If the environmental lapse rate is greater than the adiabatic lapse rate, the atmosphere is unstable.

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The statement asks about atmospheric stability in relation to the environmental lapse rate (ELR) and the adiabatic lapse rate (ALR).
First, recall two key ideas: the environmental lapse rate is how temperature naturally decreases with height in the surrounding air, while the adiabatic lapse rate is how a rising or sinking air parcel would cool or warm as it expands or compresses without heat exchange.
Now, compare ELR and ALR: if the environmental lapse rate is greater (steeper) than the adiabatic lapse rate, the environment loses temperature with height more rapidly than the rising parcel does. In other words, at some ascent height z, T_env = T0 − ELR·z and T_parcel = T0 − DALR·z, with ELR > DALR implying T_env < T_parcel.
Because the displaced parcel remains warmer (less cooled) than its surroundings at the same height, it becomes less dense than the surrounding air and continues to rise. This positive buoyancy leads to instability.
Conversely, if ELR were smaller than DALR, the environment would be warmer than or equal to the parcel, promoting stability or neutral conditions.
Therefore, with ELR > ALR, the atmosphere tends toward instability, as the rising parcel experiences buoyant acceleration due to its relative warmth compared with its surroundings.
Note: in moist or saturated conditions, the moist adiabatic lapse rate (which is smaller in magnitude than the dry adiabatic rate) would modify the exact threshold, but the general principle remains: a steeper ELR than the appropriate adiabatic lapse rate favors instability.

CLMTGR5001_002_2025_3 - Dynamics of Climate Variability and Chan PRACTICE QUIZ: Week 3 Lecture 3

If the environmental lapse rate is greater than the adiabatic lapse rate, the atmosphere is unstable.

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