Parcel lapse rates
Parcel lapse rate (per 1,000 ft):
| Parcel | Motion | Rate | Cite |
|---|---|---|---|
| Dry (unsaturated, dry-adiabatic) | Rises: cools | 3 °C | TC AIM RPA 3.2.22.8 |
| Dry (unsaturated, dry-adiabatic) | Sinks: warms | 3 °C | FAA AWH 12.2 |
| Saturated | Rises: cools | 1.5 °C | TC AIM RPA 3.2.22.8 |
Why:
- Saturated air cools slower: condensation releases latent heat, offsetting part of expansion cooling (FAA AWH 12.3).
- Water vapour in the column lowers the lapse rate experienced: moist air needs more latent energy than dry air for equal temperature change (TC AIM RPA 3.2.22.8).
Parcel temperature after lift: start - rate x thousands of ft
| Parcel | Calc (20 °C lifted 4,000 ft) | Result |
|---|---|---|
| Unsaturated | 20 - 12 | 8 °C |
| Saturated | 20 - 6 | 14 °C |
Trap: dry 3 °C rate used for saturated air; saturated is 1.5 °C per 1,000 ft.
References
- TC AIM RPA 3.2.22.8
- TC AIM AIR 1.5.1
- FAA Aviation Weather Handbook (FAA-H-8083-28)