Moisture, Humidity and Atmospheric Stability
Humidity concepts
Candidates must know the difference between relative humidity (actual vapour content relative to saturation at that temperature) and dewpoint (the temperature to which air must cool to saturate). Dewpoint depression (temperature minus dewpoint) measures how close the air is to saturation. Warmer air holds more vapour, so relative humidity changes with temperature even if the vapour content is constant - a key trap.
Lapse rates and stability
Stability depends on comparing the environmental lapse rate (ELR) with the dry adiabatic lapse rate (DALR ≈ 3°C/1000 ft) and the saturated adiabatic lapse rate (SALR ≈ 1.5°C/1000 ft, variable). If the ELR is steeper than the relevant adiabatic rate, air parcels accelerate upward → unstable. If shallower → stable. Between the two, conditional instability applies. Stability is enhanced by cooling aloft, warming the surface, or lifting the air mass; it is reduced by warming aloft or cooling the surface.
Traps
Questions confuse the SALR with the DALR, or claim that relative humidity is fixed when temperature changes. Others forget that the SALR varies with temperature and moisture.
Worked example
ELR is 2.5°C/1000 ft, air is unsaturated. Compare with DALR 3°C/1000 ft: ELR is shallower, so a lifted parcel cools faster than the environment and stability is stable. If ELR were 3.5°C/1000 ft, it would be unstable.