Temperature and Stability
Temperature and Stability is where the exam tests lapse rates and whether air will rise: absolute stability, conditional instability, absolute instability, and inversions.
The three lapse rates
- ELR: environmental, typically 2 °C/1000 ft in ISA.
- DALR: 3 °C/1000 ft (1 °C/100 m) for unsaturated air.
- SALR: about 1.5 °C/1000 ft (0.5 °C/100 m), varying roughly 1.2–1.8 °C/1000 ft; smaller when the air is warm and moist.
Stability rules
- ELR < SALR: absolutely stable.
- SALR < ELR < DALR: conditionally unstable - stable while unsaturated, unstable once saturated.
- ELR > DALR: absolutely unstable. Compare the rising parcel with the environment at the same level; a parcel that stays warmer than its surroundings keeps rising.
What changes the ELR
Steepening (more unstable): surface heating, cold advection aloft, lifting of a whole layer, daytime insolation. Reducing (more stable): surface cooling, warm advection aloft, subsidence, night.
Inversions
Radiation (clear calm night), subsidence (anticyclone, warm dry air aloft with haze trapped below), frontal, and valley inversions. Inversions mean stability, poor visibility and usually smooth air.
Worked example
Surface 25 °C, ELR 2.5 °C/1000 ft. A dry parcel lifted 5000 ft cools at DALR to 25 − 15 = 10 °C. Environment at 5000 ft: 25 − 12.5 = 12.5 °C. The parcel is colder, so the air is stable. Had the parcel been saturated it would cool at SALR to about 17.5 °C, still colder - but the correct lapse rate must be used to show it.
Traps
- Using DALR for saturated air.
- Treating SALR as a fixed constant.
- Judging stability from surface temperature alone rather than parcel versus environment.