Managing smoke in ways that prevent serious impact to sensitive areas from single burns or multiple burns occurring simultaneously requires knowledge of the weather conditions that will affect smoke emissions, trajectories, and dispersion. Once smoke enters the atmosphere, its concentration at any one place or time depends on mechanisms of transport and dispersion. By transport, we mean whatever carries a plume vertically or horizontally in the atmosphere. Dispersion simply is the scattering of smoke. Vertical transport is controlled by the buoyancy of the smoke plume and stability of the atmosphere. Horizontal transport is controlled by wind. The larger the volume of space that smoke is allowed to enter and the farther it can be transported, the more disperse and less concentrated it will become.

Not only is it necessary to anticipate the weather ahead of time through the use of climatology and forecasts, but it also is useful to monitor conditions prior to and during the burn with regional, local, and on-site observations. On-site observations are helpful because air movement, and therefore smoke movement, is influenced by small variations in terrain and vegetation cover, and proximity to lakes and oceans, which off-site observations usually cannot capture. Also, forecasts are not always accurate and last-minute changes in a burn or smoke management plan may be needed. To gain more insight into the physical process of weather in wildland areas and its effect on biomass fires, refer to the section Fire Weather.

In using weather observations, forecasts, and climate summaries effectively for smoke management there are 3 general guidelines; (1) become familiar with local terrain features that influence weather patterns, (2) develop a dialogue with a reliable local weather forecaster, and (3) ask for and use climate summaries of wind and mixing height. By combining your knowledge of local weather effects, trust and communication with an experienced forecaster, and understanding of climate patterns, it is possible to fine-tune or update forecasts to meet your specific smoke management needs.

The following sections summarize how different weather and climate elements influence smoke transport and dispersion:

• Air Pressure
• Lapse Rates
• Atmospheric Stability
• Mixing Height
• Temperature Inversions
• Wind
• Atmospheric Moisture
• Weather Forecasts
• Climate

It is helpful to understand air pressure because storms and stagnant air conditions are described in terms of low pressure and high pressure, respectively. Lines of constant pressure are used to illustrate the state of the atmosphere on weather maps, and pressure influences the expansion and contraction of smoke parcels as they travel through the atmosphere. Air pressure is the force per unit area exerted by the weight of the atmosphere above a point on or above the earth’s surface. More simply it can be thought of as the weight of an overlying column of air. Air pressure is greatest near the ground, where the overlying column of air extends the full height of the atmosphere. Pressure decreases with increasing altitude as the distance to the top of the atmosphere shortens. In a standard atmosphere, which represents the horizontal and time-averaged structure of the atmosphere as a function of height only, pressure decreases approximately exponentially with height. With 1,013 millibars (mb) being the standard atmospheric pressure at sea level, the average height of the 850 mb pressure level typically occurs at about 5,000 feet (~1,500 m), the 700 mb pressure level typically occurs at about 10,000 feet (~3,000 m), and the 500 mb height averages around 20,000 feet (~6,000 m). In the lowest part of the atmosphere (less than about 8,000 feet) pressure decreases by approximately 30 mb per 1000 feet. These are useful values to remember when analyzing meteorological data and maps for smoke management. Actual pressure is nearly always within about 30% of standard pressure.

See: Fire Weather for more information.

Lapse rate is the decrease of temperature with height. Lapse rates help determine whether smoke will rise from a fire or sink back to the surface and are used to estimate atmospheric stability. When air is heated it expands, becomes less dense and more buoyant. This causes it to rise. A parcel of air that is heated at the ground surface by fire or solar radiation becomes warmer than its surroundings, causing it to lift off the surface. As it rises, it encounters lower pressure that causes further expansion. The more air expands, the cooler it becomes. If a parcel of air becomes cooler than its surroundings, it will sink.

Cooling by expansion without an exchange of heat at the parcel boundaries is called adiabatic cooling. In dry air, rising air parcels typically cool at a rate of about 5.5 °F per 1,000 feet (~ 10 °C/km). This is called the dry adiabatic lapse rate (DALR). For example, on a clear day if a heated parcel of air begins at sea level with a temperature of 70 °F (~21 °C), it will cool dry-adiabatically as it rises, reaching a temperature of 53.5 °F (~12 °C) at 3,000 feet (~915 m).

