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A general understanding of the separate and combined effects of several weather elements on fire behavior is needed to plan and execute a good prescribed burn. Wind, relative humidity, temperature, rainfall, and airmass stability are important elements to consider because these factors influence fuel moisture. Because weather and fuel factors interact, an experienced prescribed burner can conduct a successful burn even with one or more factors slightly outside the desired range?as long as they are offset by other factors.

The following sections, from the widely used publication: A Guide For Prescribed Fire In Southern Forests (USDA Forest Service 1989), discuss the desirable ranges for weather and fuel elements that produce optimal burning conditions for both underburning and debris burning.

• Wind Considerations
• Relative Humidity Considerations
• Temperature Considerations
• Rainfall and Soil Moisture Considerations
• Airmass Stability and Atmospheric Dispersion Considerations
• Fine-fuel Moisture Considerations
• Topographical Considerations

For more detailed information on the effects of fuels and weather on fire behavior, see the following major sections of the encyclopedia:

• Fuels
• Fire Weather
• Fire Behavior

Underburning

Weather Kit

Prescribed fires behave in a more predictable manner when wind speed and direction are steady. Onsite winds vary with stand density and crown height. Wind speed generally increases to a maximum in the early afternoon and then decreases to a minimum after sunset. The preferred range in wind speed in the stand is 1 to 3 mph (measured at eye-level) for most fuel and topographic situations.

Wind speed readings for most fire-weather forecasts are, however, taken 20 feet above ground at open locations. Wind speeds in fire-weather forecasts are the maximum expected and not the average for the day. The minimum 20-foot wind speed for burning is about 6 mph and the maximum is about 20 mph. These are the most desirable winds for prescribed burning, but specific conditions may tolerate other speeds. As a general rule higher wind speeds are steadier in direction.

Steady Winds

Relatively high winds quickly dissipate the heat of a backing fire. The result is less crown scorch than from a fire backing into a low-speed wind. In-stand wind speeds should be in the low to middle range (1 to 2 mph) when heading fires are used. With high winds, heading fires spread too rapidly and become too intense. On the other hand, enough wind must be present to keep the heat from rising directly into tree crowns.

Of greater importance than wind speed is the length of time the wind blows from one direction. Persistent wind directions occur frequently during winter, especially following passage of a cold front when winds are typically from the west or northwest. As these winds slowly shift clockwise over the next few days, they become weaker and less steady. Winds with an easterly component are generally considered undesirable for prescribed burning.

However, along the coast, sea and land breezes are often utilized. Irrespective of direction, a forecast of wind steadiness should always be obtained.

The most critical areas, with regard to fuel and topography, should be burned when wind direction is steady and persistent. Relatively easy burns can be conducted under less desirable wind conditions. Topography, and local effects such as stand openings, roads, etc. may have a bearing on favorable wind conditions and should always be considered when planning a burn.

For more information on wind, see: General Winds or Convectional Winds.

Debris Burning

Winds are stronger in open areas than they are in the forest. Because there is no overstory to protect, wind is not needed to cool the heated combustion products. However, from a smoke management standpoint, the stronger the wind the better the dispersion, provided there are no downwind smoke-sensitive areas that will be impacted. When broadcast burning, eye-level winds over 3 to 4 mph can create containment problems if a heading fire is used. With piled or windrowed debris, eye-level winds of 8-10 mph can be tolerated by adjusting the firing pattern.

Wind direction may change substantially with height, but it is these transport winds that regulate the movement of the smoke column. Moderate transport wind speeds allow a convection column to develop that exhausts the smoke high into the atmosphere where it quickly disperses with a minimum impact on ground-level air quality. Before setting a fire that will generate a convection column, however, obtain information on the existing and forecast wind profiles. If an adverse profile exists, it is likely to result in an unacceptably high spotting potential. Fire behavior characteristics are associated with various wind profiles. Once the fire has died down and smoke production is from smoldering combustion, surface wind is necessary to ensure good smoke dispersion.

For more information on wind, see: General Winds or Convectional Winds.

Underburning

Daily Temperature/Relative Humidity Cycle

Relative humidity is an expression of the amount of moisture in the air compared to the total amount the air is capable of holding at that temperature and pressure. Each 20^oF rise in temperature (which often occurs during the morning hours on a clear day) reduces the relative humidity by about one-half, and likewise, each 20^oF drop in temperature (which often occurs in early evening) causes relative humidity to roughly double. When a cold front passes over an area, the air behind the front is cooler and drier than the old airmass it is replacing. The result is a drop in both temperature and humidity.

