Atmospheric instability is a state in which a displaced air parcel continues to accelerate away from its original level, either upward if it is warmer than the environment or downward if it is cooler. The American Meteorological Society describes it as a system property in which small disturbances grow rather than damp out, and in the atmosphere the disturbance in question is usually a vertically displaced parcel.
How instability arises
An atmospheric layer is unstable when its environmental temperature falls off with height faster than the rate at which a lifted parcel cools. In that case, a rising parcel stays warmer, and therefore less dense, than its surroundings and keeps accelerating upward. The steeper the environmental lapse rate compared with the parcel's own cooling rate, the more unstable the layer.
Two related flavors of instability are common in convective forecasting. Dry, or absolute, instability requires the environmental lapse rate to exceed the dry adiabatic rate, which is unusual and short-lived. Conditional instability is far more common. A layer is conditionally unstable when the environmental lapse rate lies between the dry and moist adiabatic rates, so a parcel is stable if unsaturated but unstable once it saturates and follows the moist adiabat.
How forecasters measure it
Instability shows up on the Skew-T as the shape of the environmental temperature curve compared with a lifted parcel's path. Any area where the parcel is warmer than the environment is positive buoyancy. CAPE integrates that area between the LFC and the equilibrium level and expresses the total energy available in units of joules per kilogram.
Layer-specific diagnostics such as 0 to 3 km CAPE and mid-level lapse rates describe how the instability is distributed vertically. Instability concentrated near the ground supports rapid low-level updraft acceleration, which matters for tornado potential. Instability spread deeper through the column supports taller, longer-lived storms.
Where it misleads
Large CAPE does not guarantee a storm. A profile with 4000 J/kg of surface-based CAPE and a strong capping inversion may produce nothing until either the cap erodes or a boundary provides forced lift strong enough to break it. Instability describes potential, not initiation.
The parcel chosen also matters. A surface-based instability number can overstate the day's potential when a shallow moist layer is capped from the deeper environment, and it can understate potential in nocturnal setups where the most unstable air sits above a stable near-surface layer. Reading a single CAPE value without checking which parcel produced it and how deep the moist layer is misses those distinctions.