Rising moist air (relative humidity greater than about 70%) is said to undergo a saturation-adiabatic process. The saturated adiabatic lapse rate (SALR) or moist adiabatic lapse rate is a function of temperature and water content. This is because as moist air cools its water vapor condenses, giving off latent heat in the condensation process and causing a saturated parcel to cool more slowly than a dry parcel. Near the ground in mid-latitudes the SALR can be approximated at a rate of about 3 °F per 1,000 feet (~ 5.5 °C/km). For example, on a humid or rainy day, a heated parcel with a 70 °F (~21 °C) initial temperature at sea level, will reach a temperature of 61 °F (~16 °C) at 3,000 feet (~915 m).

Lapse rates are determined by comparing temperatures between different elevations. The temperature from a ridge-top weather station can be subtracted from the temperature at a nearby valley-located weather station to calculate lapse rate. More commonly, radiosonde observations (raobs) are used to determine lapse rates. These balloon-mounted instruments measure temperature, wind, pressure, and humidity at several elevations from the ground surface to thousands of feet. Raobs are available from weather services or at several sites on the Internet twice each day: at 0000 Universal Time Coordinated (UTC) and 1200 UTC.

There are several ways of plotting raob data. Typically a pseudo-adiabatic chart is used. This chart shows measured values of temperature vs. pressure over lines of DALR and SALR. The first figure illustrates how the above examples would appear on a standard pseudo-adiabatic chart. More recently, skew-T/log-P diagrams (skew-T for short) have become popular. Instead of plotting temperature and pressure on linear, orthogonal axes, skew-T diagrams plot the log of pressure and skew the temperature axis by 45°. The skew-T/log-P view of raob data allows features of the atmosphere to be more obvious than when plotted on a standard pseudo-adiabatic chart. The second figure illustrates the above examples on a skew-T diagram. On both standard pseudo-adiabatic charts and skew-T diagrams, elevation in meters or feet (corresponding to the pressure of a standard atmosphere) may be shown and wind direction and speed with height is represented parallel to or along the right-hand vertical axis. Many other features also may be included.

See: Lapse rates in the Fire Weather section for more information.

Atmospheric stability is the resistance of the atmosphere to vertical motion and provides an indication of the behavior of a smoke plume. Full characterization of a smoke plume requires a complete estimation of the atmosphere?s turbulent structure that depends on the vertical patterns of wind, humidity, and temperature, which are highly variable in space and time. Because this can be a complex calculation, it often is approximated by estimates of static stability. The static stability of the atmosphere is determined by comparing the adiabatic lapse rate with ambient, environmental lapse rates (as would be measured from instruments on a rising balloon). By this approximation, an unstable air mass is one in which the temperature of a rising parcel of air remains warmer than its surroundings. In a stable air mass, a rising parcel?s temperature is cooler than ambient and a neutral air mass is one in which the ambient temperature is equal to the adiabatic lapse rate.

The most common way of estimating static stability is to note the slope of vertically measured temperature in relation to the slope of the dry (or moist) adiabatic line from a pseudo-adiabatic chart. The figure shows raob-measured dry-bulb and dew-point temperatures and the theoretical trajectory of a parcel being lifted from the surface. The parcel trajectory begins at the current surface temperature then follows a DALR until it becomes saturated. The point of saturation is called the lifting condensation level (LCL). Its height in meters can be approximated as 120 x (T₀ ? T_d), where T₀ is the temperature at the surface and T_d is the mean dew-point temperature in the surface layers, both in degrees Celsius. From the LCL, the parcel trajectory follows a SALR.

Throughout the depth of the diagram in the figure, the slope of the measured temperature is nearly always steeper than the slope of the adiabatic temperature, suggesting that a lifted parcel always will remain cooler than the ambient temperature, which is a sign of stability. The large distance between the measured temperature and the temperature of the theoretical parcel trajectory also gives an indication of strong stability. In a stable atmosphere, smoke emanating from relatively cool fires will stay near the ground. Hot fires may allow plumes to loft somewhat through a relatively stable atmosphere but fumigation of smoke near the ground remains common. The second figure shows smoke from a vigorous wildfire under a stable atmosphere. Smoke plumes are trying to develop but a strongly stable layer is trapping most smoke just above the ridge tops.