Preferred relative humidity for prescribed burning varies from 30 to 55 percent. Under special conditions, a wider range of relative humidities, as low as 20 percent and as high as 60 percent, can produce successful burns. When relative humidity falls below 30 percent, prescribed burning becomes dangerous. Fires are more intense under these conditions and spotting is much more likely; proceed only with additional precautions. When the relative humidity is 60 percent or higher, a fire may leave unburned islands or may not burn hot enough to accomplish the desired result.

The moisture content of fine dead fuel such as pine needles and dried grasses responds rapidly to changes in relative humidity. However, there is a time lag involved for fuels to achieve equilibrium with the moisture condition of the surrounding atmosphere. Also, previous drying and wetting will influence fuel moisture. Therefore, the relative humidity and fuel moisture must be assessed independently.

For background information on relative humidity, see: Atmospheric Moisture.

Debris Burning

Relative humidity (along with temperature) controls fuel moisture content up to about 32 percent. Liquid moisture such as rain, snow, fog, or dew must contact a fuel for moisture content to rise above 32 percent, and the increase depends upon duration as well as the amount of precipitation.

Recently-cut pine tops have a drying rate that is somewhat independent of relative humidity as long as the moisture content of fresh tops (needles still green) is above about 32 percent. Once this material initially dries to a moisture content below 32 percent, it behaves as a dead fuel and becomes much more responsive to daily fluctuations in relative humidity. The response to changes in relative humidity is much more rapid in fine dead fuels suspended above the ground than in those that have become part of the litter layer. These elevated needles and other suspended dead materials are not in contact with the damp lower litter and are more exposed to the sun and wind.

When burning piled debris, once the larger-diameter fuels ignite, increases in relative humidity have little effect on fire behavior during the active burning phase. Low humidities (below 30 percent), however, will promote spotting and increase the likelihood of fire spreading between piles.

For background information on relative humidity, see: Atmospheric Moisture.

Underburning

The average instantaneous lethal temperature for living tissue is about 147^o F. Air temperatures below 60^o F are recommended for winter underburns because more heat is needed to raise foliage or stem tissue to lethal temperature levels. When the objective is to control undesirable species, growing-season burns with ambient air temperatures above 80^oF are recommended. These conditions increase the likelihood of reaching killing temperatures in understory stems and crowns. Of course, the overstory pines must be large enough to escape injury. Larger trees have thicker bark and their foliage is higher above the flames, which allows more room for the hot gases to cool before reaching the crowns.

Temperature strongly affects moisture changes in forest fuels. High temperatures help dry fuels quickly. When fuels are exposed to direct solar radiation, they become much warmer than the surrounding air. Moisture will move from the warmer fuel to the air even though the relative humidity of the air is high. Temperatures below freezing, on the other hand retard fire intensity because additional heat is required to convert ice to liquid water before it can be vaporized and driven off as steam. Consequently, it does not take much moisture under these conditions to produce a slow-moving fire that will leave unacceptably large areas unburned.

For background information on temperature, see Temperature.

Debris Burning

For background information on temperature, see Temperature.

Underburning

Because rainfall affects both fuel moisture and soil moisture, the burn executor should have some idea of the amount of recent rainfall on the area to be burned. In winter, rainfall is fairly uniform throughout large regions of the South and rainfall data can be obtained from local weather stations. In summer, when shower activity prevails, rainfall at individual locations is much more variable. The only reliable method to determine the amount of precipitation that actually falls is to place an inexpensive rain gauge on the site.

The importance of adequate soil moisture cant be overemphasized. The preferred soil moisture is damp. Damp soil protects tree roots and microorganisms. Even when burning to expose a mineral soil seedbed it is desirable to leave a thin layer of organic material to protect the soil surface. Burning should cease during periods of prolonged drought and resume only after a soaking rain of at least 1 inch and a check for adequate soil moisture. As soil moisture conditions improve, less rain is needed before burning, but a site specific check, which includes areas adjacent to tree boles, is still needed to verify adequate soil moisture. If recent precipitation has been near average, 1/4 to 1/2 inch of rain followed by sunny skies, brisk winds, and low humidities will generally result in several days of good prescribed fire conditions with adequate soil protection.

On clay soils, such as are found in the Piedmont, much of the rainfall is lost through surface runoff, and duration is more important than amount. For example, 1 inch of rain occurring in 1/2 hour will not produce as large a moisture gain as 1/2 inch falling over a 2 hour period.

For background weather information on rain see: Clouds and Precipitation.

