Prescribed fire is a fantastic deer habitat management tool, and nothing is more important when using it than ensuring a safe, effective and contained burn. By planning and waiting to burn under the right conditions, you’ll achieve desirable habitat benefits while avoiding undesirable outcomes.
What are undesirable outcomes? The fire jumping your firebreaks and escaping the burn unit or your property. Harming valuable trees inside the burn unit with a fire that’s too intense. Harming people by smoking out a nearby town, school, hospital or highway. All of this is avoidable through planning and using weather as your guide.
The best prescribed fire conditions are sort of a “Goldilocks” zone. You need everything “just right” to burn effectively and safely. Your state forestry agency isn’t likely to issue a burn permit on red-flag days when weather conditions are too dangerous. However, even with a burn permit, you can do harm or waste your effort on an ineffective burn.
Through Learn-n-Burn courses, certified prescribed burner training, serving on the steering committee of the Georgia Prescribed Fire Council, and planning my own burns, I’ve become very familiar with a handful of weather and fuel factors that determine fire safety and effectiveness. Here’s a review of the most important and useful factors and where to find them.
Of course, this is only a general guide! The ideal ranges for each of these factors may vary slightly by region, the type of cover being burned, and your goals. Therefore, combine the following information with local training opportunities, especially any chance to help others while learning from experienced burners. Nothing beats first-hand experience. Before leading your own burn, write a plan, stick to it, and acquire the necessary prescribed fire permit.
Wind Speed and Prescribed Fire
You need wind to carry the fire, or you won’t be able to burn at all. Too much wind, though, increases fire intensity and the likelihood of escapes or timber damage. In general, you want 2 to 6 mph “in-stand” winds, meaning at eye or ground level. Heavy timber will greatly reduce in-stand winds, so always check the real conditions in your burn unit against the local forecast.
When a forecast provides “20-foot” wind speeds, that means at a height 20 feet above the top of the dominant vegetative cover. For these, a range of 5 to 15 mph is ideal. Higher winds than these ranges are a “red flag” condition for most prescribed fire and wildlife habitat goals.
Wind Direction and Prescribed Fire
You must know where your smoke is going. This one should be easy enough for a deer hunter to understand: Just equate smoke to scent! Don’t put smoke or your scent on sensitive areas. In this case, sensitive means roads, neighborhoods or any area where smoke could cause a safety issue. For every burn unit you establish, you can also determine the ideal winds for each in your planning phase – just like knowing the best wind direction for a deer-stand location. Wind direction also determines how and where you light the fire, since you usually want to start with a slow, “cool” fire that is backing into the wind.
A Word About Slope and Prescribed Fire
Wind is the most important weather factor in fire behavior, but slope can override and alter forecasted wind direction and speed – at least under the “just right” conditions for prescribed burning. Fire runs uphill much faster than downhill, so slope always trumps wind direction. Start your fire at the top of a slope even when the wind direction suggests you would create a heading fire. Also, a fire burning uphill can become more intense than a fire on flat ground under the same weather conditions. Use extra precautions and monitoring of actual wind conditions on slopes when burning in hilly or mountainous areas. Use small test fires, and adjust your burn plan accordingly.
Air Temperature
Temperature is in a roughly inverse relationship with humidity. The hotter the day, the drier the day, and vice versa. What might be ideal humidity and fuel moisture conditions at 11 a.m. when you light a fire could become too dry by 3 p.m., increasing fire intensity. Additionally, high air temperatures can increase the chances of “crown scorch” on mature trees. The main concern on hot days, however, is your health and that of your crew. There are several steps you need to take to prevent heat exhaustion. In general, do not burn when air temperatures exceed 85 degrees.
Again, there can be exceptions. Last summer on a June burn in Southeast Georgia, I saw air temperatures climb into the mid-90s in the afternoon before I completed the burn. Wind, humidity and fuel moisture conditions remained good, and we wrapped up an effective burn with no intensity issues.
Relative Humidity
An RH of 30 to 50% is common for prescribed fires. Below 30%, conditions are too dry, and “spotting” or escapes from embers landing outside your burn unit become more likely, especially if wind speed is on the high end. You can burn with RH in the 20s, but the risk of losing control of your fire is much greater. Burning above 50% is sometimes possible depending on fuels but requires more wind than normal, and you may get an inconsistent burn across the unit and have trouble keeping the fire lit.
Fuel Moisture
The ideal range for “fine fuel” moisture – meaning the smallest fuels like dry grass, dead leaves, and fallen conifer needles in the top layer of forest litter – is 8% to 15%. Any drier, and the risk of embers starting new fires across firebreaks gets too high. Plus, you risk burning down into deeper layers of litter known as “duff,” which could damage tree roots. Any wetter, and you’ll have a difficult time getting a consistent burn.