Parcel trajectories in an unstable atmosphere remain warmer than the measured environmental temperatures. During unstable conditions, smoke can be carried up and away from ground level. Downwind of the source the instability causes smoke plumes to develop a looping appearance. Obviously there are many variations between stable and unstable atmospheres that cause various patterns of lofting, fanning, coning, looping, and fumigation. Each situation shows characteristic signatures on a pseudo-adiabatic chart but some experience may be required to distinguish the subtle differences.

Because upper-air observations and observations from significantly different elevations are not always available, Pasquill (1961 and 1974) developed a scheme to estimate stability from ground-based observations. Not only is this classification system used to estimate plume characteristics; it also is used in many smoke dispersion models as a proxy for atmospheric turbulence.

Table shows the Pasquill classification criteria as modified by Gifford (1962) and Turner (1961, 1964, 1970). In this example, surface wind is measured at 10 meters above open terrain. With clear skies, the class of incoming solar radiation is considered strong, moderate, or slight if the solar altitude angle is greater than 60°, between 35° and 60°, or less than 35°, respectively. If more than 50 percent opaque cloud cover is present and the cloud ceiling height is less than 7,000 feet (~2,100m), the solar class is slight. If ceiling height is between 7,000 feet and 16,000 feet (~4,800m), then the solar class is one step below what it would be in clear sky conditions. At night, classification is based on the amount of sky that is obscured by clouds. An objective way of determining stability classification is shown in Lavdas (1986) and Lavdas (1997).

See: Atmospheric Stability in the Fire Weather section for more information.

Mixing height (also called mixing depth) is the height above ground level through which relatively vigorous vertical mixing occurs. Low mixing heights mean that the air is generally stagnant with very little vertical motion; pollutants usually are trapped near the ground surface. High mixing heights allow vertical mixing within a deep layer of the atmosphere and good dispersion of pollutants. As such, mixing heights sometimes are used to estimate how far smoke will rise. The actual rise of a smoke plume, however, considers complex interactions between atmospheric stability, wind shear, heat release rate of the fire, initial plume size, density differences between the plume and ambient air, and radiant heat loss. Therefore, an estimate of mixing height provides only an initial estimate of plume height.

Mixing heights usually are lowest late at night or early morning and highest during mid to late afternoon. This daily pattern often causes smoke to be concentrated in basins and valleys during the morning and dispersed aloft in the afternoon. Average morning mixing heights range from 300 m (~980 ft) to over 900 m (~2,900 ft) above ground level (Holzworth 1972). The highest morning mixing heights occur in coastal areas that are influenced by moist marine air and cloudiness that inhibit radiation cooling at night. Average afternoon mixing heights are typically higher than morning heights and vary from less than 600 m (~2,000 ft) to over 1400 m (~4,600 ft) above ground level. The lowest afternoon mixing heights occur during winter and along the coasts. Mixing heights vary considerably between locations and from day to day. Ferguson and others (2001) generated detailed maps and statistics of mixing heights in the United States.

Smoke plumes during the flaming stage of fires often can penetrate through weak stable layers or the top of mixed layers. Once the plume dynamics are lost, however, the atmosphere retains control of how much mixing occurs. Low-level smoke impacts increase once a convective column collapses.

The depth of the mixed layer depends on complex interactions between the ground surface and the atmosphere in a region called the planetary boundary layer (PBL). As such, it is difficult to measure exactly and there are many ways in which it is calculated. At times, it is possible to estimate the mixing height by noting the tops of cumulus clouds or the presence of an upper-level inversion, which may appear as a deck of strata-form clouds.

Typically, National Weather Service (NWS) smoke management forecast products will estimate the mixing height by the so-called parcel method. This method considers turbulence related only to buoyancy. When a parcel is lifted adiabatically from the surface, the point at which it intersects the ambient temperature profile, or where it becomes cooler than its surroundings, is the mixing height. Usually the maximum daily temperature is used as the parcel?s starting temperature and its adiabatic lapse rate is compared with the afternoon (0000 UTC) sounding profile. Conversely, the minimum daily temperature is used to compare with the morning (1200 UTC) raob for calculating morning mixing heights. If an elevated inversion (see next section) occurs before this height is reached, the height of the inversion base would determine the mixing height. If a surface inversion exists, then its top marks the mixing height. For example, the mixing height in figure 7.3 is at the top of the surface-based inversion at about 750 mb (approximately 2,400 meters or 7,800 feet above ground level).

