Atmospheric instability

Atmospheric instability is a condition where the Earth’s atmosphere is generally considered to be unstable.

As air near the ground heats, it becomes more and more unstable and becomes more and more likely to rise convectively. We now take a look at when the atmosphere is stable, less stable, and unstable, what that actually means, and how convection initiates.

STABLE AND UNSTABLE ATMOSPHERE – INSTABILITY

 

Atmospheric stability is the resistance of the atmosphere to the vertical motion of air. A stable atmosphere inhibits vertical motion. An unstable atmosphere encourages vertical motion. The stability depends on how the air temperature changes with altitude (the temperature lapse rate).

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  • Very stable: temperature increases with altitude, a temperature inversion. Air at ground level is negatively buoyant and does not rise. If air is forced to rise, it will sink back towards the ground again.
  • Stable: temperature lapse rate is less than the dry adiabatic lapse rate (i.e. it falls less than by 9.8 °C for every kilometer you go up), but the temperature does decrease with altitude. Air at ground level is negatively buoyant and does not rise. If air is forced to rise, it will sink back towards the ground again.
  • Neutral: temperature is the same as the dry adiabatic lapse rate. I.e. if the air is forced to rise, it will cool at the same rate as the temperature around it drops. It will be neutrally buoyant.
  • Unstable: temperature lapse rate is greater than the adiabatic lapse rate (9.8 °C/km). Air at ground level is buoyant and will rise.
  • Very unstable: temperature lapse rate is much greater than the adiabatic lapse rate (9.8 °C/km). Air at ground level is buoyant and will rise rapidly.