What Constitutes a Prescribed Burn?

The term “Prescribed Burn” means that scientists and firefighters must have an exact prescription to conduct a burn safely and effectively. Several factors contribute to the prescription for a burn:

Weather

Fire Ecologists and firefighters study the weather before they conduct a prescribed burn. They must examine four weather factors: temperature, wind, humidity, and precipitation.

Temperature: fire ecologists and firefighters must study expected temperatures for a prescribed burn. Why? Higher temperatures increase fire behavior by drying out vegetation, which increases the intensity of the fire. If temperatures are too low, the fire may go out. Firefighters will not conduct prescribed burns if temperatures are too hot or too cold.

Wind: wildfires are easily pushed by wind. Fire ecologists and firefighters must study the weather to know if winds are expected in the prescribed burn area. If winds are expected to be high, firefighters will not conduct the burn to keep it within its prescription.

Humidity: If the relative humidity within the burn area is too high, the prescribed burn will not burn effectively because the vegetation will be too dense or wet. Firefighters and fire ecologists usually burn when the relative humidity is low, but not too low to prevent the fire from burning out of control.

Precipitation: Rain and snow play a very important role in prescribed burning. If rain or snow is expected in the prescribed burn area, firefighters will adjust the prescribed burn time accordingly.

Smoke

Fire ecologists and firefighters must also examine the weather to determine how smoke from the prescribed burn will disseminate. Will there be wind to move the smoke away from neighborhoods, towns or cities? Will clouds move in and hold the smoke in a valley? These factors must be examined so that the public does not have to experience more smoke than necessary during a prescribed burn.

Fuels

What fuels will be burned in the prescription? Are they trees? Are they rangeland shrubs and grasses? The type of vegetation to be burned is important to both scientists and firefighters. Trees burn longer than shrubs and grasses, so firefighters must account for the fuels within the prescribed burn area so they can plan for an effective, safe burn.

Topography

Where will the prescribed burn be conducted? In mountainous terrain? On a flat rangeland? Because fire is affected by topography, fire ecologists and firefighters must examine the area before beginning a prescribed burn. If the burn is to be lit in the mountains, then firefighters must be prepared to create fire lines that can contain the prescribed burn to prevent it from moving too quickly up steep draws or in canyons. Flat ground must also be examined; as the wind typically blows on rangeland areas in Idaho, it could move fire beyond the prescribed burn area.

Unpredictable Factors

Even though firefighters and scientists examine all factors before conducting a prescribed burn, unpredictable weather patterns may disrupt the burn or cause it to move beyond its prescription. Firefighters are prepared for such incidents; they provide ample firefighting equipment in case of an emergency or will cancel/postpone the activity if the weather does not fit the prescription for a safe, effective burn.