Instead of approximating a mixing depth, physical calculations of the PBL are possible through numerical meteorological models. These calculations are more precise than the parcel method because they consider turbulence generated by wind shear as well as buoyancy. Each prognostic model, however, may calculate the PBL slightly differently as some functions are approximated while others are explicitly derived to enhance computational efficiency and the vertical resolution, which varies between models, affect PBL calculations.

When the ambient temperature increases with height, an inversion is said to be present. It usually marks a layer of strong stability. When a heated air parcel from the surface encounters an inversion, it will stop rising because the ambient air is warming faster than the expanding parcel is cooling. The parcel being cooler than its surroundings will sink. Although the heat from some fires is enough to break through a weak inversion, inversions often are referred to as lids because of their effectiveness in stopping rising air and trapping pollutants beneath it. Smoke trapped under an inversion can substantially increase concentrations of particles and gases, aggravating respiratory problems and reducing visibility at airports and along roadways.

There are three ways that surface-based inversions typically form: (1) valley inversions are very common in basins and valleys during clear nights when radiation heat losses cause air near the ground to rapidly cool: the cold surface air flows from the surrounding slopes and collects in hollows and pockets, allowing warmer air to remain aloft; (2) advective inversions are caused by cold air moving into a region from a nearby lake or ocean, usually during the afternoon when onshore lake and sea breezes tend to form; and (3) subsidence inversions can occur at any time of day or night as cold air from high altitudes subsides or sinks under a region of relatively stagnant high pressure. Valley inversions cause tremendous problems when managing long-duration fires that continue into the night. Advective inversions can surprise smoke managers who are unfamiliar with local lake- and sea breeze effects, creating poor dispersal conditions in an afternoon when typically good dispersion is expected. Subsidence inversions are difficult to predict even for a well-trained meteorologist. Figure 7.7 shows smoke caught under a valley inversion that is being transported by downvalley winds in the early morning.

Surface inversions also occur in the gaps (passes and gorges) of mountain ranges. Approaching storms usually have an associated center of low pressure that causes a pressure gradient across the range. If cold air is on the opposite side of the range, the gradient in pressure causes the cold air to be drawn through the gap, creating an inversion in the gap. Gap inversions are most common in winter but also are frequent during spring and autumn.

In addition to surface-based inversions, temperature inversions also occur in layers of the atmosphere that are above the ground surface, which sometimes are called thermal belts. Upper-level inversions usually are associated with incoming warm fronts that bring moisture and warmth to high altitudes well ahead of a storm. The inversion lowers to the ground as the front approaches. Upper-level inversions also may be associated with subsidence or surface-based inversions that have been lifted, usually by daytime heating.

See: Inversions in the Fire Weather section for more information.

Not only does smoke mix and disperse vertically, the horizontal component of wind readily transports and disperses pollutants. The stronger the wind, the more scattered particles become and the less concentrated they will be. Strong winds at the surface, however, can increase erratic fire behavior and associated emission rates. Also, significant surface winds may ?laydown? a plume, keeping smoke close to the ground for long distances.

Friction with the ground causes winds to slow down. Therefore, wind speed usually increases with height, causing a smoke column to gradually bend with height as it encounters increasingly strong winds. This pattern is complicated in regions of complex terrain, however, and it is common to find stronger surface winds in mountain passes, saddles, and gorges as air is squeezed and funneled through the gap. Forest clearings also allow surface winds to accelerate because surface friction is lower in a clearing than over a forest canopy.

Because smoke from different stages of a fire rises to different levels of the atmosphere, it is important to know wind speed and direction at several different heights. For example, smoldering smoke at night responds to surface winds while daytime smoldering and smoke from the ignition and flaming phase of a fire will respond to upper-level winds. Depending on the buoyancy of the smoke and stability of the atmosphere, winds that influence the upper-level smoke trajectories may be from just above a forest canopy to 10,000 feet (about 3,000 meters) or more. Because flaming heat can create convective columns with strong vertical motion, most smoke during the flaming portion of a fire will be carried to at least the top of the mixing height or an upper-level inversion height before dispersing. In this way, a fire hot enough to pull itself into a single convection column can reduce concentrations near the ground and knowledge of winds at the top of the mixing height or inversion level will determine smoke trajectory and dispersion. Smoldering smoke, on the other hand, has very little forced convection so it often fumigates away from a fire as it rises with daytime buoyancy. Knowledge of wind all the way from the surface to top of the mixing height may be needed to determine smoldering trajectories.

