Atmospheric Stability

Atmospheric stability is the resistance of the atmosphere to vertical motion. The distribution of temperature vertically in the troposphere influences vertical motion. A large decrease of temperature with height indicates an unstable condition which promotes up and down currents. A small decrease with height indicates a stable condition which inhibits vertical motion. Where the temperature increases with height, through an inversion, the atmosphere is extremely stable.

To determine stability conditions, temperature lapse rates are compared to dry- or moist-adiabatic lapse rates. Between stable and unstable lapse rates we may have a conditionally unstable saturated. During condensation in saturated air, heat is released which warms the air and may produce instability; during evaporation, heat is absorbed and may increase stability.

Several different lifting processes contribute to atmospheric stability, such as local heating, with wind speed, surface characteristics, warm- and cold-air advection, and many other factors. As a result, atmospheric stability varies with these factors. Atmospheric stability also varies diurnally and seasonally. We can use type of cloud, windflow characteristics, occurrence of dust devils, and other phenomena as indicators of stability.

Subsidence is the gradual lowering of a layer of air over a broad area. When it begins at high levels in the troposphere, the air, which has little initial moisture, becomes increasingly warmer with resulting lower relative humidity as it approaches the surface. If some mechanism is present by which this warm, dry air can reach the surface, a very serious fire situation can result.

Atmospheric stability is closely related to fire behavior, and a general understanding of stability and its effects is necessary to the successful interpretation of fire-behavior phenomena. Atmospheric stability may either encourage or suppress vertical air motion. The heat of fire itself generates vertical motion, at least near the surface, but the convective circulation thus established is affected directly by the stability of the air. In turn, the indraft into the fire at low levels is affected, and this has a marked effect on fire intensity. Also, in many indirect ways, atmospheric stability will affect fire behavior. For example, winds tend to be turbulent and gusty when the atmosphere is unstable, and this type of airflow causes fires to behave erratically. Thunderstorms with strong updrafts and downdrafts develop when the atmosphere is unstable and contains sufficient moisture. Their lightning may set wildfires, and their distinctive winds can have adverse effects on fire behavior.

• Determining Atmospheric Stability
• Lifting Processes
• Diurnal and Seasonal Variations in Stability
• Local Indicators of Stability
• Subsidence