Moisture, thermodynamics, stability and instability
This domain tests humidity variables, saturation, latent heating, adiabatic processes and the stability decision. You must compare the environmental lapse rate (ELR) with the dry adiabatic rate (DALR ≈ 9.8°C/km) and the saturated adiabatic rate (SALR ≈ 5–6°C/km).
Core rules
Dew point is the temperature to which air must cool at constant pressure to saturate; the spread T − Td indicates nearness to saturation. Relative humidity depends on temperature. The lifted condensation level (LCL) is near 125 m per °C of spread (about 400 ft/°C). Unsaturated parcels cool at the DALR; once saturated, latent heat release reduces cooling to the SALR. Stability: ELR < SALR gives absolute stability; ELR between SALR and DALR gives conditional instability; ELR > DALR gives absolute instability. Conditional instability is the normal thunderstorm environment.
Worked example
Surface air 24°C, dew point 18°C, ELR 8°C/km. Spread 6°C gives an LCL near 750 m. Dry ascent to 750 m: 24 − 9.8 × 0.75 ≈ 16.7°C. Environment at 750 m: 24 − 8 × 0.75 = 18°C, so the parcel is cooler and stable below cloud. From LCL to 2,000 m, the parcel cools at 5.5°C/km: 16.7 − 5.5 × 1.25 ≈ 9.8°C. Environment at 2,000 m: 24 − 8 × 2 = 8°C. The parcel is now warmer, so the layer is conditionally unstable and cumulus can grow. Moisture depth, shear and a trigger still matter.
Traps
Do not use the SALR for unsaturated air. The LCL is not the freezing level. High relative humidity does not guarantee cloud at ground level. Latent heat warms the parcel; it does not cool it.