“Pay attention to how relative humidity and fuel moisture interact with each other,” said Dr. Mark Turner, Oklahoma Wildlife Extension Specialist. “For example, if fine fuel moisture is elevated from a recent rain, you may still be able to burn effectively if RH is low. In the reverse, you might rather have higher RH during dry fuel conditions.”
Mixing Height
It’s critically important in smoke management to burn on days when smoke will lift quickly and be dispersed into the atmosphere rather than linger on the ground like fog. Mixing height helps you predict this. It’s the height below which air is being mixed by transport winds. You want a minimum mixing height of 1,650 feet. If you’re burning near a sensitive public area like a highway or community, increase your minimum to 2,200 feet.
Transport Winds
This is the average wind speed and direction between the ground and the mixing height. Ideal transport winds are 9 to 20 mph. Together, mixing height and transport winds largely determine whether your smoke lifts and disperses or remains low to the ground where it can cause safety issues in sensitive areas nearby.
Atmospheric Dispersion Index
This index of mixing height and transport winds gives you an idea of how well and how quickly smoke will be dispersed. You want a minimum daytime dispersion index of 40 to burn. The ideal range is 40 to 90.
Turner Stability Index
This is an index of atmospheric stability that’s useful. In general, very stable atmospheric conditions are undesirable because smoke won’t rise or disperse. Very unstable is also undesirable because wind conditions are too erratic. You want conditions somewhere in between. The Turner index is a scale of 1 (very unstable) to 7 (very stable). Burn when this index is at 3, 4 or 5.
Low Visibility Occurrence Risk Index (LVORI)
This index was developed in Florida as a way to predict fog-like conditions when humidity is high and dispersion low, the kind of situation when automobile accidents become more likely. LVORI is a scale of 1 to 10, with 1 being the lowest likelihood of dangerous smoke/fog conditions. Burn only on LVORI conditions of 1 through 6.
Days Since Rain
“Most fuels need a drying time, so days since rain is going to be a big factor in fire behavior whether you are burning oak litter or native grass,” said George Jensen, a Prescribed Burn Association Coordinator with the non-profit Tall Timbers. “As a general rule, grass-heavy and more open areas can sometimes be burned right after rain, but more shaded areas often need three or more days to dry out before burning can happen. But greater than 10 days can prove to be dangerous. A test fire that you can easily put out will give you an idea of what your fuels are going to do on your property.”
Prescribed Fire Conditions at a Glance
To review, here are the ideal ranges in one place:
- Wind speed, eye-level: 2 to 6 mph
- Wind speed, 20-foot: 5 to 15 mph
- Air Temperature: Under 85 degrees
- Relative Humidity: 30 to 50%
- Fine Fuel Moisture: 8 to 15%
- Mixing Height: Above 1,650 feet (above 2,220 near sensitive public areas)
- Transport Winds: 9 to 20 mph
If Available in Your Area:
- Dispersion Index: 40 to 90
- Turner Stability Index: 3, 4 or 5
- LVORI: 1 to 6
Where To Get This Information
Most of the above information can be found on the National Weather Service’s Fire Weather Dashboard. Enter your location in the search bar, and you’ll get a weekly summary and an hourly table (see below) of forecasted conditions. While some of the above indexes aren’t provided in every location, you’ll be able to get most of the critical information above.
Additionally, check the websites of your state forestry agency and your state prescribed fire council. Both will provide additional links specific to your state with local weather monitoring resources. Click here to find the website of your state Prescribed Fire Council.
Simulate Prescribed Fire Online
One of the best ways I’ve found to learn how weather factors will affect smoke dispersion at a specific location is to practice with an online simulation called “V-Smoke.” Using V-Smoke, you can plug in the actual location of your planned burn. You’ll fill in a few details about the size of your burn, the type of fuel being burned, and then you can enter any wind direction, mixing height and transport wind speed you like. Run the model, and you’ll see on a map the potential for your smoke to affect nearby areas.
In the V-Smoke map below, the yellow circle represents the area with a “moderate” level of health concern due to smoke, where air quality is generally acceptable. The orange circle represents “unhealthy for sensitive groups,” and the red is “unhealthy” for anyone. Additional levels of concern include “very unhealthy” and “hazardous.” In this example, no roads, schools, homes or other sensitive areas will be affected by a fire at this location, under these conditions.
Practicing with V-Smoke teaches you how mixing height and transport winds help disperse smoke quickly – or not. By adjusting the model’s conditions, you’ll quickly get a sense of the “just right” conditions for any location you plan to burn. Here’s a tutorial on how best to use V-Smoke, if you need it. There are other online models in addition to V-Smoke. To learn more about them, read this fact sheet from the Southern Fire Exchange.
Few things are more satisfying and fun in habitat management than implementing an effective prescribed fire. You’ll enjoy it all the more if you plan ahead and know you are burning under the “just right” conditions.