A discussion of diurnal winds, storm winds, terrain-influenced wind, inversions, and wind observations follows.

Diurnal Winds

In the absence of storms, diurnal wind patterns dominate trajectories of smoke near the ground. Diurnal patterns are caused by differences between radiational cooling at night and solar heating during the day, and by different thermal properties of land and sea surfaces that cause them to heat and cool at different rates. The differential heating causes changes in surface pressure patterns that control air movement. Slope winds and sea and lake breezes, all of which are common in wildland smoke management situations, typify diurnal patterns.

Slope winds are caused by the same mechanisms that cause valley and basin inversions. When cold air from radiation cooling at night drains into a valley or basin, it causes a downslope wind. The cold air, being denser than its surroundings, usually hugs the terrain in such a way that smoke following a drainage wind will follow contours of the terrain. During the day, heated air from the surface rises, causing upslope winds. Because daytime heating causes more turbulence than nighttime cooling, the daytime winds do not follow terrain as readily as nighttime winds, causing thermally-induced upslope winds to be less noticeable than downslope winds.

Downslope winds at night are notorious for carrying smoke into towns and across roadways (e.g., Achtemeier et al. 1998), especially where roads and bridges cross stream channels or when towns are located in valleys, basins, or near outwash plains. Downslope winds are most likely to occur when skies are clear and ambient winds are nearly calm. The speed and duration of a downslope wind is related to the strength of its associated valley inversion. Downslope winds usually begin around sunset and persist until shortly before sunrise.

Sea and lake breezes usually occur during the afternoon when land surfaces have had a chance to heat sufficiently. The heated air rises, as if lifting the overlying column of air. This causes a region of low pressure at the surface. Because land heats more rapidly than water, the differential heating causes a pressure gradient to form. Relatively cool air remaining over a lake or ocean will flow into the low pressure formed over heated land surfaces. The sea or lake breeze not only can change smoke trajectories but the incoming cool air can cause surface based inversions that will trap smoke at low levels near the ground. Also, strong sea breezes can knock plumes down, causing increasing smoke concentrations near the ground.

Storm Winds

Storms change the structure of winds entirely. Because storms often bring high instability and good dispersion, it is common to plan fires slightly ahead of an approaching storm. Knowing storm wind patterns can help anticipate associated smoke impacts. Figure 7.8 shows surface wind directions typically associated with a passing cyclonic storm. Because air flows from high pressure to low pressure (like the rush of air from a punctured tire) and storms usually have a center of low pressure at the surface, surface winds ahead of a storm in the northern hemisphere will be from the east or southeast. As the low center approaches, surface winds will become southerly to southwesterly. After the storm passes, surface winds may become more westerly or northwesterly. This pattern can cause smoke to move toward the west to northwest then north to northeast ahead of a cyclonic storm, moving toward the east and southeast following storm passage.

Each cyclonic storm usually contains at least one front (a boundary between two different air masses). A typical storm has a warm front aligned northwest to southeast ahead of the low center, a cold front trailing northeast to southwest near and closely behind the low, and an occluded front (formed when a cold front overtakes a warm front) to the north of the low. Winds change direction most rapidly and become gusty when fronts pass by. Warm fronts can bring increasing stability and cause upper level inversions, while cold fronts usually are associated with strong instability. The stronger the front, the more dramatic the wind shift and the stronger the gusts. Cold frontal passage typically improves dispersion of smoke with stronger winds and an unstable air mass that can scour away existing inversions. Smoke trajectories should be expected to change direction with the passage of a storm front and storms can cause significant changes in fire behavior and resulting emission rates. Storm fronts are not always typical, however, and the number, strength, and orientation of fronts are quite variable.

Strong winds above the influence of the earth?s surface experience forces associated with the earth?s rotation in addition to pressure gradient and other forces. This causes winds in the upper atmosphere to follow lines of constant pressure instead of moving across lines of constant pressure as surface or lower-speed winds do when air flows from high pressure to low pressure. In the upper atmosphere the pressure pattern of a typical storm is shaped like a trough (figure 7.9). As air follows the pressure contours around the trough, southwesterly upper-level winds occur ahead of the storm, becoming westerly as the storm trough passes, and northwesterly following the trough. The upper-level trough usually trails the surface low center in most moving fronts, causing smoke trajectories aloft to change directions sometime after trajectories at the surface have changed following a storm passage.

