Airmass Stability and Atmospheric Dispersion Considerations

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.