Moisture, Humidity, Condensation, and Stability
Concepts to master
Distinguish vapour pressure, mixing ratio, dew point, and relative humidity. Relative humidity depends on temperature; dew point is the temperature to which air must cool to saturate at constant pressure and is a better absolute moisture measure. Know the difference between the environmental lapse rate (ELR), dry adiabatic lapse rate (DALR, about 3 °C/1000 ft), and saturated adiabatic lapse rate (SALR, roughly 1.5 °C/1000 ft, variable). Stability is judged by comparing ELR with the adiabatic rates: absolutely unstable (ELR > DALR), conditionally unstable (between SALR and DALR), and absolutely stable (ELR < SALR).
Condensation requires saturation plus condensation nuclei. Lifting condensation level (LCL) can be estimated: spread (temperature minus dew point) in °C times about 400 ft gives cloud base above ground. Know that saturated air cools more slowly when lifted, which is why moist unstable layers produce deep cloud.
Traps
Questions often treat relative humidity as an absolute moisture measure, or forget that dew point cannot exceed temperature. Another trap is applying the DALR to saturated rising air. Watch for confusion between stability and the presence of clouds.
Rule of thumb
Cloud base (ft AGL) ≈ 400 × (surface temperature − dew point in °C). Example: temperature 20 °C, dew point 14 °C, spread 6 → base near 2400 ft AGL.