Thunderstorms, which are the result of strong convection, create much different wind patterns than cyclonic storms. Gusty, shifty winds are common at times of strong convection. Strong down bursts of wind in a direction away from the thunder cell may occur several minutes ahead the storm, while winds around the cell may be oriented towards it. Although mixing heights usually are quite high during thunderstorms, allowing for well-lofted plumes, the shifting wind directions and strong gusts can cause variable and unpredictable smoke trajectories and fire behavior in close proximity to thunderstorms.

Terrain-Influenced Wind

Surface winds are strongly influenced by small undulations in terrain that channel, block, or accelerate air as it tries to move around or over features. For example, if upper-level winds are oriented perpendicular to a terrain barrier, surface winds on the lee side of the barrier often are light and variable. Upper-level winds oriented in the same direction as a valley will enhance upvalley or downvalley winds. Cross-valley winds will be 90° different than those in the valley itself.

The combination of wind and atmospheric stability determine whether smoke will collect on the windward side of a terrain barrier, move up, over and away, or traverse the barrier only to accumulate on the leeward side. Weak winds and a stable atmosphere will enhance blocking and windward accumulations of smoke. Stronger winds in a stable atmosphere may allow accumulations of smoke in leeward valleys and basins. An unstable atmosphere allows smoke to be lifted over and above the terrain. The height, steepness, and orientation of the terrain to the wind direction determine how strong the wind or unstable the atmosphere must be to influence smoke trajectories.

Often very small-scale undulations in topography can affect smoke trajectories, especially at night when atmospheric stability keeps smoke close to the ground. Gentle saddles in ridges may offer outflow of smoke from a valley. Small streambeds can collect and transport significant amounts of smoke even with only shallow or weak downslope winds. A simple band of trees or brush may provide enough barrier to block or deflect smoke. As the urban-wildland interface becomes increasingly complex, the role of subtle topographic influences becomes increasingly important.

Higher in the atmosphere, away from the earth?s surface, topography plays a decreasing role in controlling wind speed and direction. Upper-level winds above the influence of underlying terrain are referred to as ?free-air? winds and tend to change slowly from one place to another, except around fronts and thunderstorms.

The Role of Inversions on Wind

Temperature inversions significantly influence wind direction and speed. Under many inversions there is little or no transport wind and smoke tends to smear out in all directions. Some inversions, such as advected inversions that are associated with sea breezes and valley inversions, may have significant surface wind but it usually is in a different direction to winds aloft. In these cases, surface smoke may be transported rapidly under the inversion in one direction while lofted smoke may be transported in an opposite direction.

Wind Observations

Because surface winds are strongly influenced by small undulations in terrain, vegetation cover, and proximity to obstacles and water bodies, it is important to know where a surface wind observation is taken in relation to the burn site. For example, observations from a bare slope near the ridgeline will give a poor indication of winds affecting surface smoke trajectories if most of the burn area is on a forested slope or in a valley, even if the two sites are very close. Also, if a burn site is in an east-west oriented valley and the nearest observation is in a north-south oriented valley, observed winds can be 90° different from those influencing the fire and its related smoke. Sometimes, a nearby Remote Automated Weather Station (RAWS) will be less representative of burn-site conditions than one that is farther away if the distant station is in a location that better matches terrain effects expected at the burn site.

There are four principle sources of surface wind data: (1) on-site measurements with a portable RAWS or hand-held anemometer, (2) observations that estimate winds using the Beaufort wind scale or wind sock, (3) local measurements with a standard RAWS, and (4) measurements from NWS observing stations. Because stations vary in their surroundings, from small clearings on forested slopes to open fields, and different types of anemometers are used that are mounted at different heights, wind data is very difficult to compare between one site and another. Therefore, it is useful to become familiar with measurements and observations from reliable sites and understand local effects that make data from that site unique. Also, smoke near the ground can be transported by winds that are too light to spin the cups or propeller of an anemometer or turn its tail. Frequently light and variable wind measurements actually are responding to very light winds that have a preferred direction, often influenced by surrounding topography or land use.

