Atmospheric Stability
Atmospheric stability describes the tendency of the atmosphere to resist or enhance vertical motion. A stable atmosphere suppresses vertical movement — displaced air tends to return to its original level. An unstable atmosphere encourages vertical motion — displaced air accelerates upward, leading to convection, cumulus cloud development, and potentially severe weather.
Stable vs. Unstable — What to Expect
- Stable atmosphere: Smooth air; stratiform cloud (stratus, altostratus, fog); reduced visibility in haze; widespread IFR conditions
- Unstable atmosphere: Turbulence; cumuliform cloud; good visibility between cells; heavy showery precipitation; gusty winds
Lapse Rates
Stability depends on comparing the temperature change within a rising air parcel to the surrounding environment. A rising dry parcel cools at 3°C per 1,000 ft (DALR). A saturated parcel cools at approximately 1.5°C per 1,000 ft (SALR). If the environment cools faster than the parcel, the parcel remains warmer and continues to rise — the atmosphere is unstable.
Conditional Instability
The most common and dangerous state: stable for dry parcels, unstable once saturation is reached. A trigger mechanism (surface heating, frontal lifting, orographic lifting) forces air to its lifting condensation level, after which it becomes positively buoyant and rises explosively — this is how air mass thunderstorms develop on apparently clear days.
Aviation Implications
- Stable: plan for smooth flight but expect IFR conditions, stratus, and fog
- Unstable: plan for turbulence, convection, good VFR between cells but severe hazards in cloud
- Conditionally unstable: watch for trigger mechanisms — conditions can deteriorate rapidly
On the TC Written Exam
Core meteorology topic. Know stable vs. unstable characteristics, lapse rate concepts, and conditional instability. The exam tests this in the context of cloud types, turbulence, and thunderstorm formation.
Related Terms
See also: Thunderstorm, Clear Air Turbulence, Cold Front, Warm Front.