Debris Burning

Generally, rain has a much greater effect on fuel moisture in cleared areas than under a stand because none is intercepted by tree canopies. However, fuels also dry much faster in cleared areas because of increased sunlight and higher wind speeds. This differential drying can often be used to advantage from a fire-control standpoint. Burn the cleared area several days after a hard rain while fuels in the surrounding forest are still damp. Burning under these conditions assures good soil moisture. However, when burning cleared areas, soil damage is as much a function of fire intensity and duration as it is of soil moisture. Intense, long duration fires will bake the soil regardless of the moisture present. Both the chemical and phyical properties of the soil can be altered. This type of fire should be avoided, especially on clay soils and steep slopes. These undesirable fire effects are often produced when burning windrowed or piled debris, and are one reason piling or windrowing slash prior to burning are discouraged.

For background weather information on rain see: Clouds and Precipitation.

Underburning

Stable Conditions

Atmospheric stability is the resistance of the atmosphere to vertical motion. When the atmosphere is stable, temperature decreases slowly as altitude increases (less than 5.5 ^oF per 1,000 feet). Under very stable conditions, inversions may develop in which temperature actually increases with height. The distance from the ground to the base of this inversion layer is called the mixing height. Under less stable atmospheric conditions, other factors beyond the scope of this discussion determine the height of the mixing layer. In either case, the mixing layer is defined as the layer of air within which vigorous mixing of smoke and other pollutants takes place. The average wind speed throughout the mixing layer is called the transport wind speed.

Mixing heights above 1,700 feet and transport wind speeds above 9 mph are desirable for good smoke dispersion. Some prescribed burners on the Ozark Plateau believe their fires become difficult to control when the mixing height is greater than 6,500 feet.

The old adage that hot air rises is true but only as long as it is warmer than the surrounding air. Thus, stable air tends to restrict convection column development and produces more uniform burning conditions. However, combustion products are held in the lower layer of the atmosphere (especially under temperature inversions). Visibility is likely to be reduced because of smoke accumulation. As the earth cools each night, the air near the ground is cooled more than the air above, forming a stable layer. Because this cold air is denser, it drains into low-lying areas such as swamps and bottomlands, carrying with it smoke from smoldering stumps, branches and other debris.

Unstable Conditions

When the atmosphere is unstable, the decrease in temperature with height exceeds 5.5 ^oF per 1,000 feet. Once a parcel of air starts to rise, it will continue to rise until it cools to the temperature of the surrounding air. Such conditions promote convection and rapid smoke dispersion but, if severe, can make fire control difficult.

A neutral atmosphere is one in which a rising parcel of air remains at the same temperature as its surrounding environment (i.e., the temperature decrease with altitude equals the dry adiabatic lapse rate of 5.5^oF per 1,000 feet). Smoke dispersion in a neutral atmosphere can be adequate if wind speed is sufficiently high, but higher winds can also effect fire control.

A good rule-of-thumb is to obtain forecasts of mixing height, transport wind speed, and atmospheric stability, but also to observe local indicators at the fire site. Indicators of a stable atmosphere are steady winds, clouds in layers, and poor visibility due to haze and smoke hanging near the ground. Unstable conditions are indicated by dust devils, gusty winds, clouds with vertical growth, and good visibility.

A prescribed fire generates vertical motion by heating the air. If the atmosphere is unstable, the hot combustion products will rise rapidly because of the large temperature difference between the smoke and surrounding air. The column will continue to build in height as long as it remains relatively stationary and is heated by new combustion products faster than it is being cooled. The stronger the convective activity, the stronger the indrafts into the fire. This effect increases fire intensity by producing even stronger convective activity. Eventually spotting, crowning and other indicators of erratic fire behavior develop. Such fires need to be suppressed as quickly as possible to hold damage to a minimum. With adequate planning, this situation rarely develops when underburning, using conventional ground-ignition techniques. However, when using aerial ignition techniques at the high end of the prescription window, too much area can be ignited too quickly. This action results in severe damage to the overstory. The behavior of the first row or two of spots should warn the burning boss to halt ignition and observe fire behavior before making a decision to adjust the ignition pattern, change firing techniques, or terminate the burn.

For background weather information on stability, see Atmospheric Stability.

Debris Burning

Strong convection over cleared areas burned for site preparation or slash disposal helps vent smoke into the upper atmosphere. A convection column will continue to rise until it cools to the temperature of the surrounding air or until it reaches the base of an inversion layer. A well developed convection column produces strong indrafts which help confine this type fire to its prescribed area.

