Atmospheric Thermodynamics and Stability
This domain tests whether you can predict parcel buoyancy: dry and saturated lapse rates, humidity, and the resulting stability classification.
Lapse rates
DALR is 9.8°C/km (use 10 for mental arithmetic). SALR is about 6°C/km, ranging 4-7°C/km with temperature and moisture, and is smaller because latent heat released by condensation warms the rising parcel. The ELR is the measured gradient of the surrounding air.
Stability
Compare ELR with the adiabatic rates. ELR below SALR means absolutely stable; ELR between SALR and DALR means conditionally unstable (stable for an unsaturated parcel, unstable once saturated); ELR above DALR means absolutely unstable. A steep ELR, from cold air aloft or strong surface heating, favours showers, hail and cumulonimbus; a marked inversion suppresses vertical motion and traps haze or fog beneath it.
Moisture
Relative humidity links temperature and dew point. The spread (T minus Td) shows how close the air is to saturation, and cloud base is roughly 125 m (400 ft) per 1°C of spread.
Worked example
Surface 30/20°C, ELR 8°C/km. Lift an unsaturated parcel 1,000 m: parcel temperature = 30 - 9.8 = 20.2°C, environment = 30 - 8 = 22°C. The parcel is colder and denser, so the layer is stable; because 8 lies between 6 and 9.8, it is conditionally unstable. Cloud base ≈ 2 × 125 = 250 m.
Traps
Do not use the DALR above the lifting condensation level; a steep ELR does not guarantee thunderstorms if the air is dry; remember that inversions are shown by temperature rising with height, not falling.