Atmospheric Stability and Smoke

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.