Care must be taken to ensure that all burning materials sucked into the convection column burn out before being blown downwind and dropping to the ground to act as firebrands.

Whenever a burn site is in hilly terrain, diurnal slope winds must be considered. As soon as a slope is heated by the morning sun, an upslope breeze results. This breeze will increase to a maximum (< 8 mph) during the early afternoon and end as the slope cools in the evening. As the slope continues to cool, a downslope wind will develop, reaching a maximum (< 5 mph) after midnight. This breeze will end after sunup as the slope again begins its daily heating cycle. If you ignite a fire at the base of a slope during the day, differential heating will be greatly increased. The fire will rapidly spread uphill, giving the combustion products added lift to help vent them into the atmosphere. However the nighttime downslope wind will have the opposite effect, concentrating any drift smoke in low areas.

For background weather information on stability, see Atmospheric Stability.

Underburning

Fine-fuel Moisture Chart

Fine-fuel moisture is strongly influenced by rainfall, relative humidity, and temperature. The preferred range in actual (not calculated) fine-fuel moisture of the upper litter layer (the surface layer of freshly fallen needles and leaves) is from 10 to 20 percent. Burning when fine-fuel moisture is below 6 or 7 percent can result in damage to plant roots and even the soil. When fine-fuel moisture approaches 30 percent, fires tend to burn slowly and irregularly, often resulting in incomplete burns that do not meet the desired objectives. However, when areas with very heavy fuel buildups or extensive draped fuels are burned, moisture content should be 20 to 25 percent to keep fire intensity manageable, especially if aerial ignition techniques are used. Fine-fuel moisture values obtained from National Fire Danger Rating System (NFDRS) tables on fire-behavior models are considerably less than these actual values.

Some experienced practitioners can accurately estimate fuel moisture by examining a handful of litter. However, the only sure way to tell is to take a sample and ovendry it. Tables and equations in the NFDRS and BEHAVE can be used to estimate fine-fuel moisture, but the results are invariably underestimates (because they are worst-case values designed for use in predicting wildfire behavior).

Fuel-moisture Sticks

One simple test that will give a very rough estimate of the upper-litter-layer moisture content is to pick up a few pine needles and individually bend each in a loop. If the needles snap when the width of the closing loop is about 1/4 to 1/2 inch, their moisture content is between 15 and 20 percent. If they do not snap in two, they are too wet to burn well. If they crumble into small pieces they are exceedingly dry and even if the lower litter is moist, the fire may cause damage and be difficult to control. Fuel moisture sticks that respond to weather changes like 10-hour fuels are available. With a good set of scales and proper placement of the sticks, acceptable fuel moisture estimates can be obtained just before ignition. These values will differ slightly from actual fine-fuel moistures, but are fairly representative of most southern fuel types. They are much closer to actual fine fuel moistures than are calculated or tabular values.

Lower litter should always be checked before burning to make sure it feels damp. This will help ensure that some remains, even though charred, to leave a protective covering over the soil. Generally, the moisture content increases from the litter surface down through the duff layer to the soil. Exceptions can occur after a light shower, or in the morning after a heavy dew. In these cases, fires often burn more intensely than would be expected from just looking at the upper-litter-layer moisture content. When burning on organic soils this phenomenon can have drastic consequences. If the fire dries the moist surface layer of peat, the organic soil will ignite. These fires can impact an area for many weeks in spite of control efforts, causing extensive smoke problems.

Smoke

The speed with which fine fuels respond to changes in humidity depends on fuelbed characteristics such as whether the fuelbed consists of compacted hardwood leaves or jack-strawed pine needles. Different fuel types can reach different moisture contents under the same humidity conditions. For example, grassy openings containing cured material can be burned within hours of a drenching rain if good drying conditions exist. Because of these natural variations, recommended fine-fuel moisture values are only guidelines.

On-the-ground knowledge of fuels must be incorporated into the burning prescription. Fuel moisture also influences smoke production. When very damp woody fuels burn, large amounts of characteristic white smoke are given off. Much of the visible smoke plume is actually condensed water vapor.

For background information, see Fuel Moisture.

Debris Burning

Harvested areas should be burned when fuels are dry. They will ignite easier, burn more quickly and completely, shortening the time necessary to complete the burn. Less mop-up will be required and the impact on air quality will be reduced. The short, but severe, summer droughts common throughout much of the South provide ideal burning conditions on cleared areas, provided soil moisture does not get too low.

