Stable and unstable air: cloud, ride, visibility and parcels
Stability = what lifted air does: keep rising (unstable) or sink back (stable). Sets cloud, ride, precipitation, visibility.
| Unstable air | Stable air | |
|---|---|---|
| Cloud | cumuliform (heaped) | stratiform (layered) |
| Ride | turbulent, bumpy | smooth |
| Precipitation | showery, intermittent | steady, continuous, drizzle |
| Visibility | good | poor |
| Surface wind | gusty | steady |
| Also | thermals | fog, low stratus |
Air parcel (FAA AWH 4.2.1):
- Imaginary air volume: big enough to hold many molecules, small enough for uniform properties.
- Standard tool for describing a body of air.
Adiabatic change (PHAK ch.12):
| Air | Change | Term |
|---|---|---|
| Rising | expands and cools (pressure decreases with altitude) | adiabatic cooling |
| Descending | compresses and warms | adiabatic heating |
Lifted parcel:
| State | Parcel vs surrounding air | Result | Cite |
|---|---|---|---|
| Unstable | stays warmer, so lighter: keeps rising | air turns over, cloud piles upward; mixing clears air: visibility good | FAA AWH |
| Stable | turns cooler: settles back down | cloud can only spread sideways into layers; pollutants trapped near surface: visibility poor | - |
| Neutrally stable | matches temperature and density: stays at that level | no tendency up or down | FAA AWH 13.3.2 |
Stability from lapse rate (how fast surrounding air cools with height, vs parcel rates):
| Lapse rate | Stability | Cite |
|---|---|---|
| Steeper than dry (3 °C per 1,000 ft) | absolutely unstable | TC AIM RPA 3.2.22.8; AWH 13.3.3 |
| Shallower than moist (saturated 1.5 °C per 1,000 ft) | absolutely stable | TC AIM RPA 3.2.22.8; AWH 13.3.1 |
| Between the two | conditionally unstable: stable until lifted to its LFC, then unstable | AWH 13.3.4 |
Night (TP 1102):
- Surface air generally grows more stable (radiational cooling).
- Convection cloud dissipates: turbulence much weaker.
Trap: visibility goes backwards. Unstable (rising, mixing) clears air; stable (trapped down) fouls it.
References
- TC AIM RPA 3.2.22.8
- FAA Aviation Weather Handbook (FAA-H-8083-28)
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), ch.12