Smoke as a Southern Problem

The Forest Atlas of the United States shows that the thirteen Southern states contain approximately 40% of U.S. forests – about 200 million acres. While not all of this forested land is regularly burned, the extensive forest type generally known as “southern pines” burns with a high fire frequency, about every 2-5 years. When shrublands and grasslands are added to the total, from four to six million acres of southern wildlands are subjected to prescribed fire each year. This is by far the largest acreage of wildland subjected to prescribed fire in any region of the country.

The wildland/urban interface falls within much of the range of Southern forests. Southern forests, with highest treatment intervals of prescribed fire and with the largest acreages subjected to prescribed fire, are connected with human habitation and activity through an enormous wildland/urban interface. The potential exists for significant smoke problems in this region.

Smoke and Southern Climate

Several factors regarding climate add to the smoke problem in the South. The long growing season allows time for more annual biomass production relative to other areas of the country with shorter growing seasons. Most of the Southern forests are located farther south than forests elsewhere in the country. Consequently, the sun angle is higher in the South and is capable of supplying warmth well into the late fall and early winter. Further, most southern wildlands are located at low elevations where the air is warmer. These factors contribute to the long growing season, which runs from March/April through October/November.

Abundant rainfall also encourages growth of a large number of grasses, shrubs, and trees. Most of the South receives 40-60 inches (100-150 cm) of precipitation annually. This copious rainfall, in combination with the long growing season, creates conditions for rapid buildup of both dead and live fuels. If burns are not conducted frequently, the increase in emissions from the accumulated fuels may enhance the likelihood of negative smoke impacts when fires do occur.

The coincidence of dormant-season burning with the winter rain season is a third factor contributing to nuisance smoke. Although burning is conducted year round throughout the South, a significant amount of burning is done during January through March. In a typical year, anywhere from 10-20 inches (25-50 cm) of rain will fall over Southern forests during this three-month period. In some areas of the country, the question might be, “Is it wet enough to burn?” In the South, the question is commonly, “Is it dry enough to burn?” Fires burning into moist fuel burn less efficiently and smolder longer than fires burning dry fuels. Both factors increase smoke production. In addition, less heat is produced during inefficient combustion and smoldering. Therefore, more smoke stays near the ground and increases the risk of problem smoke.

Smoke and Southern Meteorology

All thirteen Southern states have implemented burning regulations designed to limit open burning to those days when burning is considered “safe” and the risks of fire escapes are minimal. Many have implemented smoke management regulations. The need to conduct burning in a manner to reduce impacts on air quality over sensitive targets has encouraged “best practice” approaches to open burning.

Efforts to avoid smoke incursions over sensitive targets are often complicated by the highly variable meteorology of Southern weather systems during the extensive burn season. Four weather features that cause frequent wind shifts and may be accompanied by rapid changes in air mass stability and mixing height are described below.

1. Synoptic scale high- and low-pressure systems and accompanying fronts frequent the South during the winter burn season. In a typical sequence of events, the winds shift to blow from the southeast through southwest in advance of a storm, then shift rapidly to the northwest with cold front passage. Winds blow from the northwest for a day or so but gradually diminish with the approach of a high pressure system, becoming light and variable as the system passes. Then winds shift back to southerly in advance of the next storm. Low clouds, low mixing heights, and high stability often accompany low-pressure systems. Depending upon moisture availability, cold fronts may be accompanied by bands of low clouds and precipitation. Mixing heights are more favorable during high-pressure episodes. Although the movement of synoptic scale weather systems into the South can be predicted with lead times of several days, the timing of arrival of frontal wind shifts over specific burn sites is less certain.
2. Much of the Piedmont and Coastal Plain are flat and it would be expected that winds there are steady and predictable. However, the region is frequented by transient eddies that can cause unexpected wind shifts and carry smoke into sensitive areas. The vertical circulation of air that can force smoke plumes to the ground or carry smoke safely upward are well-understood, but the location, timing and strength of the vertical eddies cannot be predicted. Horizontal eddies have not been well documented, and the timing, location and intensity cannot be predicted.
3. The South has the longest coastline of any fire-prone area in the country. Thus it is axiomatic that large areas of the South are subject to wind shifts brought on by sea breezes during the day and by land breezes during the night. However, the onset, duration, and intensity of these land/water-induced circulations are not consistent from one day to the next. The region is subject at different times to warm, humid airmasses drawn northward from the Gulf of Mexico, or cold, dry airmasses drawn southward from Canada. Both systems have an impact on land surface temperatures, which results in a significant effect on the duration and extent of land and sea breezes and whether they form at all. The unpredictability of these wind systems adds to the difficulties faced by Southern land managers planning whether smoke from a prescribed burn might impact downwind sensitive targets.
4. The “flying wedge,” a wind system caused by cold air channeled southwestward along the eastern slopes of the Appalachian Mountains, can cause sudden wind shifts with large changes in wind direction and lowering of mixing heights. Although Virginia, the Carolinas and Georgia are most frequently impacted, flying wedges have been observed as far south as central Florida and as far west as the Mississippi River. “Flying wedges” occur throughout the year but are most intense, and hence bring with them strong shifting winds and lowering of mixing heights, during winter and early spring, the period of maximum wildland burning in the South.