To avoid the possibility of unnecessary damage to the site, debris should be burned as it lies (broadcast burned) rather than piled. Because fuels on logged areas receive full solar radiation, they dry before surrounding forest fuels do. It takes at least several weeks after cutting for the severed tree tops to cure. Once the needles turn a greenish-yellow and the hardwood leaves wither, the debris is ready to burn. Cleared areas can then be safely burned soon after a rain, before adjacent forest fuels dry enough to burn well. Ten-hour fuel moisture (fuels 1/4 to 1 inch in diameter, such as branches and small stems) is a better indicator of burning conditions in slash fuels than is fine-fuel moisture. Fuel moisture sticks will give excellent results. One set of "sticks" can be placed on the area to be burned and another in the nearby undisturbed forest. Let the sticks become acclimated for at least 2 weeks before reading. Many managers consider the area ready to burn when the moisture content of the sticks on the logged area reaches about 10 percent while that of those in the forest is still above 15 percent.

If the burn objective is to consume larger fuels (over 2 to 3 inches in diameter), piling will probably be necessary. Piling in wet weather should be avoided, and piles should be small and free of dirt. Fresh logging debris needs to cure for several weeks before piling, because drying conditions are exceedingly poor in the middle of a pile, especially if it is compacted or contains much dirt. Much of the smoke problem associated with burning piled debris is caused by inefficient combustion of damp, soil-laden piles. These piles may smolder for days or weeks.

For background information, see Fuel Moisture.

Slope and aspect also influence another important factor in determining fire behavior: vegetation. On most lower-slope positions and throughout north- and east-facing slopes, forest cover generally consists of hardwood species whose fuels have relatively low combustibility. Combined with higher fuel moisture and relative humidity levels, these slopes burn less frequently and with less intensity than upper-slope positions. In contrast, south- to west-facing slopes have open canopies and generally fire-tolerant vegetation. Combined with dry fuels and low relative humidity, these slopes burn more frequently and with greater intensity. There is also a change in fuel-types with elevation, from pure hardwoods on foot-slopes to mixed pine-hardwoods on mid-slopes to pure yellow pine stands on upper slope positions.

Slope steepness affects the rate of spread of a fire by influencing flame length and preheating of fuels. Flames more easily detach from a shallow slope than a steep slope. As slopes increase, radiant heating can reach more fuels and both ambient and convectional winds may force the convection column to move along the slope adding convectional heat to the preheating of fuel upslope from the fire. More rapid heating of the upslope fuels increases flame lengths and rate of spread (Rothermel 1985). See also: Effects of Topography on Rate of Spread.

Topography can also affect fire behavior indirectly by influencing surface winds, such as in the formation of slope and valley winds. See also: Effects of Mountain Topography on Surface Winds and Slope and Valley Winds.

Before or during prescribed fires, the following sources of weather information can be used to forecast safe burning conditions. Ordinarily, four sources of weather information are available:

• NATIONAL WEATHER SERVICE
• STATE FORESTRY AGENCIES
• LOCAL OBSERVATIONS
• PRIVATE WEATHER FORECASTING SERVICES

Local National Weather Service offices furnish weather forecasts and outlooks via radio and television. Spot weather forecasts are also available, but their value depends upon the forecaster's knowledge of local conditions. Inexpensive radios are also available that continually monitor National Oceanic and Atmospheric Administration (NOAA) weather-related information and forecast updates. Relying solely on the NOAA broadcasts is ill-advised because this information is not specific enough for smoke-management planning.

The best source of information including current forecasts and outlooks is generally the local office of your state forestry agency. The person you talk to can often help you interpret the forecast, give you any warnings, and pass on pertinent information such as other burns planned for that day, The prescribed burner should take full advantage of such services.

All southern state forestry agencies and national forests, as well as many military bases and private concerns operate fire-danger stations. The basic weather parameters measured at these sites are very useful. However, National Fire Danger Rating System (NFDRS) indices which are calculated from these measurements should not be used. This system was designed to provide a worst-case scenario for wildfire control over very large areas. It was not designed as a planning tool for prescribed burning!

Weather observations should be made at the prescribed burn site immediately before, during, and immediately after a fire. Such observations are important because they serve as a check on the applicability of the forecast and keep the burning crew up-to-date on any local influences or changes. Take readings in a similar area upwind of the fire to avoid heating and drying effects from the fire. Do this at 1- to 2- hour intervals, or more often if changes in fire behavior are noticed. Measurements taken in an open area, on a forest road, and in a stand are likely to differ widely. Easy-to-use belt weather kits that include a psychrometer and an anemometer are available. By using this kit and observing cloud conditions, a competent observer can obtain a fairly complete picture of the current weather.