Broadcast burning is an effective way to manage vegetation and mitigate wildfire risk on your property. Understanding how weather conditions impact the safety, feasibility, and efficacy of your burn and selecting suitable burn windows are essential elements to conducting successful broadcast burns. One element of a broadcast burn is developing a prescription, or a set of specific weather parameters under which you plan to safely and effectively conduct a burn.
When permits are required from your local fire suppression agency, they may require you to develop a written document (or burn plan) that provides a description of the burn area, target weather conditions (a prescription), burn unit preparation, crew and equipment resources needed, as well as safety and logistical factors to consider before, during, and after a burn. A well-developed plan not only ensures thorough preparation and safety but also documents your due diligence as a responsible landowner. Once you develop your burn plan and identify your weather prescription, you can look for burn windows, or specific periods (hours, days, weeks) when weather, fuel moisture, and environmental conditions allow for controlled, predictable fire behavior that align with your management objectives.
By developing a prescription and establishing a defined burn window, you can ensure the burn meets management objectives, minimize risks of escape or unintended impacts, and closely monitor forecasts as the target season or date approaches. Monitoring the weather within the burn unit can help you understand microsite conditions, daily weather patterns and can help develop your prescription. Monitoring can also mitigate the potential of having to cancel or postpone your burn at the last minute.
Developing Your Prescription
Broadcast burn prescriptions consider several weather parameters. You need to identify weather ranges for each weather parameter that are expected to meet burn objectives. Weather ranges will vary based on location, crew experience or resource availability, fuel type, and burn objectives. For examples of burn plans and associated prescriptions, visit California’s Prescribed Burn Association website (https://calpba.org/). Common weather parameters that often influence fire behavior include:
Temperature: changes in air temperature influence fire behavior. Consider both your burn objectives and vegetation/fuel type to determine the appropriate minimum and maximum air temperatures for your prescription.
Fuel moisture content: the percent water content of your fuel (e.g., live or dead vegetation) when compared with the dry weight of that material. Targets for fuel moisture ranges vary by vegetation/fuel type, desired burn intensity, and management objectives.
Atmospheric stability: is the tendency for the vertical motion of air. When conducting a broadcast burn, slightly unstable conditions are often considered favorable, as it helps lift smoke and has steady winds. However, greater instability causes wind gusts and turbulence, which can lead to unpredictable fire behavior. Atmospheric conditions also change depending on time of day. You may be able to burn during stable conditions, but because smoke-sensitive areas could be negatively affected, you should work closely with your local air district staff.
Probability of ignition (PIG): the likelihood (expressed as a percentage) that a firebrand (i.e., ember or spark) will cause ignition when it lands in receptive fuel beds. PIG is influenced and calculated by factors like temperature, relative humidity, and fuel moisture content. It is a useful measure, as it is a single value that takes many weather parameters into consideration. Probability of Ignition and the weather parameters it takes into account can be calculated using charts from the NWCG Fire Behavior Field Assessment Guide (PMS 437).
Relative humidity (RH): the ratio of the amount of water vapor evaporated into the air compared with the maximum amount possible at that temperature; typically expressed as a percentage. RH ranges required for combustion will vary by vegetation/fuel type. RH typically has a diurnal effect, opposite of temperature: as temperature increases during the day, RH values drop, and then RH slowly increases with cooling temperatures.
Seasonality: refers to the recurring changes in weather patterns that occur at different times of the year. In California, fire practitioners often conduct broadcast burns in the spring or fall when weather conditions are moderate. Seasonality may also refer to phases of plant growth and development, and some burn objectives (e.g., invasive species control) will require specific timing that favors and/or disadvantages target species. Knowing your desired management objectives and weather conditions will help you determine when to burn.
Wind direction and speed: wind direction is expressed as the direction it is coming from, while wind speed is how fast the wind is moving expressed in miles per hour. Consider topographic features during your burn planning and be wary of burning on days with highly variable winds forecasted, even if the wind intensity is within your burn prescription. When identifying an optimal burn window, remember to take into account that conditions in your burn unit may differ from local weather predictions. Even a short duration gust of wind outside your prescription can lead to containment issues. Take caution if you are observing wind gusts in your weather forecast outside of your prescription.
Finding a Burn Window
Finding a burn window involves monitoring weather patterns weeks to days before your burn to ensure there are predictable and consistent weather conditions that match your burn plan. Local knowledge and an on-site Kestrel or Weather Station can help confirm real-time conditions. Additional resources to assist with finding an adequate burn window include:
The Fire Environment Mapping System (FEMS; https://fems.fs2c.usda.gov/ui) is an interactive user interface with maps, tables, and charts that display Remote Area Weather Stations (RAWS) data. This website includes archived observations from each RAWS to track weather patterns as well as a 7-day hourly forecast of weather.
The MesoWest website (https://mesowest.utah.edu/cgi-bin/droman/mesomap.cgi?state=CA&rawsflag=3) allows you to compare observed forecasts near your burn location (from nearby weather stations, including government- and privately-funded stations) with the day’s hourly weather forecast. You can focus the map to your burn area and manually select the specific weather parameters (e.g., temperature, wind, humidity) you would like to compare and see how the weather models are performing compared with what is actually happening on the ground days before your burn.
The National Weather Service (NWS) website (https://www.weather.gov/fire/) provides 7-10-day weather forecasts and seasonal outlooks. NWS provides fire weather planning forecasts for specific locations that include forecasts for windspeed, relative humidity, temperature, and more.
The Northern California Geographic Area Coordination Center (ONCC; https://gacc.nifc.gov/oncc/predictive/weather/index.htm) and Southern California Geographic Area Coordination Center (OSCC; https://gacc.nifc.gov/oscc/predictive/weather/index.htm) provides a 7-day outlook for fire potential, a smoke forecast, and a fire weather webcast that is updated daily.
The Prescribed Fire Information Reporting System (PFIRS; https://ssl.arb.ca.gov/pfirs/index.php) is a tool developed by the California Air Resources Board (CARB) for individuals to view prescribed fire activity both locally and statewide. Both agencies and individuals can report activities using this tool. PFIRS also provides an outlook on the likelihood of permissive burn days three days in advance. This is very helpful in planning so you do not have to cancel a burn last minute due to a non-permissive burn day.
Remote Area Weather Stations (RAWS; https://raws.dri.edu/index.html) are a network of strategically placed weather stations across the U.S. that provide reports of key weather parameters including temperature, wind speed, relative humidity, and, importantly, fuel moisture content. RAWS data can be used to monitor weather trends over time because each station contains several years’ worth of archived weather data. You can use this resource to assess historical weather conditions for your region to determine a time of year when conditions may line-up to meet your prescriptions, and to better understand fuel moisture conditions in relation to current weather. If your burn unit is not located near a RAWS, the data may not provide accurate forecasts.
The Weather Underground website (https://www.wunderground.com/) provides local and long-range weather forecasts, weather reports, maps, and current weather conditions for your given location.
The Windy mobile app and website (https://windy.app/map/) provides a detailed weather forecast and live wind map. Some features are free and others are subscription-based. First, navigate to the location of your burn, then view the “Forecast for This Location” section. Depending on your subscription level, you can use the “Compare” option to view up to six different weather models. By comparing forecasts, you can gauge your confidence in wind forecasts for your burn.