It is useful to touch on weather conditions that effect smoke dispersion. Certain weather conditions must be considered during a burn to ensure the effectiveness regardless of the smoke management strategy chosen. While each of these factors is described separately below, it is important to note that dispersion is effected by a combination of these factors affecting dispersion simultaneously. The conditions described below include the following: pressure, lapse rate, atmospheric stability, mixing height, temperature inversions, and winds.
Pressure is the force per unit area exerted by the weight of the atmosphere. High pressure can cause stagnation of air, which will not allow emissions to rise, which is imperative for strategies such as scavenging.
Lapse rate is the rate of decrease of air temperature with increase of elevation The lapse rate determines how stable the atmosphere is. Atmospheric stability increases as the lapse rate decreases. As a plume of smoke rises, the pressure around it decreases, causing it to expand and cool. It will continue to rise as long as it is warmer than the surrounding air. With a low lapse rate (more stable atmosphere) a smoke plume will not rise as high and may, once it cools, sink back to the surface with the potential to impact air quality both in the immediate vicinity or downwind.
Atmospheric stability is described as the resistance of atmosphere to vertical motion. Unstable conditions allow smoke to be diluted more than stable conditions.
Mixing height refers to the height above ground level at which vertical mixing of the air occurs. Low mixing height indicates the air is stagnant, and pollutants are held closer to the ground. The lowest mixing heights often occur at night and early morning, with the highest occurring at mid to late afternoon.
When the ambient air warms with height, it causes an inversion. In this case, smoke rises until it encounters this inversion, or warm air. The smoke has been cooling as it rises, and as it hits the warm air, it sinks because it is cooler and heavier.
While other factors control the vertical movement of smoke, wind is responsible for controlling the horizontal movement of smoke. Winds are created by air pressure gradients and, without terrain influences, blow from areas of high pressure to areas of low pressure. Winds are typically light and variable when the atmosphere is stable. Due to friction from objects on the ground, wind speeds are often lower near the ground. Diurnal winds are caused by the difference in temperature between night time radiational cooling and daytime solar heating. For example, as air cools at night it becomes heavier, and drifts down valleys and drainages. This type of wind is often responsible for overnight smoke intrusions into populated areas.
Note: A revised edition of the Smoke Management Guide will be available in 2016.
Ferguson, SA. 2001. Smoke Management Meteorology. Chapter in: Hardy, Colin C.; Ottmar, Roger D.; Peterson, Janice L.; Core, John E.; Seamon, Paula. 2001. Smoke management guide for prescribed and wildland fire, 2001 edition. PMS 420-2. National Wildfire Coordination Group, National Interagency Fire Center. Boise, Idaho. 226 pp.
Whiteman, David C. 2000. Mountain Meteorology Fundamentals and Applications, Glossary p.332, Oxford University Press, Inc.