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A skew-T plot showing a morning sounding with a large hydrolapse followed by an afternoon sounding showing the cooling which occurred in the mid-levels resulting in an unstable atmosphere as surface parcels have now become negatively buoyant
Convective instability (also known as potential instability or thermal instability) occurs when dry mid-level air is drawn over very warm, moist air in the lower troposphere. This results in a hydrolapse (an area of rapidly decreasing dew point temperatures with height) where the moist boundary layer and mid-level air meet. As daytime heating increases mixing within the moist boundary layer, some of the moist air will begin to interact with the dry mid-level air above it. Due to thermodynamic processes as the dry mid-level air is slowly saturated its temperature begins to drop increasing the adiabatic lapse rate. Under the correct conditions the lapse rate can increase significantly in a short amount of time leading to convection. High convective instability can lead to severe thunderstorms and tornadoes as moist air which is trapped in the boundary layer eventually becomes highly negatively buoyant relative to the adiabatic lapse rate and eventually escapes as a rapidly rising bubble of humid air triggering the development of a cumulonimbus cloud.
Convective instability is also termed static instability, because the instability does not depend on the existing motion of the air; this contrasts with dynamic instability where instability is dependent on the motion of air and its associated effects such as dynamic lifting.
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[edit] Further reading
- Barry, R.G. and Chorley, R.J. Atmosphere, weather and climate (7th ed) Routledge 1998 p. 80-81 ISBN 0-415-16020-0