Because free-air winds are above the influence of topography, often it is possible to use an upper-level observation from some point well away from the burn site to estimate upper-level smoke trajectories. Also, surface RAWS that are mounted on the tops of ridges or mountains may compare well with free-air winds at a similar elevation. If clouds are in the area, upper-level winds can be estimated by their movement relative to the ground. High clouds look fibrous or bright white. Because the base of high clouds ranges between 5 km and 13 km (about 16,000 to 45,000 feet) their movement can indicate wind at those high levels. Midlevel clouds may have shades of gray or bulbous edges with bases ranging from 2 km to 7 km (about 6,600 to 24,000 feet). Mid-level clouds often have a strata-form or layered appearance, which may indicate the presence of an inversion. Therefore, movement of these types of clouds may closely approximate steering winds for a rising smoke plume.

In addition to observations, it is becoming increasingly common to have available the output from wind models. These data do not provide the detail of a point observation the way an individual site measurement does, but they do provide a broad view of wind patterns over the landscape. Standard analyses from the NWS use models to interpolate between observations. These products help illustrate upper-level wind patterns and typically are available for 850 mb, 700 mb, and 500 mb heights, either from a state, federal, or private meteorological service, or a variety of Internet sites. For surface winds, standard NWS analyses are helpful in regions of flat or gently rolling terrain but mesoscale meteorological models typically are needed to resolve surface wind fields in regions of complex topography. Several regions throughout the country are beginning to employ mesoscale models (e.g., MM5, RAMS, and MASS) producing wind maps with less than 15 km horizontal spacing. Local universities, research labs, state offices, and consortia of local, state, and federal agencies have undertaken mesoscale modeling efforts. Output usually can be found on a local Internet site through the NWS forecast office, a fire weather office, university, state regulator, EPA office, or regional smoke manager. Also, many smoke dispersion models have built-in wind models to generate surface winds at very fine spatial resolutions (less than 5 km grid spacings) from inputs of surface and upper-air observations or data from coarser meteorological models. Smoke dispersion models and their related wind models may be available through a regional smoke manager or EPA office (see Smoke Dispersion Prediction Systems).

See: Wind in the Fire Weather section for more information.

Because water vapor in the atmosphere reduces visibility, if smoke is added to an already humid environment, visibility can be severely degraded. Also, if the air is saturated with water vapor, particles from smoke may act as condensation nuclei causing water droplets to form. This promotes the formation of clouds or fog, which further degrades visibility. Often a deadly combination occurs during the darkness of night as smoldering smoke drains downvalley to encounter high humidities from condensing cold air under a valley inversion. The effect can be fatal, especially along transportation corridors (Achtemeier and others 1998).

Favorable conditions for fog occur when the dew point temperature is within a few degrees of the dry bulb temperature, wind is less than a few meters per second, and there is a high content of moisture in the soil. Fog is most common at night when temperatures often drop to near the dew point value and winds are most likely to be weak. Common places for fog to form are over lakes and streams and in the vicinity of bogs and marshes.

There are times when atmospheric moisture can improve visibility, however. Smoke particles can adhere to rain droplets, causing them to be carried with the rain as it falls. This ?scavenging? effect removes smoke particles out of the atmosphere, reducing smoke concentrations and improving visibility.

See: Atmospheric Moisture in the Fire Weather section for more information.

Weather forecasts typically are produced twice each day and become available within 3 to 6 hours after 0000 UTC and 1200 UTC observations are complete. This is because prognostic models require input data from the 0000 UTC and 1200 UTC upper-air observations and a few hours of run-time on a super computer. Prognostic models (progs) form the basis of most forecast products. For example, the first forecast of the day should be available by 7 am to 10 am local daylight time from Anchorage and by 10 am to 1 pm local standard time from Miami. Earlier forecasts or forecasts updated throughout the day are possible if the most recently available upper-air observations and prognostic model outputs are combined with updated surface observations. While public forecasts issued by the National Weather Service and the media are useful, they typically lack the detail needed for smoke management. For this reason, spot-weather forecasts may be requested from state, federal, or private weather services that provide predictions of critical variables that influence smoke at specified times and locations.

