On days winds are light they are also variable in direction. This creates a situation where fire and smoke are going in multiple and unpredictable directions. A steady wind direction allows the ability to predict the direction of fire and smoke. A wind of at least 4 miles per hour out of a constant direction is the best for safety, smoke dissipation, and fire crew efficiency.
A fire prescription is a set of conditions under which a fire will be set to meet land management objectives. It is based on scientific research and experience.
First, determine the management goals and objectives of the area to be burned. Next, develop a fire prescription for that area. Prescriptions may require specialized personnel, tactics, training, equipment, and firebreaks. Consultation with a fire management specialist will help in developing custom fire prescriptions that will meet the objectives of the unit.
Rule 1. For those in the process of learning to burn or with limited experience, use the 60:40 Rule. The 60:40 rule states that you burn with an air temperature of less than 60°F, a relative humidity greater than 40%, and a wind speed of 5-15 mph measured at 6 feet above the surface of the ground.
Rule 2. For those concerned about weather conditions changing out of the prescription, use the Rule of Halves. This rule is used in the field to predict changes in fire behavior. When the air temperature increases by 20°F, the relative humidity decreases by 50%. For example, if the air temperature changes from 60°F with 40% relative humidity to 80°F, the relative humidity will change from 40% to 20%. A fire that can be conducted safely at 40% relative humidity may pose a safety risk at 20%.
Rule 3. In most cases, do not burn if there is a forecasted frontal passage or wind shift within 12 hours.
Rule 4. In general, the width of firebreak on the down wind side of the area to be burned should be 10 times the height of flammable vegetation. Firebreaks are usually a combination of bare ground, mowed strips, and backfired strips. In woodlands, leaf blowers are often used to create a bareground fire break. Landscape features such as streams and roads also can make adequete firebreaks. If the firebreak is insufficient, you may experience a fire escape.
Rule 5. If the conditions are not right, including all parts of the prescription (adequate personnel, equipment, weather conditions, etc.), do not start the fire. Wait until everything is right.
Rule 6. If the fire is not going well, put it out.
Rule 7. Do not leave the fire until it is completely out, that is no visable smoke for at least one hour.
Trees and brush should never be cut, dozed and piled prior to a prescribed burn. This will complicate the burn and increase spotfire chances. Research has shown that brush piles can cause spotfires up to 500 feet down wind. If piling is necessary, push piles 300 feet into the burn unit, keep piles small and keep an eye down wind of them during the prescribed burn.
Burning brush piles in the summer when surrounding plants are green.
The best time to burn brush piles is after the prescribed fire has been conducted. Burn when the area is black or let the burn unit green up. Safe times to burn brush piles include early summer months when surrounding grass is green and and well hydrated (May and June for much of the Great Plains). Burning in the winter with dormant fuels and dry conditions can cause problems, even when burning brush piles with snow on the ground. Large brushpiles can burn for days, sometimes weeks, and when the snow melts the spotfire risk increases.
Burns with high temperatures and low humidity are for experienced fire bosses and crews. Look back at the general guidelines for prescribed fires.
Research has shown that if a prescribed fire is conducted with relative humidity under 25% there is a 100% probability of a spotfire occurring. There is only a 46% chance of a spotfire when the humidity is between 25% and 29%. Spotfires significantly decrease when prescribed fires are conducted when the relative humidity is higher than 40%. This does not suggest that prescribed fires cannot be conducted under 40% humidity, but is one of the many decisions that needs to be made when planning a prescribed burn.
Burning under good smoke dispersion conditions.
Burning on days that have good smoke dispersion can reduce smoke effects. Before beginning a prescribed fire, the effects of smoke downwind must be considered. Burning so that smoke disperses away from sensitive areas (residential areas, hospitals, highways, and airports) must be considered in the burn plan. If smoke is a concern, consider burning dry fuels over moist fuels. Backfires also tend to produce less smoke than flank or headfires.
For more detailed information about prescribed fire prescriptions, view this fact sheet: http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2704/NREM-2878web.pdf
Weather Conditions for Controlled Burns
Although controlled burns can be conducted under pre-determined weather conditions, burns can be unsafe if the experience of the burn boss and crew is inadequate.
