Atmospheric stability

Atmospheric stability is the atmosphere’s tendency to resist or amplify vertical displacement of air. It depends on how an air parcel’s temperature changes relative to its surroundings as it rises or sinks.

Environmental lapse rate: The rate at which the surrounding atmosphere’s temperature decreases with altitude. It sets the environmental temperature against which a displaced parcel is compared.
Potential temperature: The temperature an air parcel would have if brought adiabatically to a standard reference pressure. Its vertical variation is a compact indicator of dry atmospheric stability.
Atmospheric convection: Vertical atmospheric motion driven by buoyancy differences between air parcels and their surroundings. Unstable layers allow convection to grow, while stable layers inhibit it.
Conditional instability: A state in which unsaturated air is stable but saturated air is unstable to vertical displacement. It shows how the same temperature profile can resist dry ascent yet support moist convection.
Dry adiabatic lapse rate: The rate at which an unsaturated air parcel cools as it rises without exchanging heat. Comparing it with the environmental lapse rate identifies stability for unsaturated parcels.
Equivalent potential temperature: A measure related to the temperature a moist air parcel would reach after condensation and latent-heat release. Its vertical profile helps diagnose stability when water vapor affects parcel buoyancy.
Capping inversion: A stable layer that suppresses rising convection until sufficient heating or lifting breaks through it. It can delay thunderstorms and then permit rapid storm development once removed.
Absolute instability: A state in which both saturated and unsaturated parcels become buoyant when displaced upward. It is the strongly unstable limit of the atmospheric stability spectrum.
Moist adiabatic lapse rate: The temperature-change rate of a saturated rising parcel, reduced by heat released during condensation. Its lower, variable rate explains why moisture can make an otherwise stable layer unstable.
Adiabatic process: A thermodynamic change in which a system exchanges no heat with its surroundings. Rising and sinking parcels often change temperature approximately adiabatically.

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