Even though there are increasing numbers of numerical guidance tools, weather forecasting still is an art, especially in places with few observations or where there are complex local interactions with terrain, water bodies, and vegetation cover. The primary source of smoke weather forecasts remains the National Weather Service. Their rigorous training, fire weather program, and state-of-the art equipment and analysis tools help maintain a unique expertise. Most NWS fire weather forecast offices now issue special dispersion and transport forecasts. In addition to NWS forecasters, many states maintain a smoke management program with highly skilled meteorologists. Also, the number of inter-agency fire weather offices and private meteorological services is growing and can provide reliable forecast products specifically designed for smoke management. Whatever the source of a forecast, it is helpful to combine the forecast with your own general understanding of weather conditions by reviewing the many satellite pictures, current observation summaries, and prognostic model output products now available on the World Wide Web. In this way, apparent trends and local influences can be determined and the need for last minute changes can be recognized more quickly. For example, increasing afternoon cloudiness in the forecast may have indicated an approaching storm that was predicted for the following morning. If clouds do not increase when predicted, however, it could be suspected that the storm has been delayed or it was diverted elsewhere. A check with the forecaster or updated satellite picture may confirm the suspicion and the management plan may be altered.

Because the atmosphere behaves chaotically, the accuracy of a weather forecast improves as time to an event shortens. For example, it is possible to provide an indication of storminess within 30 to 90 days. A storm passage, however, may not be predicted until about 14 days in advance with about 2 days accuracy. Within 5 days, 1-day accuracy on storm passage may be possible. Increasing accuracy should be expected within 48 hours and the timing of storm passage within 1/2 hour may be possible with 12 hours advance notice. Spot weather forecasts usually are available 24 to 48 hours in advance of a scheduled burn. This allows a smoke manager to anticipate a potential burning window well in advance. Specific timing, however, should not be made before 2 days in advance if the situation is highly dependent on an accurate weather forecast.

Our increasing knowledge of air-sea interactions is making it possible to predict some aspects of weather up to a year in advance as certain regions of the country respond to the El Niño Southern Oscillation (ENSO). Precipitation and temperature during winter and spring are most strongly related to ENSO. Relating key factors for smoke management such as wind and mixing height or stability is more difficult, especially during summer. Nevertheless, an ENSO-based seasonal prediction gives prescribed burners an idea of general weather conditions to be expected, thereby helping prioritize scheduled burns and decide if marginal days or weekends early in the burning season should be used or whether a more optimum season will ensue.

Climate simply describes the prevailing weather of an area. Understanding climate patterns can help develop long-range smoke management plans or adapt short-range plans. For example, afternoon mixing heights in most coastal regions of the United States are typically lower than the interior because moist, marine air is relatively stable. This means that there may be fewer days with optimum dispersion along the coast than interior. It usually is windier along the coast, however, and burns might be scheduled in the early morning if offshore breezes are desired to reduce smoke impacts on cities and towns.

It is possible to infer climate just by local proximity to oceans, lakes, rivers, and mountains. Also, vegetation cover can give an indication of climate. Desert landscapes, with a lot of bare soil or sand, heat and cool rapidly, causing them typically to have high daytime mixing heights and very low nighttime mixing heights. Natural landscapes of lush green forests tend to absorb sunlight while transpiring moisture, both of which help to modify heating and cooling of the ground surface. This can reduce daytime mixing heights and keep nighttime heights relatively high, with respect to deserts. Also, the structural deformation of trees often indicates high winds, where the direction of branches or flagging point away from prevailing wind directions.

Quantitative summaries of climate can be obtained from the state climatologist or Regional Climate Center (RCC), many of whom also maintain informative Internet sites and can be reached through the National Climatic Data Center (NCDC). It is most common to find temperature and precipitation in climate summaries. Monthly or annual averages or extremes are readily available while climate summaries of daily data are just beginning to emerge. For example, a recently generated climate database by Ferguson and others (2001) provides information on twice-daily variations in surface wind, mixing height, and ventilation index over a 30-year period.

We know that there are year-to-year variations in climate (e.g., ENSO) so at least 10 years of weather data are needed to obtain a preliminary view of climate in a particular area. There also are natural, ?decadal? patterns in climate that last from 7 to 20 years. Therefore, it is appropriate to acquire 30 to 50 years of weather observation data for any reliable climate summary.

See: Fire Climate in the Fire Weather section for more information.