Temperature, Humidity, Stability and Lapse Rates
Lapse rates
- DALR (dry adiabatic): 3 °C/1000 ft (~9.8 °C/km) for unsaturated rising air.
- SALR (saturated adiabatic): ~1.5 °C/1000 ft, less because latent heat is released.
- ELR (environmental lapse rate): the actual measured rate.
Stability
Compare ELR to the adiabatic rates:
- ELR < SALR → absolutely stable (air resists vertical motion).
- ELR > DALR → absolutely unstable.
- SALR < ELR < DALR → conditionally unstable (depends on moisture).
Humidity
Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure. As air cools, RH rises; when RH = 100%, condensation begins at the dew point. The dew point spread is a quick stability/moisture indicator: small spread → high RH → cloud/fog likely.
Worked example
Surface temp 20 °C, dew point 14 °C. The spread is 6 °C. Using the dry adiabatic rate, air rising cools ~1 °C per 100 m, so the condensation level is about 600 m (roughly 2000 ft) - the cloud base. A rough rule: cloud base (ft) ≈ 400 × spread (°C) ≈ 2400 ft.
Traps
Students often apply DALR to saturated air. Remember SALR is smaller. Also, stability depends on the environmental rate, not on the parcel's rate alone.