Smoke and Southern Highways

Several million acres of Southern wildlands are burned each year, the vast majority without incident. However, smoke and smoke-induced fog obstructions of visibility on highways sometimes cause accidents with loss of life and personal injuries. Several attempts to compile records of smoke-implicated highway accidents have been made. For the 10-year period from 1979-1988, Mobley (1989) reported 28 fatalities, over 60 serious injuries, numerous minor injuries and millions of dollars in lawsuits. During 2000, smoke from wildfires drifting across Interstate 10 caused at least 10 fatalities, five in Florida and five in Mississippi. In their study of the relationship between fog and highway accidents in Florida, Lavdas and Achtemeier (1995) compared three years of accident reports that mentioned fog with fog reports at nearby National Weather Service stations. Highway accidents were more likely to be associated with local ground radiation fogs than with widespread advection fogs. Accidents tended to happen when fog created conditions of sudden and unexpected changes in visibility.

There are several reasons why smoke on the highways is a serious problem in the South, some of them interrelated.

Road density

The density of the road network in the South is far greater than in other wildland areas in the country where prescribed fire is in widespread use. The difference in road density between generally forested areas in the west and in the south exists primarily because of land use history. While Western forested lands have always been in forest, in the Southern area, roads and communities remain essentially unchanged from the old agricultural South.

Population in wildland areas

The population dwelling near or within Southern wildlands is greater than that in other areas of the country where prescribed fire is in widespread use. Many people live in close proximity to Southern forests; many more live in areas interfacing fire-prone grasslands and shrublands. Southern States are becoming more urban, and the numbers of tourists driving to resort areas along the Gulf coast, the Atlantic coast, and the Florida peninsula are increasing. Therefore, the number of accidents related to smoke and fog can only be expected to increase.

Climate and meteorology

Factors of Southern climate and meteorology combine to produce airmasses that entrap smoke close to the ground at night. Smoke is most often trapped by either a surface inversion or inversion aloft. This is a condition in which temperature increases with height through a layer of the atmosphere. Vertical motion is restricted in this very stable air mass. Although most inversions dissipate with daytime heating, inversions aloft caused by large-scale subsidence may persist for several days, resulting in a prolonged smoke management problem

Most smoke-related highway accidents occur just before sunrise when temperatures are coldest and smoke entrapment has maximized under a surface-level inversion. The high sun angle during the burn season contributes to warm daytime temperatures. Near sunset, under clear skies and near calm winds, temperatures in shallow stream basins can drop up to 20 degrees F in one hour (Achtemeier 1993). Smoke from smoldering heavy fuels can be entrapped near the ground and carried by local drainage winds into these shallow basins where temperatures are colder and relative humidities are higher. Hygroscopic particles within smoke can assist in development of local dense fog. Weak drainage winds of approximately 1 mile per hour (0.5 m/sec) can carry smoke over 10 miles during the night—far enough in many areas to carry the smoke or fog over a roadway.