People new to burning should consider using the 60:40 rule. The 60:40 rule refers to restricting burn conditions to air temperatures less than 60 degrees Fahrenheit and relative humidity (Rh) greater than 40% with a wind speeds of 5-15 mph measured at 6 feet above the surface of the ground.
Rule of Halves
The rule of halves is an easy way to remember how to predict changes in fire behavior when the weather changes. When the air temperature increases by 20 degrees Fahrenheit, the relative humidity decreases by 50%. For example, if the air temperature changes from 60 degrees Fahrenheit with 40% relative humidity to 80 degrees Fahrenheit, the relative humidity will change from 40% to 20%, making the spotfire risk increase dramatically. In most cases, do not burn if there is a forecasted frontal passage or wind shift within 12 hours.
There are many weather tools available. Make sure and check the wind direction, speed, temperature and relative humidity a few days before the burn, and right before the burn. It is very helpful to take notes of weather conditions during the burn. This can help people learn how to predict fire behavior in the future. On-site weather meters can be very helpful in taking up-to-date weather.
If conditions are not correct, including all parts of the prescription, do not start the fire. If the fire is not going well, put it out. Do not leave the fire until it is completely out, which means no smoke detected for at least one hour.
View this fact sheet for more detailed weather information regarding spotfires: http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2704/NREM-2878web.pdf
Written Prescribed Fire Plan
Photo: Fire boss discussing the plan with the burn crew prior to a prescribed fire (by Samuel Roberts Noble Foundation staff).
A prescribed fire plan details the prescription necessary to safely and effectively accomplish burn goals on a site. A thorough written plan is one of the primary differences between a prescribed fire and a controlled burn. A primary advantage of a written plan is it requires the preparer to thoroughly plan the prescribed fire. A written document is important to communicate the plan to a burn crew, prepare for contingencies, minimize liability, and insure the many aspects of a fire are thoroughly considered such as: goals, site preparations, weather conditions, fire and smoke sensitive considerations within and near the burn, labor and equipment needs, neighbor and civil authority notifications, ignition sequences, and appropriate records. Although initial preparation of a thorough prescribed fire plan requires some effort and time, future burns on the same site usually require only minor modifications to the initial plan.
Items addressed in a prescribed fire plan
- Preparer’s name
- Date of latest revision
- Map of burn site
- Goals or objectives for the burn
- Description of burn site including topography, fuel types and fuel loads
- Description of firebreaks and their preparation
- Fire boss name
- Minimum burn crew number and skills
- Minimum equipment needed to conduct the burn safely and protect the crew
- Plans to protect fire sensitive areas within burn site
- Plans to minimize fire and smoke impact to sensitive areas outside burn site
- Applicable permits
- Civil authority and neighbor notification plans and records of contact
- Potential burn dates and times
- Suitable weather conditions such as temperature, relative humidity, wind speed and direction
- Suitable soil moisture conditions
- Suitable atmospheric mixing and transport conditions
- Written ignition plan
- Contingency plans for spot fires, escaped fires, equipment breakdowns and other problems
- Mop-up and post-burn monitoring procedures
- Records of weather forecast(s) examined prior to the burn
- Records of actual weather conditions during the burn
Assistance preparing written prescribed fire plans and examples of them are available from several sources:
- State extension services http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2563/E927web.pdf and www.extension.iastate.edu/forestry/publications/PDF_files/PM2088A.pdf
- Natural Resources Conservation Service http://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17745.wba
- The Nature Conservancy www.tncfiremanual.org/burnplan.htm
- Samuel Roberts Noble Foundation www.noble.org
- State natural resource agencies such as http://paprescribedfire.org/pa%20burn%20plan%20template%20inst.doc
Written prescribed fire plans do not have to be as detailed as some examples at the above websites, but should address the items mentioned in this article. All prescribed fires should have a written plan. Unfortunately, this has not been the case and a lack of planning is one of the primary reasons that many controlled burns develop into out-of-control burns.