acre. Adequate fuel loads are necessary to achieve certain goals with a burn. Fuel load requirements differ for various burn goals. If the goal is a continuous burn across a unit, continuous fine fuels of at least 1,500 pounds per acre are necessary. If brush control is a primary goal, 3,000 pounds per acre or more may be needed. Excessively coarse or volatile fuel loads, such as many large eastern red-cedars, may require a very skilled burn crew, very wide fireguards and extra fire suppression equipment. Sufficient fuel loads allow a burn to occur under more diverse weather conditions.

Weather includes wind speed, air temperature and relative humidity. Wind is the driving force that steers a fire and governs its speed. Wind is affected by topography and temperatures. Wind speed influences the rate of spread and fire intensity. It does this by increasing available oxygen and bending the flames, which preheat unburned fuel. Wind also aids in the movement of firebrands or embers.

Air temperature directly affects the ease of fuel ignition and indirectly affects fire behavior, wind, rate of fire spread, fuel moisture and atmospheric stability. The main thing to remember about air temperature during a burn is its impact on relative humidity. For every 20°F increase, relative humidity decreases by 50 percent. If a burn is started at 40°F and 60 percent relative humidity and later the temperature rises to 60°F, relative humidity will drop to 30 percent. Most prescribed burns are conducted from 25 to 60 percent relative humidity. A fire should not be ignited when relative humidity drops below 20 percent because the difficulty of controlling a fire and the chance of spot fires occurring increase greatly. Air temperature can also play a major role in smoke dispersion. At night, temperature inversions can trap smoke in low-lying areas. These inversions are created by cold dense air near the ground not mixing with warmer air above which traps smoke near the ground.

Topography, fuel loads and weather are just three of many environmental factors to be considered before conducting a prescribed burn. Other considerations include coarse fuels, fuel volatility, season of burn and smoke management, just to name a few. When writing your burn plan, take these factors into consideration.