In order to implement a safe and successful prescribed burn, it is important to understand how various factors influence fire behavior. Wind, relative humidity, temperature, soil moisture, fuel moisture, air mass stability, and topography are important elements to understand and consider when planning and implementing a burn. These elements influence flame height, rate of fire spread, how smoke produced from the burning vegetation will dissipate, and the overall success of meeting the burn objectives.

Wind - Prescribed fires behave in a more predictable manner when wind speed and wind direction are steady. Wind speed generally increases to a maximum in the early afternoon and then decreases to a minimum after sunset. Ideal transport wind speed, wind measured at 5 meters above ground level, should range from 10 to 25 km/h for good smoke dispersion. The 5 meter wind speed, mentioned above, is the wind speed typically forecasted by local weather stations. However, the preferred surface wind, or wind speed at eye level, should range from 2 to 5 km/h for most fuel and topographic situations. When conducting prescribed burning in vast, wide-open spaces, wind speed at surface or eye level can approach the 5 meter wind speed because there is nothing in the way to slow the surface winds down.
Of greater importance than wind speed is the length of time the wind blows from one direction. Persistent wind directions occur most frequently following the 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, topography, natural firebreaks, and locations of smoke sensitive areas may have a bearing on which wind directions are most favorable. Regardless of wind direction, wind steadiness is very important and should be forecasted to occur throughout the planned burn time.

Relative Humidity - Relative humidity has a strong influence on the moisture content of the vegetation (fuels) being burned. As relative humidity decreases, fuels become drier. As relative humidity increases, fuels retain more moisture and are less apt to burn. 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. When a cold front passes over an area, the air behind the front is cooler and drier. The result is a drop in both temperature and humidity. Preferred relative humidity for prescribed burning ranges from 30 to 55 percent. When relative humidity drops below 30 percent, prescribed burning can become dangerous. Fires are more intense under low humidity ranges and sparks from burning fuels may be transported outside the burn area and ignite surrounding vegetation, causing unwanted spot fires. When relative humidity exceeds 65 percent, a fire may leave large unburned islands or may not burn hot enough to achieve the desired results. For most situations where a landowner is conducting a prescribed burn without professional assistance on-site, preferred relative humidity should range from 40 to 65 percent.
 
Temperature - Temperature can also strongly influence the moisture content of fuels being burned. High temperatures help dry fuels quickly. In addition, when fuels are exposed to direct solar radiation (sunlight), they become much warmer than the surrounding air. Moisture will move from the warmer fuels into the air even though the relative humidity of the air is high. Air temperature can also directly impact the heat intensity of the fire. Cool fires are typically not hot enough to kill woody vegetation. Under most prescribed burning objectives, the air temperature for a late winter to early spring burn should range from -7 C to 20 C. When the objective of the burn is to control woody vegetation, air temperatures above 16 C may be necessary to raise woody stem tissue to lethal temperature levels.

Rainfall and Soil Moisture - Because rainfall affects both fuel and soil moisture, it’s important to have a good idea of how much rain has recently fallen on the area to be burned. The importance of adequate soil moisture can’t be overemphasized. Damp soil protects the root zone of grasses, forbs and trees from being killed during a fire. It also protects soil microorganisms. Even when burning to expose a mineral soil seedbed it is desirable to leave a thin layer of organic material to protect the site from erosion. Prescribed burning should cease during periods of drought and resume only after a good soaking rain of at least 25 mm. On clay soils, much of the rainfall is lost through surface runoff, therefore, duration of the rainfall is more important than the amount that falls. For most prescribed burning objectives, the soil should be damp to moderately wet.

Fuel Moisture - Fuel moisture, especially the moisture content of fine fuels such as grassy and weedy material, is strongly influenced by relative humidity, temperature, and rainfall.
Fine-fuel moisture content should range from 10 to 20 percent for optimum burning conditions. A rough estimate can be obtained by taking the relative humidity (RH) and dividing it by 2: (RH ÷ 2 = FFM). When fine-fuel moisture is below 6 or 7 percent, burning can result in damage to plant roots, microorganisms and even the soil. When fine-fuel moisture nears 30 percent, fires tend to burn slowly and irregularly, often resulting in incomplete burns that do not meet the desired objectives. Fine-fuel moisture is usually at its lowest value when the maximum temperature has been reached for the day (usually in the late afternoon). As the sun sets, the temperature drops and the relative humidity increases. Fine-fuel moisture can also vary considerably depending on the height of the vegetation. Typically, moisture content will increase from the upper portion of the vegetation down to the litter layer. However, a light rain or morning dew following a dry spell can give the false impression that the litter layer and underlying soils are also moist. The bottom of the litter layer should always be checked prior to burning to make sure it feels damp.

Airmass Stability - Atmospheric stability is the resistance of the atmosphere to vertical movement and has an important influence on smoke management. A prescribe fire generates vertical air movement as the air is heated. If the atmosphere is unstable, the hot combustion products and smoke will rise rapidly and disperse into the upper atmosphere. Unstable atmospheric conditions promote rapid smoke dispersion. Indicators of unstable conditions include wind gusts, clouds with vertical growth, clear skies, and sometimes dust devils. Under stable atmospheric conditions smoke will be held close to the ground and can cause severe smoke problems and reduced visibility. Indicators of stable conditions include poor visibility due to haze, layered clouds, no wind, or very steady (not gusty) low wind.
 
Topography - Topography, or the lay of the land, also influences fire behavior and is the most constant of the environmental elements. It is much easier to predict the influences which topography will have on a fire than the influences of fuel characteristics and weather.

Aspect, slope and terrain are the three characteristics of topography that can influence fire behavior. Aspect refers to the direction a slope faces. This determines the amount of heating the fuel gets from solar radiation, as well as the condition and types of fuels present. South and southwest slopes are normally more directly exposed to sunlight, and generally have sparser fuel loads, higher temperatures, lower humidity, and lower fuel moisture. North and northeast slopes generally receive less direct sunlight, and typically have heavier fuel loads, lower temperatures, higher humidity, and higher fuel moisture.

Slope is the degree of incline of a hillside. Fires burn more rapidly uphill than downhill. The steeper the slope, the faster and hotter the fire burns. This is because the fuels above the fire are brought into closer contact with the upward moving flames. Heat from the flames reduces fuel moisture and allows the fuels to catch on fire quickly. Conversely, a fire started at the top of a slope will move down slope slower and cooler.

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Terrain refers to the shape, or lay of the land, and can influence the direction and rate of fire spread. Fire in steep narrow ravines can easily spread to fuels on the opposite slope by radiant heat and wind blown sparks. Likewise, fires started at the bottom of ravines may react similar to a fire in a chimney. Air drawn from the bottom of the ravine will create very strong upslope drafts. These upslope drafts will spread the fire rapidly and result in extreme fire behavior that can be very dangerous. In addition, fires immediately adjacent to woodland edges may be affected by wind eddies that may move the fire in the opposite direction of normal wind flow.