Stability, Instability and Lapse Rates
Comparing lapse rates
Stability is decided by comparing the environmental lapse rate (ELR) with the dry adiabatic lapse rate (DALR, ~3°C/1000 ft) and saturated adiabatic lapse rate (SALR, ~1.5°C/1000 ft). If the ELR is less than the SALR, the air is absolutely stable: a displaced parcel returns. If the ELR exceeds the DALR, it is absolutely unstable. Between the two, the air is conditionally unstable: stable if the parcel is unsaturated, unstable once it becomes saturated. Exam questions love this conditionality.
Triggers and indicators
Instability is increased by surface heating, cooling aloft (cold advection, upper troughs), lifting of a whole layer and the arrival of moist low-level air. It is decreased by surface cooling, warming aloft (subsidence inversions) and drying. Stable air gives layered stratiform cloud, steady precipitation and smooth flying; unstable air gives showery precipitation, cumuliform cloud, turbulence and good visibility between showers.
Traps and worked example
Candidates often forget that an inversion is extreme stability, and that the SALR applies only while the parcel is saturated. A common trap is assuming a high ELR always means thunderstorms: you also need moisture and a lifting trigger.
Worked example: surface 25°C, 5000 ft temperature 5°C. ELR = (25 - 5) / 5 = 4°C per 1000 ft. That exceeds the DALR (3°C/1000 ft), so the layer is absolutely unstable. If the 5000 ft temperature were 12°C, the ELR would be 2.6°C per 1000 ft, between SALR and DALR: conditionally unstable.